FIG. 52-60.—TYPES OF DRY FRUITS Fig. 52. The strawberry. The fleshy part consists of the modified upper end of the flower stalk or receptacle, while the true fruits are the dry achenes on or embedded in the surface and popularly called the seeds. Fig. 53. A three-celled capsule splitting lengthwise as in the common Iris. Fig. 54. Fruit of the cocklebur, the hooked prickles of which are admirably adapted for clinging to the fur of animals. Fig. 55. Pods of a plant of the Mustard family, which split down both edges, unlike the true peas, which split down only one edge. Fig. 56. Two types of achenes of the daisy family tipped with plumed bristles, greatly aiding their carriage by the wind. Fig. 57. Common garden pea—a typical legume. Note that it splits only on one side. Fig. 58. The samara or two-winged fruit of the maple. Fig. 59. The samara or single-winged fruit of the ash. Fig. 60. The dry two-pronged and bristly fruit of the unicorn plant (Martynia), admirably adapted for dispersal by animals.FIG. 52-60.—TYPES OF DRY FRUITSFig. 52. The strawberry. The fleshy part consists of the modified upper end of the flower stalk or receptacle, while the true fruits are the dry achenes on or embedded in the surface and popularly called the seeds. Fig. 53. A three-celled capsule splitting lengthwise as in the common Iris. Fig. 54. Fruit of the cocklebur, the hooked prickles of which are admirably adapted for clinging to the fur of animals. Fig. 55. Pods of a plant of the Mustard family, which split down both edges, unlike the true peas, which split down only one edge. Fig. 56. Two types of achenes of the daisy family tipped with plumed bristles, greatly aiding their carriage by the wind. Fig. 57. Common garden pea—a typical legume. Note that it splits only on one side. Fig. 58. The samara or two-winged fruit of the maple. Fig. 59. The samara or single-winged fruit of the ash. Fig. 60. The dry two-pronged and bristly fruit of the unicorn plant (Martynia), admirably adapted for dispersal by animals.
Fig. 52. The strawberry. The fleshy part consists of the modified upper end of the flower stalk or receptacle, while the true fruits are the dry achenes on or embedded in the surface and popularly called the seeds. Fig. 53. A three-celled capsule splitting lengthwise as in the common Iris. Fig. 54. Fruit of the cocklebur, the hooked prickles of which are admirably adapted for clinging to the fur of animals. Fig. 55. Pods of a plant of the Mustard family, which split down both edges, unlike the true peas, which split down only one edge. Fig. 56. Two types of achenes of the daisy family tipped with plumed bristles, greatly aiding their carriage by the wind. Fig. 57. Common garden pea—a typical legume. Note that it splits only on one side. Fig. 58. The samara or two-winged fruit of the maple. Fig. 59. The samara or single-winged fruit of the ash. Fig. 60. The dry two-pronged and bristly fruit of the unicorn plant (Martynia), admirably adapted for dispersal by animals.
dispersal of their seeds. Greater food value to birds and animals overcomes this in some kinds, and another help is that some fruits of this sort are covered with hooked prickles or barbs (Figures54and60). The common weedy burdock, the barbed fruits of which may often be found sticking to the fur of animals in great quantities, is a case in point. There are whole groups of plants that rely on this method for seed dispersal, notably the avens, tick-seed, tick trefoil, and many shrubs in the tropical regions.
Where the fruits are neither barbed nor very good to eat, and so apparently doomed to be more or less permanent stay-at-homes, nature has provided some of them with the proper equipment for flight through the air. Winged fruits like the maple are to be seen on any windy day during their season scurrying before the breeze, and consequently spreading their kind over considerable distances. In the maple there are two wings, joined at the base where the seeds are embedded in the wings, and the fruit is known as asamara(Figure 58), or key fruit, from a slight resemblance to an old-fashioned key. Ash trees bear fruits that are a slight modification of this type and may be carried considerable distances by the wind (Figure 59).
In the dandelion, daisy, and nearly all its thousands of relatives, this faculty of setting sail in the air has been carried to the greatest perfection, just as we saw its flowers were. In this family of plants, the largest in the world, the fruit is mostly tipped or surrounded by a small collection of very fine bristles. The fruit, known as anachene(Figure 56), is so light that with the added buoyancy of this tiny collection of down it can be transportedgreat distances. Some have been known to fly hundreds of miles in severe storms, and, as we shall see in the chapter on plant distribution, these tiny plant balloons have played a conspicuous part in spreading their kind over the face of the earth. Cat-tails also, together with many other plants, have this faculty and make up by its possession for the lack of fleshy or otherwise desirable fruits that might be carried. Allachenesare not winged, those which dot the surface of the strawberry being imbedded in the luscious flesh, which is not really fruit at all. Only the achenes on the strawberry are true fruits, the fleshy part being merely a development of the upper part of the flower stalk and not of the ovary (Figure 52).
Fruits, then, cannot be restricted to the common understanding of them. They are transformations of the ovary, in which or upon which seeds are nursed, and upon which most plants depend for the dispersal of their seeds. We shall see later on how fruits have fulfilled their destiny, how some are fit for their true function only when they have been eaten by birds, and when some digestive juice has released them from the impotence they would suffer without being eaten, how a whole forest has been changed in the West by the busy activity of squirrels upon the fruits and seeds of a single kind of fir tree; how the fruit of the coconut palm has been spread throughout the tropical world because it can float in the sea securely protected from injury from salt by the impervious coverings of its fruits.
As the final stage in the development of all plants is their seed, with the dropping of which they bidgood-by to their fellows, it is not perhaps remarkable that in the seed of all flowering plants is the germ for the new generation. To seeds which may be as small as the mustard, so often mentioned in the Bible, or as large as the coco de mer, or double coconut, from the Seychelles Islands, often fifty pounds or over, is intrusted by cunning nature the one final and most important act in the whole kingdom of the plant world. Nearly all plants would die off forever if seeds did not have in them the germ of life, apparently quite dead, but actually only dormant. This living germ may persist for years, sometimes even a hundred years, and yet with the proper conditions it never fails to sprout.
Seeds have inside them a tiny plantlet folded and ready to grow when the seed splits to release it. Also, in the seed is stored up food to sustain the new plant until such time as its own roots begin to act. This young plantlet is known as theembryo, and to this all actions of the seed are subservient.
As the seed splits, and the young plant develops its first leaves and rootlets, there is shown one of the most remarkably uniform tendencies in plant life. In all plants with net-veined leaves the young plantlet starts life with two leaves, orcotyledons, as these first leaves are called, and this whole group of plants are thus known asdicotyledons. In plants with parallel-veined leaves the young plantlets start out with a single cotyledon and are therefore calledmonocotyledons. In only the pines, spruce, and a few other evergreen trees the seedling plants have several cotyledons and are known aspolycotyledons. All the flowering plants in the world belong to one of these groups, so that merely to see the germinating seed tells the story at once. The linking ofparallel-veined leaves and a single seed leaf, and net-veined leaves with two seed leaves, is also associated with very definite arrangement of their flower parts, their method of growth and other characters. Something has already been said of this in the discussion of stems and leaves, and more will be found in the chapter on plant families. No more beautiful example of the plan or scheme of nature is to be found than these characteristics of all plants, and in seeds we find the first hint as to which army the plant will join, under which banner it will fight, and under what generalship it will develop. Nothing tells us so much as these first seed leaves, pushing their way up through the soil and revealing, as they burst above ground, to what place in nature their destiny will consign them.
Flowering plants, which make up the bulk of the vegetation of the earth, have been discussed in some detail, not only because they furnish us with all the things that make life possible, but also because they show perhaps better than anything else the division of labor, all striving for one end. Roots, the food gatherers. Stems, the framework for the foliage and its means of reaching the light, or as a storage house for reserve food. Leaves of many kinds, all factories working night and day to make the necessary food. Flowers of every hue and shape to lure insects, or by other means secure union of male and female. Fruits to ripen the result of this mating of the sexes. And, finally, the seed carrying with it the yet unborn life. Each part occasionally losing itself in order that the end may be accomplished, many of them changing their form or even their function where that is of advantage, all in their separate ways doing their task, the end of whichthey cannot see, and the fruits of which they will never enjoy. Nowhere is it so true as in plants that to save oneself there must be the capacity to give oneself. Untold millions of leaves fall, or trees crash down, or seeds are developed, each fulfilling their destiny which is to insure the perpetuation of their kind. As we shall see later on, there are many mistakes, many apparently futile attempts, thousands are wiped out that one may be saved, and in the past multitudes have gone out forever. Yet the result of it all is the plant world as we know it to-day, each kind struggling to increase its sphere of influence, or to cover more of the earth’s area. The combat between different kinds is inexorable, yet the capacity for sacrifice on the part of different organs, in order that a certain individual kind may win, is literally beyond belief.
In the light of what has been said about flowers it may well be questioned how anything can be a plant and still have no flower. The fact is that flowers as we commonly understand them are unknown in the plants about to be discussed, but that whatcorrespondsto a flower, and performs thefunctionof a flower all plants must and do have. In the case of most flowering plants the possession of flowers is one of the beauties of nature in its most resplendent mood, while in the so-called flowerless plants the functions of flowers are performed by tiny microscopic organs, even the existence of which has been only recently discovered. Because flowering plants produce their sexual organs in such a gorgeous setting, for all the world to see their matings they have been calledphanerogams,which means literally visible marriage, while the flowerless plants which perform similar functions in more secret ways are calledcryptogams, meaning hidden marriage.
Thesecryptogamsor flowerless plants occur in far greater numbers in the world than flowering plants, but their size in most cases is very much less. Many individuals are so small, as in the case of bacteria, that a single one can only be seen after it has been magnified many hundreds of times by the microscope. Of the cryptogams some of the largest, certainly the most beautiful, and probably the best known are
In nearly all woods one may find delicate feathery plants with graceful, usually much divided leaves that nearly always start up from the ground like a slowly opening, but somewhat fuzzy coil. (Figure 62.) Ferns, at least most of those that grow in America, uncoil their leaves in this way, almost without exception. The accompanying figure shows the procedure, and in addition to this character one may hunt in vain for flowers.
While they bear no flowers we already know that nature could not leave them with no means of reproduction without abandoning them to a childless old age and the consequent extinction of the race of ferns. So far from the truth is this that ferns make up a goodly proportion of the world’s vegetation, and there are many hundreds of different kinds known. The lack of flowers, of course, explains why ferns do not bear seeds which are matured in a fruit or ripened ovary.
On the back of the leaves of most ferns, along or near the edges of the finer subdivisions, one may
FIG. 61-63.—COMMON WOODLAND FERN Fig. 61. A general view. Fig. 62. Its uncoiling spring condition. Fig. 63. The back of one of the smaller divisions of the leaf showing the collection of spore cases (sori). These are sometimes borne on special leaves, but in most of our American kinds on the backs of ordinary foliage leaves.FIG. 61-63.—COMMON WOODLAND FERNFig. 61. A general view. Fig. 62. Its uncoiling spring condition. Fig. 63. The back of one of the smaller divisions of the leaf showing the collection of spore cases (sori). These are sometimes borne on special leaves, but in most of our American kinds on the backs of ordinary foliage leaves.
Fig. 61. A general view. Fig. 62. Its uncoiling spring condition. Fig. 63. The back of one of the smaller divisions of the leaf showing the collection of spore cases (sori). These are sometimes borne on special leaves, but in most of our American kinds on the backs of ordinary foliage leaves.
find, at the proper season, collections or rows of tiny, usually brownish dots. These contain often thousands of microscopic objects known generally asspores, and from this fact the dots are called spore-cases, or more technicallysori. (Figure 63.) The process by which new plants are formed is a
Aerial Roots of Fig Trees Hanging Over the Edge of a Cave in the Rain Forest, San Lorenzo, Santo Domingo. (Photo by the author. Courtesy of Brooklyn Botanic Garden.)Aerial Roots of Fig Trees Hanging Over the Edge of a Cave in the Rain Forest, San Lorenzo, Santo Domingo.(Photo by the author. Courtesy of Brooklyn Botanic Garden.)
Aerial Roots of Fig Trees Hanging Over the Edge of a Cave in the Rain Forest, San Lorenzo, Santo Domingo.(Photo by the author. Courtesy of Brooklyn Botanic Garden.)
Venus’s Flytrap, an Insectivorous Plant of the Southeastern United States. The fringed valves of its leaves close together when an insect alights between them. (Courtesy of Brooklyn Botanic Garden.)Venus’s Flytrap, an Insectivorous Plant of the Southeastern United States.The fringed valves of its leaves close together when an insect alights between them. (Courtesy of Brooklyn Botanic Garden.)
Venus’s Flytrap, an Insectivorous Plant of the Southeastern United States.The fringed valves of its leaves close together when an insect alights between them. (Courtesy of Brooklyn Botanic Garden.)
somewhat complicated one, but the spores in these brown dots are the agency which makes reproduction possible, and the actual mechanism of it, one of the most interesting achievements in plant life, will be described in the chapter on “How Plants Produce Their Young.†Sometimes the spores are not borne on the backs of ordinary foliage leaves but on special leaves that bear, very often, nothing else.
Ferns are much like ordinary flowering plants; except for their lack of flowers, they have all the root, stem, and leaf characters of their more showy neighbors. While most of them have compound leaves, even sometimes twice or thrice compounded, a few have simple, narrow leaves without teeth, and one kind in tropical America has threadlike leaves. In many tropical rain forests, so called from their dripping wet condition, ferns form large trees, and these tree ferns are among the most graceful and feathery of all plants. There are, too, a few climbing kinds—one, called the climbing fern, is a native of the eastern United States. Then there is the walking fern, that seems to upset the statement that plants do not move as animals do. It sends out delicate runners that, rooting at the tips, form new plants, often several feet from the parent plant.
The characteristic of having, even in the simplest form, stems, leaves, and roots, with all that this implies in their internal structure, marks them off at once from all other flowerless plants. In ferns there is always some internal equipment for carrying food from one part of the plant, the roots, to another, and this ability is possessed by virtue of ducts or vessels through the stem and leaves. This system, found in all flowering plants and ferns, butnowhere else in the plant world, is called thevascular system,or literally, a vessel system. We shall see how important was the acquirement of this system of vessels, when we get to the chapter on the History of the Plant Kingdom. Its appearance upon the earth marks as important a stage in the development of plants as the dawn of a definite backbone did upon animal life.
Ferns, then, arevascular cryptogamsbecause they do have conducting vessels in their stems, and they produce their young by a process of hidden marriage which will be described later. All other cryptogams or flowerless plants are without this system of vessels and are called thereforenon-vascular cryptogams. Numerically they are tremendously important; upon them depend many manufacturing processes like bread making, brewing, and all arts using fermentation. But they are hardly recognized as plants by the general reader, and because of their size and the necessity of studying them with a microscope in order to understand their structure they will be treated here only briefly.
The remaining flowerless plants, having no duct system in their make-up, are, as we know, callednon-vascular cryptogams. This is a general term for a very large group of plants, some quite obvious and well known like a mushroom, for instance; others so small or of such uncertain structure that they are not even well known by experts. This great mass of plant life, more numerous than all the other kinds of plants combined, contains many different forms, some of which are of gigantic size. A single plant of a certain Pacific Coast seaweed regularly exceeds in length the height of the tallest
FIG. 64-67.—TYPES OF FLOWERLESS PLANTS Fig. 64. A moss plant. Fig. 65. A mushroom, a common type of the fungi, which include also puffballs, molds, and many disease-causing microscopic organisms. Fig. 66. A common seaweed, a representative of the algæ, which include the green scum on the top of ponds, and the kelp from which fertilizer is now being made. Fig. 67. A lichen, a common cryptogamous plant on logs and rocks. Our native kinds are usually grayish-green in color.FIG. 64-67.—TYPES OF FLOWERLESS PLANTSFig. 64. A moss plant. Fig. 65. A mushroom, a common type of the fungi, which include also puffballs, molds, and many disease-causing microscopic organisms. Fig. 66. A common seaweed, a representative of the algæ, which include the green scum on the top of ponds, and the kelp from which fertilizer is now being made. Fig. 67. A lichen, a common cryptogamous plant on logs and rocks. Our native kinds are usually grayish-green in color.
Fig. 64. A moss plant. Fig. 65. A mushroom, a common type of the fungi, which include also puffballs, molds, and many disease-causing microscopic organisms. Fig. 66. A common seaweed, a representative of the algæ, which include the green scum on the top of ponds, and the kelp from which fertilizer is now being made. Fig. 67. A lichen, a common cryptogamous plant on logs and rocks. Our native kinds are usually grayish-green in color.
known trees. And yet other inhabitants of the water, certain kinds that float freely, are microscopic in size. The latter occur in such enormous numbers that their tiny decomposed skeletons after dropping to the bottom of the sea form the diatomaceous earth, so much used in polishing machinery. The commercial product now comes from deposits of these skeletons laid down in pastages, which, due to changes in the land and water surfaces of the earth, are now found in Virginia, Nevada, California, and in Bohemia. All these must have been in the bed of waters long since gone, which teemed with these microscopic organisms. To-day there are over ten thousand different kinds known, yet so small are they that their dimensions are measured in thousandths of an inch!
Somewhat lower in the scale of life—and by this we mean simpler in structure—than the ferns are themosses. (Figure 64.) There are thousands of different kinds, but everyone is familiar with the collective growth of the commoner sorts which makes the velvety mossy carpet in our woods. The individual plants are small, but in many kinds sufficiently large to be seen without a microscope. Most important of all, practically every one of them has the ordinary green color of the better known plants, and as we shall see in the section devoted to “Leaves as Factories for the Making of Food,†that stamps them at once as plants, if other things did not.
Mosses are almost infinite in their habits, some growing on the dry rocks or trunks of trees, many growing in moist woods, some in the water, and immense quantities of certain kinds in bogs. The peculiar bog mosses, known as sphagnum, play an important part in forming peat and perhaps coal. While mosses are otherwise not of much commercial importance, they are among nature’s most beautiful ground covers, carpeting many a nook and dell with a soft, velvety, almost cushionlike growth.
Although they are rather small, they appear to have a somewhat definite stem and tiny leaflike appendages of it, without, however, having thevascular system found in all ferns. Mosses might almost be considered miniature ferns, of which they are perhaps only simple ancestors. Their vegetative or green parts vary much in shape, size, and the arrangement of the tiny leaflike appendages, and while most of them are a beautiful bright green, nearly all the bog or sphagnum mosses are rather ashy gray in color. In most of the typical mosses there arises from among the vegetative growth of them a slender stalk, at the top of which is a small capsulelike organ. This contains the spores, and it is upon this long slender stalk and its spore-filled capsule, really marvelous in its internal structure and mechanism for the discharge of the spores, that mosses depend for their reproduction. As in the case of the ferns this process will be considered later, along with that of some other plants. This whole story of how plants produce their young, perhaps the most fascinating of any part of the study of plant life, is so fundamentally a part of their history and shows nature in her most maternal moods, that a special chapter will be devoted to it. There we shall see, as a whole, how these vastly different acts of fertilization and reproduction are, in different groups of plants, all responses to that insistent command for life, more life, in a never-ending stream.
The chief characters to remember about mosses are that they are very simple, but practically always green plants that have some differentiation into stem and leaf; that, while they have no vascular system, their structure and particularly the mode of reproduction suggests that they are not very distant from the ferns, and quite likely simple ancestors of them. These characters are of more importancethan appears on the surface, as we shall presently see, for they mark mosses off from many other nonvascular flowerless plants which have quite different structure and altogether different mode of life.
If you will turn to the chapter on Plant Behavior and read particularly the sections on “Leaves as Factories for the Manufacture of Food†and “Borrowing from the Living and Robbing from the Dead,†you will see in the food habits of the plants there noted the great difference that exists between plants, like mosses and ferns, that have green coloring matter in them, and those we are about to mention that never do. The lack of this green coloring substance tells us at once that plants of this sort live only on the dead remains of other plants. In the case of these nonvascular flowerless plants there are certain modes of growth that, in some forms at least, are always associated with this scavenger-like food habit.
The common mushroom (Figure 65) is the best known of that large group of plants, called generally fungi, which produce no green coloring matter, have no leaves attached to a stem, andalwayslive on decayed vegetable, or sometimes inhabit living animals, even man himself. The mushroom with its brownish stalk and buttonlike dome is familiar enough, but there are literally thousands of different kinds, a common sort forming “brackets†on the trunks of trees. While perhaps everyone would recognize these as plants, peculiar as they are in their often weird shapes and unusual as they nearly always are in their color, there are many minute kinds of fungi that scarcely anyone would even think of as a plant, and yet for better or worse they are incomparably the most powerful plants in theworld. For upon these microscopic fungi man depends for many things. It is certain kinds of them that make the manufacture of cheese possible. They turn milk sour (pasteurizing milk is merely stopping their work), give to yeast its power of “raising†bread, all brewing depends upon them, every process of fermenting the juice of fruits for wine making or for whatever else, the decay of wood—all these processes and scores of others, whether for the good or evil of mankind, depend upon the work of these plants, any one of which is so small that a single individual must be magnified hundreds of times to detect it. Many of them are the “germsâ€â€”better called bacteria—that cause diseases like tuberculosis, cholera, typhoid, anthrax, and diphtheria. All surgeons wage incessant warfare against a host of them that attack wounds and form pus. They live in our intestines and have much to do with digestion, and unhappily with indigestion, so that we may be said to carry about with us a whole flora of them! Nearly all the diseases of plants, like the blight of potato and the rust on wheat, are caused by them. Some other kinds live in the soil, and many flowering plants depend absolutely for getting their food upon the work of these fungi. Unfortunately their minute size and consequently obscure mode of life demand technical skill and the use of the microscope to detect them, so we must leave them here, always keeping in mind that these smallest of all plants are charged with a power for good or evil; so far as man’s life is concerned, greater perhaps than all other plants.
While most fungi, particularly those familiar ones like mushrooms and puffballs, are inhabitants of the land, the remaining group of nonvascularflowerless plants are nearly all water plants. Most of the better known ones live in the sea, and as the wrack or tangle washed up on the shore we recognize them as seaweed. Thealgæ(Figure 66), which is a general name for such plants—and they live in the sea, in fresh water, and even on dry land—are, so far as structure is concerned, the simplest of all plants.
Those that are fastened to rocks are often beautifully colored, much branched, and many kinds bear small bladders that act as buoys. These coast seaweeds are generally of different colors, those nearest the surface being generally greenish, the deeper water kinds reddish or brown. None of these seaweeds are found at great depths, because the really deep parts of the ocean are almost, if not quite, dark. Seaweeds, and in fact all the algæ, have green coloring matter in them, even where this is masked by reds and browns, as is the case in some particularly showy kinds. As you will find in the section on “Leaves as Factories for the Making of Food,†no plant with green coloring matter can live in the dark. That is why seaweeds are not found in the great deeps of the sea, some of which are several miles below the shore line along the coasts, and are so cold and dark that neither plants nor animals can grow in them.
Those seaweeds that grow along the coast, and are uncovered by the retreating tides, are well known by everyone, but by far the greater number of algæ float without anchorage of any kind. One kind that has been torn from its anchorage occurs in such enormous quantities that off the coast of America it has formed literally a floating island composed entirely of dense mats of a species of seaweed. Thisplace, known as the “Sargasso Sea†from the name of the seaweed forming it, was the terror of old mariners and Columbus’s ship was fouled in it for two weeks. The area occupied by the weed is several hundred miles long and wide, and while old sea yarns about ships being caught in it and never escaping are gross exaggerations, it is certainly one of the most curious of plant growths, due entirely to a nonvascular cryptogam.
Of those kinds that are never anchored the number is legion, and in addition to those forming the diatomaceous earth, already mentioned, there are many more. They form almost the only food of hosts of creatures of the sea, but because of their floating freely in the water, the consequent difficulty of collecting them, and their unusually minute size, little is likely to be known of them, except by the experts.
Other algæ are always found in fresh water and form the scum found on stagnant pools. Individuals of any of these are so minute that, while under the microscope they are of the greatest beauty, their structure must remain for most of us a sealed book.
We have now traced, in only the briefest fashion, the outlines of what plants are, reversing the order of nature in beginning with those most complex but best known, the flowering plants. As we shall see later, these are the climax of prodigal nature and are to be considered the end rather than the beginning of plant life on the earth. Then, and still more briefly, have we stopped to see those less known plants that produce no flowers, such as the ferns, mosses, fungi, and finally the seaweeds or algæ.These are all to be considered as the ancestors of flowering plants, the ferns the nearest to them and the algæ probably the most distant relatives. The development of plants from the minutest alga up to our most gorgeous flowering plant, is an infinitely slow and painful process. With many mistakes, with its pathway strewn with the wreckage of forlorn hopes and false starts, it is incomparably the most dramatic story in the plant world. Some of its details will be told in the chapter on the “History of the Plant Kingdom.â€
Nor can we leave the discussion of what plants are without some mention of the thing that really makes up their structure, whether it be a microscopic bacterial organism or the Big Tree of California. For the unit of all animal and plant life is thecell. In its simplest form it is merely a minute sac with a definite wall and inside the wall is a substance known asprotoplasm, literallyprotos, first, andplasma, thing formed. It is protoplasm that forms the living tissue of all plants and animals; it is life itself. No one has ever succeeded in making any, notwithstanding that many learned men have tried for years. Its inclosure in the cell wall, its power of self-division and consequent multiplication of the units, make up those first things about which most of us can never know much, but the end of which we recognize in the beauty of plant life all about us. For only under the highest powers of the microscope may cells be seen and studied. Just as bankers reckon mills as a definite unit of a cent, and yet none of them has ever seen a mill, so we must think of cells as the definite unit of all living things, although most of us will never see a cell. But, unlike the mill, cells may be seen by those equippedto see them, and this study, the development and grouping of them to form all the varied objects that inhabit the plant world, is known ashistology. It is literally the internal history of plants and animals, and lies quite outside the scope of this book. What we must never forget is that whatever knowledge we have gained, either from the foregoing account of what plants are, or from our observation of them, is, after all, only a partial notion of them, as unsatisfactory as our estimate of what people really are, from merely looking at the outside of the houses in which they live. The outer form we may know and admire, the inner substance must ever remain for most of us a secret treasure house the value of which is certain, but the key to which we do not possess.
PRACTICALLY all that has been said in the first chapter relates to what plants are, their organs, or what we may call the architecture or plan of their framework. But what they do with this elaborate structure is as important as what we do with a house that may contain every modern improvement but is never a home until these things have been put to use. One of the chief concerns of any architect is to see to it that the house has as much sunshine by day and as attractive illumination by night as possible. Nature, that greatest of all architects, also sees to it that plants get the utmost necessary sunlight, but for a much more important reason than the mere attractiveness of sunshine, be that ever so beautiful. For light, the life-giver of all green things, is so absolutely essential to plants that experiments to grow them in the dark have always failed, and many gardeners now use electric light in greenhouses in order to prolong the short daylight of winter. It is the lack of light that makes celery blanch.
Plants grown in the house inevitably turn toward the windows, even plants growing against a wall turn their leaves away from it—nowhere can one find living green things that do not find the light as surely and persistently as men try to get their foodor their mates. Many examples of this could be given and must have been noticed by everyone.
Sometimes seeds germinate under a barn floor for instance, and the puny pale little plantling reaches out slender stems, all of which turn, as a compass turns to the north, to perhaps a crack of light in one corner of the building. We have already seen how the search for light will carry the slender rattan palms of India hundreds of feet to the topmost leaves of the forest. Individual plants, and, as we shall see later, whole forests make desperate efforts to get to the light. We know already, that the struggle for light is just as bitter as the struggle for food by roots. And finally if, as we have many times proved by experiments, plants die when grown in a dark room, what is it that light does for plants and how is a process carried on that everything leads us to think is of the greatest possible importance? Quite obviously it is not the mere beauty of sunshine dancing upon the landscape, as entrancing a picture as that may be any summer afternoon, with the play of sunshine and shadow on the tracery of foliage. That green color of the foliage, the almost universal green of so much of the earth’s vegetation, restful to tired eyes, providing us with the most pleasant shade, has wrapped up within it the secret of just what sunshine does for plants. For under the magic of light acting upon this greenery one of the most important industries in the world, the manufacture of food, is constantly going on.
It must be clear enough from the start that to call a leaf a factory for the making of food forces us to decide at once whether this is a mere way ofspeaking, or whether, incredible as it may seem, anything as thin as a leafcanreally produce food. As we eat lettuce, and millions of cattle graze every day, leaves as food producers win handily on that score. But to understand how food is produced in such a tiny factory demands that we walk about in it for a bit, study the inside of it and especially its many small chambers within which is not only the machinery, but some of the finished product stored up for later use.
Unlike modern factories there are many entrances, from any one of which we can begin our tour of inspection. On the under side of nearly all leaves and on the upper side of some there are scores or even hundreds of small pores calledstoma, so small that only with a microscope can they be seen. These entrances through the factory wall, are carefully guarded by a pair of watchmen whose business it is to see neither too much dry air gets in nor too much of the product of the factory gets out. They see to it, also, that waste products are thrown out at the proper time. These watchmen, orguard cells, as they are called, are constantly on the job, work almost automatically, but their chief function is connected with the proper ventilation of the place, and will be discussed later under “How Plants Breathe.â€
Once past the entrance it is obvious that we are in one of the strangest of all factories, for none of the rooms are truly square or oblong and their irregularity as to outline would drive your average foreman into profanity. Yet they are certainly divided into distinct classes, at least as to size and as to what the rooms contain. Some are apparently filled with nothing but air and have direct connection through the stoma with the outdoors. These are calledintercellular spaces. Others, and these are most important, are filled mostly with the green coloring matter that gives the leaf its color. This substance is known aschlorophyll, its individual units aschloroplasts, or literally, chlorophyll bodies. Quite independently of these chlorophyll cells or rooms, or the intercellular spaces which correspond to halls, there are some large and many small tubes. These are the veins of the leaf and their finer branches and by their direct connection through the stem to the roots, serve as the ducts through which some of the raw materials are brought into the factory.
This green coloring matter or chlorophyll is perhaps the most important substance in nature. Without it all except a very few plants would die, and even in those beautifully colored leaves like coleus or caladium chlorophyll is always found, but in these colored leaves it is merely obscured by other coloring substances. It is in the chlorophyll that the ability resides to take the inorganic substances through the roots or from the air, and by the aid of sunlight transform them into organic substances like starch and sugar. Nothing else in all nature can do it; without this faculty, which the commonest green leaf possesses, the earth would prove uninhabitable within a single year. Just what chlorophyll is chemically is not yet thoroughly known, but the thing of chief interest is that it is hardly ever found in parts of the leaf not exposed directly to the sunlight, and that during the autumnal coloring and before the fall of the leaf chlorophyll is carried to other parts of the plant, and quite possibly stored for use the following season.
While the composition of chlorophyll is not surely known, iron is certainly one of its constituents, asplants deprived of iron lose their green color. It also is known to contain oxygen, carbon, hydrogen, and nitrogen, but merely to catalog what we know about its make-up does not tell us that it is a living green substance and that sunshine sets it in motion. Just exactly how light acts on chlorophyll no one really knows; we merely know that it does so act and that the result is one of the marvelous secret processes of nature, perhaps like the secret of life itself forever hidden from man. In our tiny factory, then, we have raw products coming from the roots and through the stoma from the air; machinery of the most efficient type, for chlorophyll works night and day, and constantly renews itself while producing the finished products; energy from the sun; and finally the complete manufactured products which are foods in the shape of starch and sugar. During the growing season there is no banking of the fires, no stoppage of this most important of all industries, no strikes or lockouts. Each part of the whole works smoothly and with the nicest precision—in fact so perfectly does this process keep on going, so complete is the orderliness of the place, and so regular are the completed products turned out, that no modern factory manager or workman but can learn something from a rather close study of this smallest but most efficient factory in the world.
Some of the raw products are delivered to the leaf from the roots where they have been absorbed by another process that will be considered a little later. These consist of water and the inorganic substances dissolved in it, popularly called sap. Carbon and oxygen come mostly from the air, sometimes separately, more often in the form of a combination called carbon dioxide which is one of the chief constituentsof the gas thrown off by man as he breathesout. Now these inorganic substances, contained in the sap or derived from the air, are literally mixed by the chlorophyll and form, always with the aid of sunlight, substances known ascarbohydrates, the commonest example of which is sugar. Some form of sugar is one of the earliest results of this process, but sugar is quite easily dissolved in the sap which has contributed to its manufacture, and the excess sugar is thus removed. Otherwise it would clog the machinery and prevent the production of fresh supplies. This first step in the manufacturing process has not inaptly been calledphotosynthesis, the meaning of whichphotos, light, andsynthesis, combining by means of, suggests in a word the necessity of light and the combination of the inorganic substances mentioned above. Of course this process of photosynthesis is not as simple as the brief account of it suggests, for it is actually a complicated chemical process only part of which is yet understood. It is fairly certain that it goes step by step; it is quite certain that the beginning is inorganic and the end organic compounds like sugar. Something is known also of the wear and tear on the chlorophyll, its waste products, and how it keeps itself not only fit but provides for its own constant renewal. One of the excess or by-products in this initial manufacture of sugar is oxygen. This is either used in other ways by the plant, or more generally it is thrown off through the stoma into the outer air. Oxygen, as one of the necessary constituents of the air that man breathesin, is thus thrown off, while, as we have seen, carbon dioxide, a poisonous gas which we breatheout, is a necessity for this manufacturing process in all green plants.Hardly any trick of nature so completely fulfills the wants of animal and plant life as this mutual exchange of by-products—in the case of animals it is the waste of respiration, in plants it is the wastage of sugar making and some other changes that go on in the plant just after this stage.
The amount of sugar made, carbon dioxide taken in, and oxygen given off by this process suggests that while leaves may be very tiny factories they are among the most efficient in the world. Assuming an area of leaf surface equal to about a square yard the amount of sugar made would be about one-third of an ounce in a day or nearly three pounds in a single growing season. Carbon dioxide withdrawn from the air would average from the same area of leaf surface about two gallons a day or over three hundred gallons for the season. As an equal amount of oxygen is given off by the leaf, it becomes clear that as all of this interchange must go through the stoma the functioning of these and their guardians must be nearly one hundred per cent perfect. As we shall see a little later, they perform still other duties with even greater perfection. When we stop to reflect what an absurdly minute fraction one square yard of leaf surface is to the total leaf surface in the world, we come to some realization of the gigantic proportions of this process of manufacturing sugar and exchange of gases mutually useful to animals and plants. While in the United States most of the leaves fall in the autumn, the great bulk of the vegetation of the world holds the greater part of its leaves all the year, notably in the vast evergreen forests in the north, and of course practically all tropical vegetation. Chlorophyll in such places works continually and what the totalof sugar production may be no man can even guess.
Sugar, although the first step in the process, is not the final one, and the leaf has still other tasks to complete. Some of the sugar is used up in the process of renewing the chlorophyll, some of it is moved to other parts of the plant where in sugar cane it forms the world’s chief sugar supply; but the remainder is transformed into starch, a substance that is not dissolved by the water of the sap, and is therefore capable of permanent storage either in the leaf itself or in other parts of the plant, notably in the tubers of the potato, the solid part of which is nearly all starch. The conversion of sugar to starch, which is really a means of contriving to properly store the product of the factory, is done by certain ferments known as enzymes. Just what enzymes are or even how they work is not well known, but apparently they have the faculty of converting certain substances like sugar, and in the process they neither use up nor materially change their own composition. It is certain that the conversion of sugar to starch is an elaborate chemical process, but it is accomplished by these enzymes, the very existence of which has only recently been discovered. Enzymes not only do this, but they convert starch which is insoluble into a kind that may be dissolved and thus carried to different parts of the plant. Upon this power depends the storage of starch in roots, tubers, seeds, or wherever else it is found in the plant, and it is of course upon this power man depends for the food supply of the world. Wheat or corn, potatoes, rice, all the foods that are rich in starch produce none in that part of the plant harvested by man. All of it has come by the processwhich is only sketched in its briefest outlines in the foregoing paragraphs. All of it must come from that green coloring matter of nearly all plants which, while mostly confined to leaves, is not always so. And wherever chlorophyll is found this process goes on even in the simplest plants. Because it is so overwhelmingly a characteristic of leaves and, as we have seen, leaves are the one organ of the plant upon which man pins his only hope of future food supply, the leaves of all plants may be truly likened to a factory the work of which is never ending, the product of which the leaf will never use, but the result of which has far-reaching consequences to us all.
Now that we understand the importance of light to all except a very few plants, and its very close relationship to the green coloring matter of all leaves, many things about the arrangement and position of leaves, and indeed of the whole plant, may be understood, which, without this knowledge, seems the result of mere caprice or chance. It would seem as though the habit of plants growing toward the light, and against the pull of gravity, a character almost universal, no matter from what mountain declivity or rocky cliff it may spring, might be the result of the “pull†exerted by light on the green coloring matter in the leaves. While light does aid in plants having a generally erect habit it is not the cause of it, as we have many times proved by experiments. As a seed sprouts and the roots go down into the earth, the shoot, before it has broken through the surface and whilestill in the dark, always grows upward. This property of growingin two opposite directions at the same time, the roots always with gravity and the shoot nearly always against it, is known asgeotropism. In the case of vines or other trailing plants there is the same tendency exhibited, even though the plant is not erect. We must think of geotropism as a growth habit of all plants, not caused by light, for it has been shown to act in the dark, but of the greatest advantage to all plants in their initial start toward the light. If this were not the case, it may be imagined into what chaos the vegetable world would be thrown. We are so accustomed to roots going down and shoots going up that we are not apt to think of it as the result of two antagonistic growth habits, the true cause of which is not understood, the result of which is common knowledge. Geotropism is one of those mysteries with which the book of nature is crowded, and merely to describe it and realize its force is by no means to arrive at its true inwardness.
But, quite independently of this peculiar growth habit, the stems and often whole plants do show response to light and many times the response, in its effects, cannot be distinguished from geotropism. Perhaps the most homely illustration of this is the common house geranium which, no matter how often it is turned, always grows toward the window, and if not turned at all becomes hopelessly lopsided, with the leaves all bending sharply toward the light. Trees growing on a cliffside, while always growing upward, nearly always may be seen bending away from the cliff where light is scarce and toward the unobstructed light. The position of hundreds of twigs and branches on any tree have been dictated by their exposure to light, and the habit of practicallyall trees in the forest of being clear of branches for many feet from the ground is another illustration of the profound effect of light. In the latter case the taller the trees the farther from the ground are the first branches, and in the big trees of California the first branches are frequently over a hundred feet from the ground. In their young stages all these trees were furnished with branches, the leaves of which in their day performed their appointed tasks. But in the strife and hurry of the crowns of the forest to overreach their neighbors these lower branches, from the bottom upward, gradually die off. So inexorable is the plant’s demand for light, that these lower branches, in spite of being nearest the source of their food from the roots, are doomed to be killed. Nature plays no favorites and these lower branches, once the pride and support of the young tree, are ruthlessly dropped off when they can no longer play the game. This wholesale slaughter of lower branches in a forest, more complete than any pruning by man could ever be, gives us, if the story of the factory leaf has not already done so, some conception of the part played by light in the plant world.
The shade of certain trees is so much denser than others that they have been planted for this purpose, notably the horse-chestnut and Norway maple. Foresters have long recognized this difference in trees and it would be strange if nature had not taken advantage of it also. If certain trees can still maintain themselves in the forest without producing a dense crowd of leaves, such as the silver maple for instance, they would have a decided advantage over a tree like the sugar maple which casts a much denser shade. A walk through any forest will showscores of examples of trees that live and produce seeds by virtue of the fact, not that they demand all available light, as their more vigorous neighbors do, but that by a compromise, by an almost diabolical cunning, their light demands, and of course their leaf exposure, have been cut down to a point where the tree can grow in a place impossible for trees that lack this ability. It is, of course, not a trick which any individual tree can perform at will. Rather is it a characteristic found in all individuals of certain kinds, where the comparative disadvantage of making less food and having less leaf exposure is more than overcome by the enormous advantage of being able to fight their way into a forest that would otherwise be impossible for them. We shall see, in the chapter on Plant Distribution, how this peculiar response to light has had effects of considerable significance upon forests, particularly after forest fires, lumbering, or other disturbance of the natural conditions. Trees in the forest, and the shrubs and herbs under them are not the quiet stately things about which the poets are so fond of singing. They are places, on the contrary, of intense warfare, and perhaps some of the greatest casualties occur in the battle for light.
Leaves, as being the most directly involved in the matter of utmost exposure to light, show the greatest amount of response to it, by their shape sometimes, by their position nearly always, and very often by the character of their leafstalks. In many herbs the first young leaves are relatively short-stalked, while as the plant grows upward the lower leaves are progressively longer stalked, which is a direct response to the fact that the upper leaves take their full share of light, leaving little or nothing for the lower ones.To avoid complete shading their leafstalks are often many times the length of their more fortunately placed neighbors above them. In those plants like the garden primrose or common weedy plantain, which bear all their leaves in a close cluster or rosette at the level of the ground, we see an almost fiendish cleverness in their earlier and later habits of growth. When the leaves first start, as they nearly always do among grasslike vegetation in which these plants usually have to fight for a chance of life, the leaves grow straight up, so that they may get above the level of the surrounding grass. Once there, and the precious light an assured fact, they gradually flatten out their leaves to form a rosette, of course cutting off the light from the grass about them and killing it just as certainly as though it were pulled up by the roots. Hundreds of different kinds of plants do this, apparently with the utmost cruelty to their inoffensive neighbors, with whom they start upon nearly equal terms in the race for life. If they began at once to spread out their rosette while it was still in its small spring state, the upward pointing grasses would smother it, and as if in anticipation of this the leaves grow up with the grass, only to flatten out when the proper time comes for them to show their true colors.
Light not only affects leaves in their habits of growth but it actually causes movements in some leaves which are as regular as clockwork. The best known cases are those in the pea family and wood sorrels, all of which bear compound leaves. During the day these leaflets are spread out in the ordinary way and catch the light, but at sundown, as though this were a quite useless exertion for the night, they fold up and the leaf “goes to sleep.†On cloudy daysthey partly fold up, as if in recognition of the fact that for their business of getting light it is an off day; but also if the sun comes out they hurriedly expand their leaflets. It is not yet certain whether these apparently intelligent movements of leaves in relation to light are of any real advantage to the plant as a whole or not. They are surely one of the most interesting things to watch and may be seen in locust trees and wood sorrel any night.
Just as we can have too much of a good thing, it is possible for plants to have too much direct sunlight. In open spaces, where the struggle for life centers not about the fight for light but over other matters, we find leaves actually protecting themselves against too much exposure, and by a variety of ingenious ways. The texture of the upper or lower side, the kind of hair growing on their surface, and the number and size of their pores, are the most usual ways of leaves arming themselves against an oversupply of the one thing that their neighbors in the cool forest fight to the death to obtain. There seems to be a fatality against which plants, like ourselves, are nearly helpless. Their attempts to overcome it, again like our own struggles against an apparently overmastering fate, develop those characteristics that insure survival to the fittest, death to the puny or unaccommodating.
We could hardly leave the subject of light and plants’ relation to it without mentioning, perhaps, the most remarkable case of adaptation to peculiar light conditions. All those aquatic plants that grow beneath the surface of the water need and get much less light than ordinary land plants. But from the island of Madagascar comes the lace leaf or water yam, which grows in quiet pools that are mostly inthe depths of the tropical forest. Add to the dense shade cast upon the gloomy surface of such ponds the amount of light naturally lost in its passage through the water, and we get some notion of the singularly secluded home of this aquatic plant. What, now, is nature’s response to these peculiar conditions? How do the leaves of this well-shaded inhabitant of quiet pools behave? Their leaves are about a foot long and three or four inches wide, quite unnecessarily large for a submersed aquatic, but they consistwholly of veins. There is no “meat†to the leaf, none of that soft, green tissue so familiar in ordinary leaves. The conditions under which it is doomed to live almost seem as if it recognized the futility of having a broad expanse of the usually constituted leaf blade to expose to a light which is not there. It is significant that this skeletonized condition is permanent, the leaf functions much as ordinary aquatic leaves do, but its network of quite naked veins almost seems a mute protest against its fate. The delicate, lacelike “foliage†of this aquatic adds a touch of beauty to one of the most curious plants in the world.
If we could stretch an apparently impervious membrane, like the inner white skin just inside an eggshell, or a piece of parchment, and so form a wall through the middle of a glass box, and then pour into one of the compartments pure water and in the other a mixture of water and molasses, a very curious result would follow within a comparatively short period. We should find that presently there would be a gentle filtering of the water through the membranetoward the molasses water, and similar gentle current in the other direction. In other words, fluids of different density, if separated by a membrane, tend to equalize each other. This equalization may not be very rapid, and at first it will be more speedy from the less dense to the more dense, but eventually it will make the different fluids of a common density. This purely mechanical property of the equalization of fluids separated by a membrane is known asosmosis, and it is upon the possession of the equipment necessary for this that roots depend for getting food and water from the soil.
In our discussion of roots in Chapter I, we found that they end in very fine subdivisions, which are themselves split up into practically invisible root hairs. These root hairs are the only way that roots can absorb the food and water in the soil, and they are able to do this because they are provided with a membrane which permits osmosis to act between the solution inside the root hair and the water in the soil. The solution in the root hair is mostly a sugary liquid, some of that surplus sugar made in the leaves, and it is denser than the soil water, so there is apparently nothing to prevent an equalization of the liquids on different sides of the membrane. If this actually happened, as it would in the case of the simple experiment noted above, then roots would exchange a fairly rich sugary liquid for a much more watery one, and we should find that plants did not get their food from the soil, but really have it drained away from them by osmosis. But nature has a cunning device for stopping such robbery, which is prevented by the membranes of root hairs being only permeable to the extent of letting waterin, not permeable enough to allow sugar toescape. As we have seen, osmosis is a purely mechanical process which, if left to operate without interference, would not aid but injure the plant. Surely, nothing with which plants are provided is so important to them as this delicate membrane of the root hairs which, while allowing osmosis to act in a one-sided fashion, preserves to the plant the sugary liquid that alone makes the absorption of soil water possible.
As root hairs are very much alive and work constantly, they must be provided with air, without which no living thing can exist. And here, again, it seems as though nature, with almost uncanny foresight, had deliberately planned for this requirement of roots. And, in this case, not by interfering with a physical process by an adjustment of plant structure, but by the arrangement of soil particles and the way in which water is found in all soils. Soil particles, even in the most compact clay, do not fillallthe space occupied by the soil as a whole. There are tiny air spaces all through the soil, which insures a constant supply of fresh air. That is one reason why gardens are cultivated, to see to it that plenty of fresh air is allowed to permeate the soil. Around the finest soil particles there is always an almost incredibly thin film of water, which is renewed as soon as it is lost by its absorption by the root hairs or by evaporation. This renewal of the water film is itself a mechanical process, called capillarity, best illustrated by putting a few drops of water on a plate and placing on them a lump of sugar. The water will spread all through the lump of sugar in a few seconds and the capillarity that forces it up through the lump is the same as sees to it that the tiny film of water surrounding the finest soil particlesis constantly renewed from the lower levels of the soil.
Little do we dream, as we walk over the commonest weed, that buried at its roots are these delicate arrangements for securing food and water. Osmosis allowed to act so that the “exchange†of liquids is all to the advantage of the plant, capillarity providing a constant water supply, and the very piling together of the soil so contrived that the life-giving air filters all through it—does it not seem as if all this were, if not a deliberate plan, certainly a more perfect one than mere man could have devised?
If you will turn back for a moment to the beginning of the description of how plants get their food, you will find that in osmosis the weaker liquid tends to permeate the denser one more rapidly than the denser one does the weaker. As we have just seen, the sugary liquid in the root hairs is denser than the soil water outside, and, furthermore,none of it is allowed to escape. This comparatively greedy process of taking everything and giving nothing results in a constant flow of soil water into the root hairs. When the flow of liquids in osmosis is not at once equalized, a gentle pressure is brought to bear to make them so. This is what is calledosmotic pressure, and it is this pressure that forces the absorbed liquid through the roots and part way up the trunk of even the tallest trees. While we have just said it is a gentle pressure, that is true only in the case where the osmosis has free play, and the pressure is stopped with the perfect mixing of the two liquids. But what if they can never mix? What may not the accumulated osmotic pressure amount to in such a one-sided process as goes on in roothairs with everything coming in and nothing going out. Cut-off stems, with a pressure gauge attached to them, indicate that in some plants the pressure is from 60 up to 170 pounds!
Another result of this pressure is that it keeps leaves and the fleshy stems of plants in their ordinary position. The actual solid part of nearly all leaves is scarcely 5 per cent of their bulk and all the rest is water. The constant pressure of this water from the roots is sufficient to keep leaves comparatively stiff and rigid, how stiff is quickly realized if the pressure stops and the leaf wilts or withers. Sometimes this osmotic pressure, particularly during rainy weather, becomes so great as to cause injury to the plant, the splitting of tomatoes and occasionally of plums, being due to it. This osmotic pressure, together with the extra pull given by the leaves, is sufficient to account for the rise of water to the tops of the tallest trees. The tallest trees in the world are certain kinds of blue gum in Australia which frequently reach a height exceeding 300 feet. What the combined osmotic pressure and leaf pull must be to carry such a heavy thing as water to such a great height is easier to imagine than to calculate.
The root hairs, then, by the process already described, absorb the water from the soil, but plants can no more live on water alone than we can. As we have seen, the membrane in the root hairs cannot allow the passage of even the tiniest particle of solid matter. In fact the root hair itself is so small that it can only be seen through the microscope, and of course the membrane is smaller still. Plant foods, then, can never be solids, but must always be such materials as can be dissolved in water. The chief of these are chemical substances, such as lime, potash,nitrogen, magnesium, phosphorus, sulphur, and iron. Hydrogen is also necessary, but as this makes up half the composition of water, there is a permanent supply of that provided by the soil water. These things make up the great part of plant foods taken in through the roots, and it is from these that leaves, by a process you already understand in its essential details, manufacture sugar and starch.
But neither starch nor sugar, important as they are to the plant, and absolutely necessary as they are to us, are the only things made by plants. Leaves may well be called factories, but plants are themselves the most wonderful chemical laboratories, beside which any built by man are as play-things. For plants, by processes too complicated to be explained here, work over their accumulation of starch and sugar, recombine some of their constituents, and store up in various parts of the plant the results, which are often such food ingredients as protein. This is the really essential food substance in wheat, as it is in eggs and meat. No chemist has ever succeeded in making a single scrap of it, yet it is such an everyday occurrence in practically all plants that it, with starch and sugar, forms the great food supply of the world. Not protein alone, but all the amazing plant products like the oils from the olive and the resin from pine, rubber, the drugs of plant origin, even tobacco—all these and hundreds of others are made by plants from those few simple foods absorbed through the roots, literally pumped up to the leaves and there, under the magic of sunlight, combined and recombined, worked over and changed utterly in their make-up. Nothing could be more perfect than the marshaling of forces and contrivances to secure the result; letthere be even the least bungling, and for us the world would cease to be worth fighting for.
Nor does the work of plants stop here. If it did, they would be not unlike a commission merchant who had gathered from the four corners of the earth a supply of eggs only to find he could not or more likely would not sell them all at once, and yet had failed to provide himself with proper storage. Plants, too, have times in their life when adequate storage is necessary for them. So true is this that unless there is food enough stored in seeds to give a start to seedlings before their own roots begin to work, they would die almost at once. In seeds and in many nearly dormant parts of plants these foods are stored away for future use. The tubers of potatoes and all our root crops, like beet and parsnip, are common examples of this. Even the manufacture of wood in the trunks of trees is a storage appliance on the part of the plant, for wood is just as much one of the food products of a plant as wheat or rice.
With such a beautifully perfected mechanism for getting food it might seem as though all plants would be satisfied to lead that life of independence for which they are so splendidly equipped. Some of them, however, are like men in one respect: there seems to be no end to the chase after getting something for nothing. Those that stand on their own roots, get their food honestly, and take nothing for which they do not make prodigal returns, make up the great bulk of the vegetation of the earth. Their independence has dubbed them with the title