Chapter 14

Fig. 16. Bacillaria paradoxa

This Bacillaria paradoxa (fig. 16) differs from the precedingspecies in its motion; each half of the row of frustules moves in an opposite direction on each side of a central stationary frustule, and the alternate motion is so regular as to time, that if in advancing, the frustules meet with an impediment, they wait till the proper time comes for their retreat. The jerking motions of the Naviculæ are ascribed by Prof. W. Smith to forces acting within the plants, originating in the vital operations of growth, by which the surrounding water is drawn in at one end of the frustule, and expelled at the other.

Some species of diatoms are so universal that they are found in every region of the globe; others are local, but the same species does not inhabit both fresh and salt water, though some are found in brackish pools. The ocean teems with them. Though invisible as individuals to the naked eye, the living masses of the pelagic diatoms form coloured fringes on larger plants, and cover stones or rocks in cushion-like tufts; they spread over the surface as delicate velvet, in filamental strata on the sand, or mixed with the scum of living or decayed vegetable matter floating on the surface of the sea; and they exist in immense profusion in the open ocean as free forms. The numbers in which they exist in all latitudes, at all seasons, and at all depths—extending from an inch to the lowest limit to which the most attenuated ray of light can penetrate, or at which the pressure permits—are immeasurably in excess of what we have been in the habit of assuming. Temperature has little to do with the distribution of diatoms in the tropics; it decreases with the depth at a tolerably fixed rate till it becomes stationary. It increases in the polar regions with the depth, and approaches the standard, which is probably universal, near the bed of the ocean.

Nothing can exceed the vividness of colour or massiveness of the endochrome or soft internal matter of thefloating diatoms, that matter which diminishes their specific gravity and makes the plant buoyant which otherwise would be weighed down by its silicious coat. At those periods in which the structural and reproductive phenomena proceed most vigorously, their position in depth must be fluctuating; hence they approach and vanish from the surface. Their growth is perfected by the heat and light which penetrates the sea in calm weather.

Diatoms are social plants crowded together in vast multitudes. Dr. Wallich met with an enormous assemblage of a filamental species of Rhizoselenia, which is from six to twenty times as long as it is broad, aggregated in tufted yellow masses, which covered the sea to the depth of some feet, and extended with little interruption throughout six degrees of longitude in the Indian Ocean. They were mixed with glistening yellow cylindrical species of such comparatively gigantic size as to be visible to the naked eye.

Other genera constitute the only vegetation in the high latitudes of the Antarctic Ocean. Dr. Hooker observes, that without the universal diffusion of diatoms in the South Polar Ocean, there would neither be food for the aquatic animals, nor would the water be purified from the carbonic acid which animal respiration, and the decomposition of matter, produce. These small plants afford an abundant supply of food to the herbivorous mollusca and other inhabitants of the sea, for they have been found in the stomachs of oysters, whelks, crabs, lobsters, scallops, &c. Even the Noctiluci, those luminous specks that make the wake of a boat shine like silver in a warm summer night, live on the floating pelagic diatoms, and countless myriads are devoured by the enormous shoals of salpi and other social marine animals.

The silicious shells of the diatoms form extensivefossil deposits in various parts of the globe, containing species which have long ceased to exist, and others that are identical with those still alive even in their most minute and delicate engravings. The polishing slate of Bilin in Bohemia, which occurs in beds 14 feet thick, and the Tripoli and Phonolite stones on the Rhine consist entirely of the silicious coats of diatoms, while the city of Richmond in Virginia stands upon a marine deposit of the debris of diatoms 13 feet thick, and of unknown extent. Near the Mediterranean, very extensive strata, consisting almost entirely of marine Diatomaceæ, alternate with calcareous strata chiefly formed of Foraminifera, the latter being a race of microscopic mollusca. The fossil Diatomaceæ at Oran in Algeria are particularly perfect and beautiful. In many of these deposits existing species are found.

The trade winds bring over large quantities of dust mixed with diatoms, which sinks through the upper into the lower current, blowing over America, and at last falls in Europe. Professor Ehrenberg found that this dust contained chiefly true American species, many of which were identical with forms existing at the bottom of the Antarctic Ocean, where an area of 4,800 square miles was discovered by Sir James Ross skirting the volcanic coast of Victoria Land, consisting of the remains of these microscopic plants, which have deposited their silicious valves at death for countless generations, producing geological changes of enormous magnitude; while a still greater area of sea-bed in the North Atlantic is the perpetual grave-yard of myriads of microscopic mollusca. Thus the Supreme Being, whose power is stupendously manifested in the motions of the celestial bodies, creates generations of infinitesimal creatures, adorns them with exquisite beauty, and makes them His agents to form future continents.

The Confervaceæ are a numerous tribe of pretty littleplants, usually of a green colour, growing in fresh and salt water, on moist ground, wet rocks, and thermal springs. There is scarcely a gently running stream in which they may not be seen, like bunches of green threads, attached to stones and waving in the current; some are so soft as to become almost a mass of jelly when taken out of the water. They are sometimes branched, but more frequently simple, formed of cylindrical cells, joined in a single long row by their flat ends, and they increase in length by the bisection of their terminal cells.

Cell multiplication in Conferva glomerata:—A, portion of filament with incomplete separation ata, complete partition atb;B, the separation completed;C. formation of additional layers of cellulose wall.

In unicellular plants bisection is an act of reproduction; in the multicellular Confervæ it is an act of growth and extension which is accomplished as follows:—The terminal cell of the plant grows to twice its length, the matter within the primordial cell spontaneously divides into two equal parts, and both the film and cellulose coat which cover it, bend round, and form a double layer or cellulose division between them. This cellulose layer extends over the whole exterior of the primordial cell, so that the new cellulose division or septum becomes continuous with a new layer which is formed throughout the interior of the cellulose wall of the original cell. In this manner two perfect cells are formed out of one, and as the extreme cell may undergo the same process, the growth of the plant may be continued indefinitely. Branches are sometimes formed by buds springing from any part of the stem; thoughapparently so different, it results from the subdivision of the cell which produces the bud.

Fig. 18. Zoospores.

The Confervaceæ are generally reproduced by zoospores. In most cases the endochrome within a cell divides itself into numerous segments, each of which becomes a minute zoospore, and escapes into the water through a rupture in the cell wall. This is the case in a very graceful genus of Confervæ, of which the Chætophora elegans is a species. It consists of filamental strings of cells, ending in a capillary bristle, with lateral branches like narrow fronds. It is reproduced by zoospores. One half of each zoospore is round, opaque, and full of matter; the other half hyaline, and tapering to a beak furnished with four cilia. It frequently happens in this genus of Confervaceæ, where the filaments are divided at equal distances into little joints or compartments, that the zoospores issue from the terminal cell first, then from the next, and so on in succession till the upper part of the branch is left empty, while the lower part is still forming zoospores. After moving in water for a time, the zoospores retreat to a shady place, fix themselves to some substance, and begin to grow. These plants rapidly cover a large surface of water; for each individual cell may produce 100 zoospores, and as the development and dissemination of them continues during the whole summer, one plant may yield an enormous number.

The Sphæroplea annulina is a rare and very remarkable Conferva, whose cinnabar-coloured spores make the surface of the water in which it floats, like a pool of blood. It has no root, being merely a filament with capillary extremities, formed of elongated cells joinedend to end. The spores only grow on the filaments that are exposed to the sun and air; the filaments that are below the water are green and barren. The spores are filled with red matter, grains of starch, and red oil, the outer or cellulose coat being so plaited, that the spore looks like a red star with white rays.

When a spore germinates, it produces a minute cell ending in capillary fibres, which increases in length by the continual bisection of its central cells, while the other Confervæ grow by the bisection of those that are terminal. During this growth, the red contents of the spore are so changed by a remarkable succession of chemical processes, that the primordial cells in the filament of the young plant are filled with a colourless viscous matter, an aqueous liquid, granules of starch, and chlorophyll. In some of the cells the starch disappears, while the green matter and the other materials arrange themselves into a series of rings, alternating with empty spaces or vacuoles. After a time, the green changes to a yellowish red, and then each ring in succession resolves itself into a multitude of minute active particles, which move with incredible velocity in the void spaces of the cell, till at last the whole cell swarms with them. They are analogous to the pollen of flowering plants, and thence are called spermatozoids. Their form is cylindrical, thick, broad, and yellow at one end, sharp at the other, with a colourless beak, and long cilia. The parent cell is at last pierced by their united efforts, and out they rush in great confusion into the water; some whirl round their centres, others swim in a circle, many describe cycloidal curves by a series of leaps, and a few swim in straight lines.

During the preceding changes another process is in progress, within what may be called female cells. In these the starch, mixed with green matter and aplastic substance, arrange themselves also into green and vacant rings, and after various and complicated changes, each green ring forms itself into a kind of plastic primordial free cell, which, after being fertilized by the moving bodies, gets a stronger coat. The green matter becomes first of a red-brown colour, then red; and after leaving the parent cell it is invested with a plaited cellulose coat, and becomes a star-like resting spore which may produce a new plant. No cryptogamic plant exhibits a greater variety in the modes of action of the vital forces, none more activity in the motile powers. In some of these Confervæ, the moving male filaments, or spermatozoids, instead of escaping singly from their prison cell in confusion into the water, are discharged in a mass enclosed in a capsule furnished with cilia, which moves with its lively burden like a zoospore, till a lid falls off which sets them free.

Some pretty plants allied to the Confervæ are called Batrachospermeæ, from the resemblance which their beaded filaments bear to the spawn of a frog. They are all inhabitants of fresh water, chiefly of gently flowing streams, and are so flexible that they yield to every movement of the water, and when taken out of it are like a mass of jelly. Their colour is usually a brownish-green, but sometimes it is of a reddish or bluish purple. The central stem of the plant, though originally formed of a single row of large cylindrical cells placed end to end, gets an investment of cells, or rather branches, which ultimately becomes a thick cylindrical stem, bearing, at nearly regular intervals, whorls of short radiating branches, each composed of rounded cells, arranged in a bead-like row, and sometimes branching again. Some of the radiating branches grow out into transparent points, which may possibly be antheridia, and contain motile bodies; for within certain cells in other branches resting spores are found, which are agglomerated andform the large dark globular masses that are seen in the midst of the whorls.

The Hydrodictyon utriculatum is another allied plant of singular structure, which grows in fresh-water pools in the midland and southern counties of England. It resembles a regularly reticulated green purse, from four to six inches long, and is composed of a vast number of tubular cylindrical cells, which adhere to one another by their rounded extremities, the points of junction corresponding to the knots or intersections of the network. Each of these cells may form within itself from 7,000 to 20,000 gonidia, which at a certain stage of their development are observed to be in active motion in its interior; subsequently, by mutual adhesion, they form into groups which lay the foundation of new net-plants, when set free by the dissolution of their envelope. Besides these groups, there are certain cells which produce from 30,000 to 100,000 more minute bodies of a longer shape, each of which is furnished with four long cilia, and a red spot. These escape from their cell in a swarm, move freely in the water for a time, then come to rest, and sink to the bottom, where they remain, heaped together in green masses. Their future fate is unknown, but they are believed to be male filaments similar to those described, and are generally called spermatozoids.

The Nostochineæ are either an assemblage of cells loosely united into numerous green chaplets, or distinctly beaded filaments, generally twisted, and occasionally branched; they are imbedded in a firm gelatinous frond of different form, sometimes globular, sometimes spreading in branched masses, often of considerable size. They are frequently seen on damp shady walks in gardens: they shrink to a film in dry weather, and reappear so suddenly in rain that they have been called fallen stars. They are reproduced by spontaneousdivision of their filaments; the segments escape from the gelatinous mass, move slowly in the direction of their length, after a time come to rest, secrete a gelatinous envelope, and not only grow in length by transverse bisection, but split longitudinally into new filaments which are separated by their gelatinous secretions. These movements, discovered by M. Thuret, are evidently intended to disperse the plant.

Vesicular cells, destitute of endochrome, sometimes furnished with cilia, and of a larger size than the others, are occasionally seen at the end or middle of a filament of the Nostocs, sometimes situated at intervals along their length; and near to these are sporangial cells, a little larger than the ordinary cells. From analogy, it is believed that the vesicular cells are antheridia, and that the sporangial cells contain germs which, after being fertilized by the spermatozoids, are set free and become resting spores. In some species, the sporangial cells are oblong, and contain vividly green matter; in others, the cells are elliptical and brown.

The species are widely distributed. Hormosiphon arcticus, a species consisting of a modification of cellulose, abounds to such a degree in the herbless polar regions, that it affords a welcome variety of food. Each plant lies on a small depression of the snow, which covers the soft and almost boggy slopes bordering the arctic seas, but it is carried by the winds in every direction, rolling over the snow and ice to a distance of several miles. Two northern species of Nostoc were found by Dr. Hooker in Kerguelen’s Land, growing on wet rocks near the sea; one of them was the common Nostoc commune. Other species occur in the warm springs in India, as well as in the arctic and antarctic regions, and an aquatic species is much used in China as a wholesome food. The genus Monormia forms floating masses of jelly on the surface of brackish water. The necklaces are of vast length, and,together with the jelly in which they are imbedded, wave with the slightest motion of the water. Floating masses grow on large ponds or lakes, which give the water a green tint.

The structure of the Oscillatoriæ is microscopic. They are minute filiform plants closely allied to the Nostocs; and consist of transparent colourless tubular filaments containing colour cells of various forms, more or less separated from each other, and visible through their transparent tubes; the colour is usually some shade of green, yellowish, or purple. In the genus Rivularia these tubular filaments have a globular transparent cell at the base, and are closely packed into little balls, either forming small groups, as in the Rivularia nitida, or singly attached to stones and rocks. In Rivularia nitida, the filaments radiate from a centre. Some Oscillatoriæ form velvety cushion-like patches upon rocks, others are attached in tufts as parasites to other sea weeds, while many are arranged in free or attached stratified bundles. Lingbya furnishes a beautiful specimen of the latter. The filaments in the stratified group are usually much twisted and interwoven, and some of them exhibit singular oscillating motions, as the Oscillatoria littoralis and spiralis, Spirulina tenuissima and others; one end of the filaments remains at rest, while the other extremity is in constant vibration. With a microscope the movement in some species is seen to be from side to side like a pendulum, in others it is spiral or twisting, and when a fragment of the plant is set free when vibrating the movement is progressive. If a fragment be put into a glass of water, its edge in a little time becomes fringed with short filaments radiating from central points with their tips outwards. They soon detach themselves from the fragment by their oscillations, and as their vibrations continue after they are free, they swim with a spiral motion to the edge of the water, and even ascend theglass till arrested by the dry part above.[35]During these motions there is a corresponding alteration in the form of the filamental tubes believed to arise from rhythmical periods of vital contractibility, which are affected by light and heat, because the motions are more rapid in sunshine than in shade; besides, they are checked by strong chemical agents. Some of the species have a tuft of delicate cilia at the extremities of their filaments.

The free stratified bundles contain the simplest form of the Oscillatoriæ. Each filament is a straight or slightly curved chain of cells, full of coloured matter, and enclosed in a common transparent colourless tube. Multiplication takes place in these by division; when about to multiply, two adjacent coloured cells, or the two halves of a divided cell, recede from one another, and the outer tube contracts at the point of division, and separates them into two distinctly new filaments. Sometimes the transparent outer tube does not yield, so that the divided parts retain their places in the tube, which dilates when these new parts are again divided. The manner of division varies with the species, and the generic characters of the Oscillatoriæ depend upon the different conditions of the external tube, and the form and arrangement of the coloured cells within it. The tube often contracts to the finest point during division, and frequently consists of distinct coats, the number of which increases upwards, sometimes with such regularity as to produce a beautiful streaked effect. Like their allies, the Oscillatoriæ are reproduced by zoospores. While these parts are growing, but especially during their dissolution, the endochrome undergoes various changes of colour, staining the water they die in, and rendering it putrid; some of the common kinds emit a strong odour of sulphuretted hydrogen.

In the compound gelatinous Oscillatoriæ, the jelly is of very different degrees of tenacity. The mass of the Dasyglœa is so slippery that it can scarcely be taken hold of; Rivularia nitida (fig. 19) is equally so, its tubes being so thick and tender. Many species of the genus Rivularia have a peculiar mode of oblique alternate branching; species of that genus grow on the stems of aquatic plants, on rocks in rapid streams, on cliffs when washed by cataracts, or sometimes in calcareous water, in consequence of which crystals of carbonate of lime are deposited on their substance. The Rivularia nitida occurs among Algæ exposed at low tides, and a species of another genus floats on fresh-water lakes like green stars.

Fig. 19. Threads of Rivularia nitida.

The Oscillatoriæ are found in every part of the world, most abundantly in the temperate zones. They chiefly inhabit fresh water, but these minute plants attain their greatest size in the sea. Numerous species grow in warm springs, and one species, Trichodesmium erythræum (fig. 20), spreads for many square miles over the surface of the Indian seas in faggots of red-brown threads, like fragments of chopped hay; the same species is said to abound in the Red Sea also.[36]

The Conjugatæ are fresh-water plants of numerous species, which have almost the same structure as the Confervæ, but the green endochrome within the cells of their articulated threads is more highly organized, and the manner of reproduction is altogether different and very peculiar.

These plants consist of strings of cylindrical cells joined end to end by their flat ends, and generally float freely on or near the surface of still water, especially when buoyed up by the bubbles of gas which are liberated from them by the heat and light of the sun. In the early stage of their life, while as yet the cells are undergoing multiplication by self-division, the endochrome is diffused pretty uniformly in each cell; but as the plant approaches towards maturity, it undergoes various modifications, according to the species. In some it consists of large granules disposed in rows; in others it is formed into broad spiral bands with large granules in binary or stellar groups placed at intervals on it; and, in the œdogonium capillare and others, the granules are united in spiral lines which cross one another and form a network.[37]

Fig. 20. Trichodesmium erythræum.

The act of conjugation by which spores are formed, usually takes place between the cells of two distinct parallel filaments which happen to be adjacent to each other, and all the cells of the two filaments generallytake part in it at once. The cells that are opposite to one another put out little protuberances, which come into contact with each other; the intervening partitions disappear, so that a tube is formed which establishes a free communication or passage between the cavities of the conjugating cells. In the genus Mesocarpus and others, the conjugating cells pour their endochromes into a dilatation of the passage that has been established between them, and it is there that the matter mingles to form a spore or embryo cell. But in the Zygnema (fig. 21), which is the commonest form of these plants, the endochrome of one cell passes entirely over into the cavity of the other, and within the latter the two endochromes coalesce into a single mass, round which a firm coat is developed, and it becomes a spore. All the cells of one filament are thus left empty, while spores are formed in all the cells of the other.[38]Sometimes cells in the same filament conjugate, and occasionally the endochrome in a cell divides into two parts, each of which becomes a spore.

Fig. 21. Conjugation of Zygnema quininum:—A. two filaments in the first stage of conjugation;B, completion of the act of conjugation.

Some of the spores are quiescent, others have cilia and are motile, but both after a time become attached at one end by two or three root-like fibres, and grow intofilaments by repeated bisection. According to the observations of M. Itzigsohn, the endochrome in certain filaments of Spirogyra breaks up before conjugation into little spherical aggregations, which are gradually converted into nearly colourless spiral filaments, having an active spontaneous motion, and therefore corresponding precisely to antherozoids. With the exception of South America, the Conjugatæ are widely dispersed in warm and temperate climates.

The genus Vaucheria may be assumed as a type of the Siphoneæ, whose essential character is, that the plant consists of one single tubular cell, however branched and complicated its form may be. The Vaucherias form tufted masses of branching tubes, filled with bright green granular matter, on mud and damp soil; they abound in fresh-water pools, and some grow in the sea. When about to produce fruit, the extremities of some of the tubes swell out in the shape of a club, in which a portion of the green matter collects, takes a darker hue, and is separated from the rest by a transparent space and a new envelope. After various changes, the darker green matter forms itself into a zoospore, which is so active that it breaks open the top of its club-shaped cell, and comes into the water; sometimes several come, one after another. They are egg-shaped, with a colourless beak, and as their whole body is bristled with cilia, they leave a long current in their wake when they swim, which they do with such impetus that they are flattened against any obstacle they meet with, even to the discharge of their green endochrome. They escape from their cell about eight in the morning, move for two hours, then come to rest, and begin to grow into a new plant.

M. Pringsheim discovered another mode of reproduction in the Vaucherias, which are monœcious plants, that is to say, the same plant produces snake-likefertilizing spermatozoids and female germ cells. For example, the Vaucheria sessilis consists of one long branched cell; on the same side of it two swellings appear near to each other, one of which elongates, curls round like a horn, and is soon filled with snake-shaped filaments having long cilia at their thin end, with which they move rapidly both within the horn, and after they come out of it into the water. They are perfectly colourless, and correspond to the pollen of flowering plants. The other protrusion which swells into a globose germ cell, and which corresponds to the pistil of a flower, contains a mass of green endochrome, which, after being fertilized by the snake-like filaments, becomes a primordial cell which has no motion, but after having secreted a strong coating of cellulose, it sinks to the bottom of the water, becomes a winter or resting spore, and lays the foundation for a new generation of plants. The resting spores produce new forms, while the zoospores, like buds, only multiply the type of the individual plant with all its peculiarities.

The marine genus Bryopsis grows in New Zealand, the Falkland Islands, and the seas about Cape Horn. The species are mostly parasites on other Algæ, and produce innumerable zoospores. The genus Codium is found in high latitudes, and appears under four different forms on the British coasts; one of these inhabits turfy banks exposed to the spray of the sea, the others grow in deep water, or on rocks never uncovered but at spring tides. Species of this genus are found as far south as Kerguelen’s Land, and in most of the intervening latitudes. The Caulerpas inhabit the warmer districts in the northern hemisphere, and furnish five species in New Zealand. The numerous species afford almost the whole food of turtles on many coasts, and other genera furnish nutriment to a host of smaller animals.[39]

The Achlya prolifera is also a unicellular plant, much smaller than the Vaucheria, but whether an Alga or a Fungus is not very clearly settled. To the naked eye it appears as a cluster of colourless threads on dead flies floating in water, on the gills of fishes, and sometimes on frogs. With a microscope the tufts are seen to consist of tubes extending in all directions, filled with a nearly colourless granular matter, the particles of which are seen to move slowly in streams along the walls of the tubes, the currents sometimes anastomosing with each other. When the plant is about thirty-six hours old, the endochrome begins to accumulate in the dilated ends of the tubes, and is cut off from the remainder by a transverse division, the motion of the particles being still visible in the part cut off. The endochrome breaks up into a number of long masses, each of which acquires a cell wall and two cilia, and begins to move about within the parent cell; when mature they are set free by the rupture in its wall, and germinate, and produce a facsimile of the parent. It appears that, in some species, the transverse dividing film becomes convex as soon as the motile bodies are discharged, a new fertile articulation is formed and new motile spores are set free, and this process is continued till the vital powers of the plant are exhausted. The Achlya has resting spores, which may remain long in the water without change, but if a dead insect be put into it, they fix on it and germinate immediately. It is supposed that these resting spores are fertilized by filamental bodies. The Achlya prolifera goes through all its changes in an hour and a half or two hours. It is found in the thermal springs at Vichy, Nevis, and Vaux, where it contains an alkaline iodide.

The whole of the plants which have been described in the preceding pages belong to the group of green Algæ, although many are inhabitants of fresh water.The structure of the marine Algæ is entirely cellular. Deprived of vascular tubes, they can have no circulation of sap, consequently they derive their nourishment by absorption throughout their whole surface from the medium in which they live, for their root, or rather fulcrum, only serves to fix them to the rocks and stones to prevent them from being buffeted by the waves. Since solar light and heat decrease rapidly with the depth, each family of Algæ has a zone peculiar to itself. The first zone extends from high to low water mark, and is inhabited by plants periodically exposed to the atmosphere, to the direct light and heat of the sun, and occasionally to rain. Some of the Algæ that are long left dry are believed to derive some nourishment from the substances to which they are fixed. The second zone, which extends from low water mark to a depth of fifteen fathoms, is the region of the great marine forests which encircle the globe in both hemispheres. Other two zones follow at greater and greater depths, but all are divided into various minor regions, below the last of which the Algæ decrease as the depth increases, till, as far as we know, vegetation ceases altogether; that depth, however, must be very great, as diatoms are sometimes found, and in great quantities, three hundred fathoms deep.

The marine Confervas, like those growing in fresh water, are slender-jointed filaments formed of one series of cells joined end to end. The cells become more or less flattened on the surface of contact, while the side walls retain their natural curvature, which may be cylindrical or oval. The filament may, therefore, be cylindrical or beaded. The cells are almost always longer than they are broad, and for the most part equal and similar in the same plant, although there are exceptions to uniformity of size. The cells contain a transparent liquid through which minute solid particles of variousshades of green are pretty evenly scattered. The conversion of these particles into zoospores has already been described. Since these Algæ have no roots, and the cell wall no opening, each cell of a Conferva elaborates independently the nutriment it absorbs from the water. Some species form a fleecy layer over rocks, and on the bottoms of salt-water pools and estuaries, others extend in bundles in salt-water ditches, and some are found on rocks, between tide marks, rising in long, straight, stiff, and wiry tufts, from three to eight or twelve inches high.

The genus Hormotrichum, which forms tufts several inches long, of bright grass green, differs from the Confervas in being soft and gelatinous, and even more by its mode of increase, which, however, is still by zoospores. The H. collabens may be taken as the type of this genus. It forms a long and large tuft of soft gelatinous and slippery filaments of glossy green. The joints of the filaments are once, or once and a half, longer than they are broad, and the green granular matter within them is collected into a round sac or sporidium in the centre of each, and after being converted into zoospores, the sac comes through a rupture in the joint into the water, opens, and sets the zoospores free.

The genus Cladophora, which has twenty-five species in the British seas alone, forms tufts of jointed filaments from four to eight, ten, or even twenty inches high. In some species the filaments are rigid, bristly, and wiry; in others they are soft and silky; but they are always richly, variously, and sometimes densely, branched and rebranched. In some the branches and branchlets are forked, in others tripartite; the Cladophora pellucida, which is a rigid, wiry plant, combines both these forms.

The genus Bangia consists of purple filamentous jointed and unbranched Algæ, which are distinguished from all others by the microscopic arrangement of theirendochrome, which is enclosed in little cells placed according to a definite plan within the transparent and tubular joints of the filaments. In the Bangia fuscopurpurea, whose blackish purple tufts, several inches long, cling closely to the rocks near high-water mark, the tubular joints contain rows of minute colour cells radiating from a centre. In the narrow filaments there is but one colour cell in a joint, but in the broader filaments there are from three to five, forming a tesselated line across it. In this plant one spore is produced in each joint. The Bangia ciliaris forms a scarcely perceptible rosy pink fringe of hair-like jointed filaments on the Zostera marina, and also on other Algæ. The filaments are not more than the tenth or fifth of an inch long, consequently their joints are most minute, yet the microscope shows that they contain from two to three colour cells set as if radiating from a centre, and that the granular endochrome in each cell is converted into two zoospores. The Bangia ceramicola, which forms purplish pink tufts on small Algæ in rock pools, differs from both of the preceding. The joints of its filaments are once or twice as long as they are broad, and contain colour cells like long upright lines. By aided vision zoospores are seen to be formed within the linear colour cells, then the cells run together into a globular mass, which bursts through the cell wall, leaving the joint empty. The whole genus is soft and sometimes gelatinous.

The Enteromorpha genus is characterized by a cylindrical and tubular stem and branches. These plants form two groups, one whose filaments and branches swell from a narrow base upwards and terminate in a blunt extremity, while in the other group the tips of the branches are pointed. The Enteromorpha intestinalis, which is an inhabitant of many seas, has a thin membranous, tubular, cylindrical, and unbranched stem, inflated upwards into a broad round head, being moreor less wrinkled and curled throughout. Downwards it tapers to a fine thread, and although attached at first, at last it becomes floating. Several of these plants rise from the same root, sometimes to the height of two feet, at others not more than an inch, and they are of every width, from the tenth of an inch to three inches, their colour being grass green. The typical form of the other group is much branched, and all the branchlets are finely pointed.

The three genera Codium, Bryopsis, and the marine Vaucherias are all soft plants characterized by their filaments being tubular, however much they may be branched. They agree also in being reproduced by zoospores developed from the green matter within little sacs attached to the exterior of their filaments. The species of the genus Codium differ much, although formed of similar elements. In the C. tomentosum, which is from three to twelve inches long, the dark green stem is thicker than a crow’s quill and much branched; while Codium Bursa, on the contrary, is a dark green round spongy lump of tubular filaments, densely interwoven and matted together. These masses, which are from one to eight inches in diameter, become hollow when old, and different sizes and ages grow together in a group.

The Bryopsis is a yellowish green tubular plant, from two to four inches high, plumed like a feather, and sometimes replumed. It is a rare plant in England, and grows on the larger Algæ in deep water.

The Vaucheria marina forms soft limp tufts of hair-like filament filled with bright green matter, which often runs partially out. It is from one to three inches high, and has a few long upright branches, to which are attached small stalked pear-shaped sacs containing zoospores. Both this plant and the Vaucheria velutina, grow on muddy shores.

Fig. 22. Ulva latissima:a, portion of ordinary frond;b, cells in which the endochrome is beginning to break up;c, cells from the boundary between the coloured and colourless portion, some containing zoospores;d, ciliated zoospores;e, development of zoospores.

The Ulvas, which are the grass green layers seen on all our coasts, originate in the simple vegetable cell, whatever form their foliaceous fronds may ultimately assume. When the cell is divided in one direction only, a confervoid filament is the result; and if the filament should increase in breadth as well as length, according to a determinate law, a ribbon-shaped frond may be produced; but when the original cell is divided into four cells, and each of these four and all their successors undergo similar division, the increase being as the series 1, 4, 16, 64, &c., a membranous expansion is formed, in which all the cells are firmly attached to one another, and every portion is the exact counterpart of another. The cells of the Ulvas frequently exhibit an imperfect separation of the granular endochrome into four parts preparatory to multiplication by double division, and the entire frond or leaf shows the groupsof cells arranged in clusters containing some multiple form of four, as infig. 3, page 171.

The frond membrane of the true Ulvas, as that of the Ulva lactuca, is formed of but one layer of cells; the frond itself is thin as cambric paper, almost transparent, and of a pretty light green. When young it is a puckered inflated bag, which afterwards bursts and opens into a flat, ribless, wavy, more or less rounded expansion, three to six inches long, and as many broad. This plant, which is attached to the rocks between the tide marks on our shores, is rare in the Mediterranean; nor is it so common in Britain as the Ulva latissima (fig. 22), which is cosmopolite, and abundant everywhere. It is found as a ribless irregular expansion of a full bright green in deep water, and of a yellow apple green when in shallow water, and exposed to the light. The base and stem are very short, and the frond, which the microscope shows to be formed of two layers of cell membrane, spreads so rapidly into crisp wide-lobed foliations, that the parts often overlap each other in stiff bulging folds. It is from six inches to a foot in height, and from three to twelve inches wide. The frond of the Ulva Linza is also formed of two layers of cells, but so small and so closely pressed together that the two layers can only be detected by the microscope. This plant, which is from six inches to two feet long, is a ribless, narrow, ribbon-shaped expansion with curled wavy edges tapering to a base, and either blunt or pointed at the top. Its colour is the same as that of the Ulva lactuca.

In the Ulvas, which are multilocular plants, some cells are selected to bear fruit, and others not. The granular endochrome of these chosen cells divides into several parts, which are at first in close contact and at rest; then they become restless, acquire four or a greater number of cilia, and pass through a fracture in the cell wall into the water, in which they swim freely as zoospores.After a time they come to rest, attach themselves to some object, and begin to grow. The walls of the cells which have thus discharged their endochrome in the form of zoospores, remain as colourless spots on the frond. The whole colouring matter of a portion of the frond may escape as zoospores, leaving behind it nothing but a white membrane. With a microscope, this process may sometimes be observed in all the different stages of its progress.

Every full-grown Ulva has its own precise and definite form, but whatever that may be, the young plants on their first appearance from the shore are in all respects similar to Confervas; the top cells soon divide, and a plane or sac-like frond is formed.

Certain Ulvas, which have a yellow tint, produce small zoospores with only two cilia, but in the Ulva bullosa and the Ulva latissima four zoospores are produced in the same cell, each having four cilia. The same fructification prevails also in the purple Ulvas—Porphyra laciniata and vulgaris. The latter is seen in winter and the early spring, covering the rocks near high water mark, with its tiny bright purple lanceolate leaves. Later in the season it grows into a flat narrow ribless frond with a pointed end, and about two feet long, the margin of the frond becoming waved and plaited as the plant increases in growth. At a later period, it is seen mixed with the Porphyra laciniata, which is a ribless flat frond of a dull purple; sometimes it is very thin, divided or torn, and occasionally growing in a circle round its root. Both forms are sold as laver.[40]

TheRhodospermeæ, Florideæ, or Rosetangles, are the most beautiful of the marine vegetation. No sea plant surpasses them in delicacy and grace of form or richnessof colouring, but the most beautiful are seldom seen, because they grow below the line of ebb tides, or under the shelter of other sea weeds in the rock pools left at low water, their crimson tints being deepest when sheltered from strong light. The Rhodosperms, which have representatives in every sea, are much more numerous than the green Algæ both in genera and species. Thirteen orders, comprising sixty-seven genera, inhabit the British coasts. Many are exceedingly minute, forming patches and velvety cushions on rocks and other Algæ; a vast number have jointed filamentous fronds, while others consist of tubular filaments, and many exhibit a shrub-like collection of firm branches; some are flat and foliaceous expansions without a midrib, either thin and delicate, or thick and strong, while a very brilliant group of both narrow and spreading fronds possess a midrib as a distinguishing character. The structure of the frond varies from a simple membranous to a cartilaginous or even horny substance, caused by a greater development of the cellular tissue, which in the higher kinds of Florideæ divides the epidermal layer or skin from the parenchyme or spongy matter within.

The mode of reproduction by tetraspores, as well as by simple spores, distinguishes the Rhodosperms from the other two great divisions of the marine Algæ. These bodies are produced by the division of the red or crimson endochrome into four parts, which remain in the cells till they acquire an envelope; their form, which is much varied, depends upon that of the endochrome. Some are produced by the breaking up of a globe of endochrome from the centre into four pyramidal segments; or should the endochrome be elliptical, by dividing it into four by three parallel segments, or a mass may be divided into four by horizontal and vertical sections. Some of these are represented, greatly magnified, infig. 23. The tetraspores are lodged in wart-like excrescences,immersed either partially or wholly in some part of the frond.

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