In addition to the works referred to in the last chapter, the student should consult Pfeffer'sPhysiology, pp. 86-149, and pp. 410-441. With reference to water cultures, Sachs'Lectures, XVII., may also be consulted. The standard work on ash constituents of plants is Wolff,Aschen-analysen, 1871 and 1880, an indispensable book of reference in this connection, though there are others, quoted in Pfeffer, where further literature may also be found.
In addition to the works referred to in the last chapter, the student should consult Pfeffer'sPhysiology, pp. 86-149, and pp. 410-441. With reference to water cultures, Sachs'Lectures, XVII., may also be consulted. The standard work on ash constituents of plants is Wolff,Aschen-analysen, 1871 and 1880, an indispensable book of reference in this connection, though there are others, quoted in Pfeffer, where further literature may also be found.
Soil not a dead matrix—Organic materials—The living organisms of the soil—Their activities—Their numbers and importance. Abandonment of the notion that chemical analysis can explain the problem.
Soil not a dead matrix—Organic materials—The living organisms of the soil—Their activities—Their numbers and importance. Abandonment of the notion that chemical analysis can explain the problem.
It is customary to regard the soil, between the particles of which the root-hairs of plants are distributed, as if it were merely a dead matrix of smaller or larger pieces of rock, such as sand, gravel, stones, etc., and organic remains, such as bits of wood, leaves, bones, etc., with water and air in their interstices. As matter of fact, however, soil is a much more complex body than was suspected until comparatively recent times.
It is, of course, beyond the scope of this book to go into the different varieties of soils, their structure or arrangement, and the chemical nature of their constituent rocks and the débris mingled with the latter. For the same reason I must pass over the curious properties of soils in relation tothe solutions they yield to water in contact, the manner in which they retain some of these solutions and allow others to pass easily, and the remarkable double decompositions which go on in them. Moreover, I must assume as known the chief physical properties of ordinary soils with respect to the phenomena of capillarity, absorption of heat, action of frost, and so forth.
But all ideas as to the nature of soil based merely on the study of its chemistry and physics are misleading, and it is in just the establishment of this truth that modern discoveries in Agricultural and Forest Botany have played so important a part.
From the facts that organic débris is found chiefly at the surface of the earth, and that the smallest particles are held in suspension by the water near the surface, it is comprehensible why such organic remains abound in the upper parts of the soil, where the rootlets with their absorbing root-hairs are also found, because they must have oxygen. The rule is, therefore, that an ordinary soil consists of upper strata, rich in organic materials and in oxygen, and a subsoil, poorer in these substances.
Among these organic materials are countless myriads of living beings, especially fungi and bacteria, which require oxygen and organic materials for their subsistence, and it depends on the open or close, moderately moist or damp, warm or cold nature of the soil, and on some obviously connected factors, how far down these aërobic organisms can thrive. As we go deeper down theybecome fewer and fewer, and gradually disappear, and (neglecting certain anaërobic bacteria of putrefaction) they are rarely found in marked abundance more than a few inches below the surface soil.
These aërobic fungi and bacteria are the great agents of continued fertility of a soil, and it is they which, living and multiplying in the moist and well-aerated warm interstices of a rich open soil, carry out the useful destruction of organic matter, breaking it up into mineral and gaseous bodies, which are then dissolved in the water bathing the root-hairs or escape into the atmosphere. In this work of destruction they are aided by the oxygen of the air and the solar heat: their own fermentative action is also accompanied by a marked rise of temperature, and the carbon-dioxide and other products of their activity all go to complicate the chemical changes going on in the soil around the roots.
Duclaux has calculated thatAspergillus niger, a common mould fungus, can break down organic substances, such as carbohydrates, at such a rate that a metre cube of the fungus would decompose more than 3000 kilogr. of starch in a year, and this may serve as an example giving some idea of the possibilities in soil.
Analyses of waters containing large quantities of organic matter, as they enter such open soils as those referred to, compared with the drainage water after passing through the upper strata, show that the carbonaceous and nitrogenous materials are broken down to more or less completelyoxidised simpler compounds, and that the following chief changes result. The ammonia and some other nitrogenous bodies remain behind in the soil, as also do the phosphoric acid and much of the potash; whereas large quantities of nitric and nitrous acids, together with much sulphuric acid, chlorides, and calcium salts pass away in the drainage. These facts are obviously highly important in agriculture.
Experiments on sewage farms have shown also that the upper soil retains most of the bacteria of the sewage. Koch found at Osmont, near Berlin, that whereas the different sewage waters contained numbers so enormous that each cubic centimeter probably held 38,000,000 germs, the different drainage waters held only 87,000 per c.cm.; and the whole process of water-filtration through sandy soils depends on these well-known facts.
Recent experiments in connection with soil-filtration, however, bring out the further facts that the oxidations which organic matters undergo in the soil—and without which they are useless to the higher plants—are enormously enfeebled if the upper layers of soil are sterilised, so as to deprive them of the myriads of aërobic bacteria, fungi and yeasts which they normally contain, and there can no longer be any doubt as to the importance of the biology of the soil in connection with the preparation of materials suitable for absorption in solution by the root-hairs of agricultural and other plants.
The researches of the last ten years havebrought to light a long list of forms, comprising yeasts, such as Hansen'sSaccharomyces apiculatus, fungi and bacteria which live and grow in the soil, finding their water and food supplies in the interstices, and under conditions which we now know to be very diverse. They are usually more numerous, in species and individuals, in cultivated farm and garden soils than in woods, prairies, and untilled lands; but the geological nature of the strata, the closeness and otherwise of the soil, its damp or dry character and its average temperature (which depends on many things besides latitude or altitude) and other factors co-operate to rule their distribution and numbers. The fact that cultivated land is so well supplied with manures, air, etc., is of great importance in relation to their relative abundance there, and it is extremely probable that the use of artificial manures lessens their numbers considerably as compared with land on which stable and other animal manures are employed.
A list of the soil-bacteria which have been isolated and more or less carefully cultivated and examined would comprise about fifty species; but it is certain that, as at present classified and named, many more species are to be discovered in any ordinary soil.
The fungi are apparently even more numerous than the bacteria, and we may rest satisfied for the present with the general statement that the life-actions of the myriads of individuals of these organisms in the soil completely alter the questionof soil-water as understood by the last generation of agriculturalists.
But there is another aspect of this question of soil-organisms which has grown in importance of late to such an extent that we are more than ever justified in regarding the biology of soil as far more vital to the interests of the plant than its physical or chemical properties. With many of the fungi in the soil the roots of plants have to compete—just as plant competes with plant—for water, salts, and other food-materials. The toadstools which are so conspicuous in fields and forests spring from mycelia which ramify in the ground, and are busily breaking down the remains of other organisms, and just such fungi are known to store up relatively large quantities of salts of potassium and phosphorus—the very salts which are so valuable to crops and occur so sparingly in most soils, but which the extensively spread fungus mycelia can gradually accumulate. Some of these fungi, moreover, are more active in their antagonism, and actually attack and pierce the roots as destructive parasites, but I pass these by for the present, as they form the subject for further consideration when we come to the diseases of plants.
It is obvious that the competition of fungi with root-hairs for mineral salts, oxygen, etc., may be at times acute, and it is extremely probable that cases of so-called sterility of soil, where a particular soil is found unsuitable for a crop, may sometimes be due to this over-competition.
The researches of recent years, however, and especially those of Frank, Winogradsky, Hellriegel, and Stahl, have brought to light a series of relationships between certain of these soil-organisms and the higher plants which place the matter of soil-biology in quite new lights.
On the one hand it has been discovered that groups of bacteria are the active agents in bringing about the destruction of organic nitrogenous matter with the formation of ammonia, in oxidising this ammonia to nitrous and to nitric acids, which combine with bases in the soil to form the corresponding salts; while, on the other hand, other forms can decompose the nitrates and reduce them to nitrites, or set free ammonia or even nitrogen from them. Moreover, there are certain species which can fix the free nitrogen of the atmosphere, and start the cycle of up-building of this inert element into the complex higher compounds we term organic. It is impossible to over-estimate the importance of these processes of nitrification and denitrification going on in the soil about the root-hairs of the higher plants.
But, in addition to this circulation of nitrogen in the soil, it turns out that the life-actions of bacteria, and not mere chemical decompositions, are largely responsible for the circulation of carbon, of iron, of sulphur and other elements formed from the decomposition—also by bacterial and fungal agency—of animal and vegetable remains in the soil.
Even more startling are the biological relationsin the soil between the absorbing roots of the higher plants and some of these bacteria and fungi, for it has now been established beyond all doubt that certain fungi enter the living roots and there flourish not as mere destructive parasites, but as messmates not only tolerated by the plant, but even indispensable to its welfare. It is probable that nearly half the plants of our fields, moors, and forests entertain such fungi in their root-tissues. The curious, and long-known nodules on the roots of leguminous plants—peas, beans, clover, etc.—are filled with bacteria which enable these plants to avail themselves of the free nitrogen of the air, and so enrich the soil with nitrogenous substances.
The roots of most forest trees, orchids, and plants of the moorlands, meadows and marshes are similarly occupied by fungi, which in some way convey salts—probably especially phosphates and potassium compounds—to the plant in return for the small tax of organic carbon-compounds it exacts from the latter. In some cases at any rate, as Bernard has lately shown, the very existence of the plant depends on its seedling roots obtaining this advantageous attachment and co-operation (symbiosis) of the fungus immediately on germination.
These remarks must suffice to illustrate this part of my subject, and to emphasise the statement that the question whether a given plant can be grown in a given soil, is by no means one of simply the physical and chemical constitution of the latter. The plant will have to run the gauntlet of a long series of vicissitudes broughtabout by the presence or absence, relative proportions and vigour, and specific nature of the organisms in the soil at its roots, and it is easy to see that many cases of disease may be due to the absence of advantageous bacteria or fungi, or to circumstances which disfavour their life, as well as to the predominance of competing organisms.
It will now be evident that the old points of view must be abandoned, and with them, especially, the widely prevalent notion that chemical analyses of the plant and soil can explain the real problems of agriculture.
It was of course an enormous advance in the science when, thanks to the splendid labours of the chemists, at the end of the last century and the beginning of this, we obtained that preliminary knowledge of the constitution of the air, and of the composition of the water, acids and salts, etc., which plants require for their food-materials and life-processes. Much was gained by De Saussure's establishment of the fact of oxygen respiration, though we now understand by the term something very different from, and much more complex than, what he understood by it, as, also, much had been gained by the previously acquired knowledge of the gas-exchanges in carbon-assimilation: nor must we forget the services of those who proved, by laborious analyses, continued for long periods, what chemical compounds are found in the tissues of plants, and in the soils at their roots and the atmosphere which surrounded them. We must also remember many othercontributions which have been furnished, and are still being furnished by the chemist; and I for one hope that his labours will continue to go hand in hand with those of the physiologist.
But, when all due honour is paid to the scientific chemist, it must still be allowed that his problems are different from the real problems of agriculture. To take one set of instances alone. The chemist can analyse a given soil or a given manure, and can even go a long way towards making them, but his analyses do not tell us what conditions are necessary in order that their ingredients may be presented to the roots so as to be absorbed and become built up into the plant. Chemistry told us that carbon was fixed from the air, but physiological experiments determined how this meant the synthesis of certain definite carbohydrates—this, too, in the face of the powerful authority of the chemist Liebig, who supposed that the vegetable acids were the results of the assimilation of carbon. Wolff, De Saussure, and other chemists have done yeoman service in showing that different plants, growing in the same soil, contain different proportions of mineral substances; but it was by means of water-cultures, and other physiological researches, such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch on root-hairs, that the puzzling question of selective absorption, by means of the living root-hairs, came into the arena of our knowledge.
In every case—and, as already said, I am not undervaluing the work done—the chemist has leftus only on the threshold of the real problem. He has stood outside the factory in which the real work we want to know about is being carried on, and has told us of so many tons of this material being carried in at the gates, and of so many tons of that coming out; he has even burnt down the factory, and all its contents and machinery, and has then told us how many tons of the various materials were there at the time; but this is not what we want, valuable as the information is, and still more will be. What we want, and what we expect to obtain, is more information regarding what is done with the materials in the factory: what machinery they are put into, and how they are put in: what stages they go through, and how the stages follow one another: what wear and tear has to be endured, and how we can step in and stop the working of the machine for our own benefit at the best possible time.
The physiologist proceeds empirically, by experimenting with the living machinery. He recognises the parts and their structure, and tries to find out what they are doing: he knows that the laws of physics and chemistry cannot be traversed, but he sees these laws at work under special and very complex and peculiar conditions. He therefore, as the results of his experiments, sets new questions—or old questions under new conditions, if you like—and undoubtedly wants the help of both chemist and physicist; or, if it is preferred, the chemist and physicist may attack the problems, but they must familiarise themselves with thepeculiar mechanism of the organism concerned, and cannot hope to attain success without experimenting with it. I confess it seems to me as reasonable to look upon scientific agriculture as a branch chiefly of chemistry as it would be to look upon horse-breeding or pigeon-rearing from the same point of view; and why the professed chemist's advice is regarded as so comforting and final in the one case and not in the other is one of those mysteries which seem inherent in human nature.
The central point in agriculture is the plant: get the most out of it—the energy-winning machine which alone can keep the animals and everything else connected with the farm going—and all the rest follows. The old agriculture has taken a gloomy view of things, and especially on account of a large variable which it blames for many ills, namely, the season or climate. Perhaps the old agriculture has not sufficiently recognised that Nature grows plants in accordance with the fact that variation is not peculiar to the weather: if the seasons vary, so do fruit and other produce and the plants which yield them; and since man cannot hope to control the one variable, possibly relief will be found in doing more, within his limits, towards controlling others.
In any case he cannot hope to succeed without study of the physiology of the plant.
An admirable short account of soil in its relation to root-hairs is given in Sachs'Lectures, XV.; but for a more exhaustivetreatment of the subject of soil the reader is referred to King,The Soil(Wisconsin, 1895), or Warrington,Lectures on the Physical Properties of Soil(Oxford, 1900);Larbalétrier,L'Agriculture(Paris, 1888), chapters II. and III. There is also a very good account in Bailey,The Principles of Agriculture(London, 1898), chapters I.-III.With reference to the organisms in soils and the decompositions they bring about, the student should consult Kramer,Die Bakteriologie in ihren Beziehungen zur Landwirthschaft(Wien, 1890), and Lafar,Technical Mycology(Engl. edition, 1898), sections V., VIII., and IX.
An admirable short account of soil in its relation to root-hairs is given in Sachs'Lectures, XV.; but for a more exhaustivetreatment of the subject of soil the reader is referred to King,The Soil(Wisconsin, 1895), or Warrington,Lectures on the Physical Properties of Soil(Oxford, 1900);Larbalétrier,L'Agriculture(Paris, 1888), chapters II. and III. There is also a very good account in Bailey,The Principles of Agriculture(London, 1898), chapters I.-III.
With reference to the organisms in soils and the decompositions they bring about, the student should consult Kramer,Die Bakteriologie in ihren Beziehungen zur Landwirthschaft(Wien, 1890), and Lafar,Technical Mycology(Engl. edition, 1898), sections V., VIII., and IX.
The crossing of varieties of wheat, etc.—The essentials of fertilisation—Rimpau's experiments—Hybrids and selected varieties.
The crossing of varieties of wheat, etc.—The essentials of fertilisation—Rimpau's experiments—Hybrids and selected varieties.
In the more hopeful view of the case which the new agriculture will have to take, it will recognise the physiological truth that since the living plant is the important and variable machine which constructs the produce looked for, and since that machine will work best in proportion as its needs are properly satisfied; therefore in cases where the needs of a given type of the machine cannot be efficiently provided for, it will be well to select some other type which will take what supplies and conditions can be offered. Of course, this is already recognised to a certain extent, as is implied in the practices of "rotation of crops," selection of "pedigree wheats" and mixtures of "pasture grasses," and in decisions as to the quality of land according to the kinds of weeds found on it, andso forth; but I am convinced that the agriculturist of the future—and the same applies to the horticulturist, planter and forester—will have to concern himself more systematically with the working and the variability of the plant, and particularly with what Darwin termed Variation under Domestication, than has always been the custom in the past. The subject of the plasticity of cultivated plants, and especially of hybrids, is in one sense an old one; but much work is being done which proves, as such work is apt to do, that very much more may be done by well-planned experiments on the selection of new varieties raised by hybridising and cultivation.
In illustration of this point, a short summary of some of the results of crossing different species of wheat, barley, oats, peas, beet, etc., may serve to show what has been gained and what may be hoped for in these directions. It should be stated that much has been done and is being done in this country as well as abroad, as witness English varieties of corn, peas, and potatoes, and the recent experiments on crossing various kinds of maize in America.
The hybridiser grows his cereals, etc., in pots until ready for crossing, and then takes them into the laboratory, removes the weaker spikelets, and takes out the young stamens from the flowers left on the plant. The female plant is then ready, and the flowers covered with paper caps. The pollen, obtained by a clean wet brush from the plant chosen as the father, is then carefully placed inposition on the stigmas, and the caps replaced. The pollination is repeated occasionally, and care taken that no uncrossed flowers develop later. In this way a few seeds or grains are got to start with.
This would be the place to introduce an account of the enormous advances made by the botanists of the last decade or two in the study of the microscopic phenomena of fertilisation. Without going into details—which would more than occupy all the space at command—I may recall the discoveries of Strasburger and his pupils, and of Guignard, which have supplemented the earlier discoveries of De Bary, Cohn, and Hofmeister, by establishing the facts that the essential point in fertilisation is the fusion of two nuclei, and the bringing together in the fused mass of two extremely minute thread-like coiled bodies, the so-called chromatosomes or filaments, one of which is derived from the male and the other from the female parent. The particulars as to the marvellous adaptations to secure the union of these two infinitesimally minute threads, their behaviour immediately before and after union, and many other points must be passed over, as I have only space to emphasise the one crowning discovery that these tiny filaments of nuclear substance are the material carriers of all the hereditary properties of the parents to the young plant which their union initiates.
It must not be supposed that the above statements are based on any meagre foundation of facts. The attraction of the fusing nucleated masses had been demonstrated over and overagain by Tulasne, De Bary, Strasburger and others; but Pfeffer brought the matter to a crisis by discovering the attractive (chemotactic) substance emitted in given cases, and by collecting the fertilising bodies by its means into artificial tubes.
The fusion of the nucleated bodies in the sexual act was observed by Strasburger in the living plant a few years ago, and numerous later observers have confirmed it. Meanwhile all the stages of approach and contact of the essential filaments of the nuclear substance have been traced, as also all the stages of the transference of half of each filament, male and female, into each of the first two cells of the very young embryo-plant.
Moreover, the essentials are found to be the same in the animal kingdom also, and the bearing of all these discoveries on the phenomena of reproduction, variation, and heredity in living organisms has been and is of the highest importance, for they support, control, explain and correct so many of the splendid results of Knight, Kölreuter, Sprengel, Hildebrand and Hermann Müller, and in every direction throw side-lights into the crevices of that magnificent structure, the theory of Natural Selection, erected for all time by our countryman, Charles Darwin.
To return now to experiments on crossing. It is found that the first products of the crossing appear exactly alike; they may have characters intermediate between those of the father and mother, or they may resemble one more than theother, but all the seeds of the same cross do it in the same way.
On then sowing the seeds of the plants produced from this first cross, variations begin to appear. Most of the progeny revert to one or other of the parent forms, others show all conceivable combinations of their characters, and a few may give rise to entirely new characters. In succeeding generations the reversions are preponderant, and, supposing no care is taken to prevent it, the whole of the offspring gradually go back to the ancestral type.
Some important consequences result, however, if systematic care is brought to bear on the matter. This tendency to variation in the second generation of crossed plants has often been noted, and it bears out very distinctly the conclusions to which Darwin came.
The hybridiser takes advantage of this variation, as others have done, to select some forms and rigidly suppress others, in order to obtain well-marked varieties of the plants he experiments with. In illustration, I may take the following from Rimpau's account of his experiments on crossing wheat: By crossing a white English long-eared, dense wheat, and celebrated as a heavy cropper, with a red, looser German wheat, remarkable for its resistance to winter cold, Rimpau hoped to obtain a variety uniting both the above qualities. As regards the property of resistance, he failed, and he eventually gave up the attempts in face of the advantages offered by theso-calledSquare-heads, which then came into the market. His experiments, even with the above varieties, are worth noting, however, for they show how promising the results of carefully conducted crossing and selection may be.
The crossing was done in 1875, in both directions. In 1876 the few grains obtained were found to yield plants almost all alike, with the long loose ear of the German parent, but the paler colour of the English wheat.
In 1877 the plants, obtained by sowing the finest grains, were found to consist of pure white, pure red, and of forms which appeared to vary and revert in all possible degrees as regards colour, density, and other characters intermediate between these.
By carefully separating the closest and densest white wheats from the closest and densest red ones, he got in 1878 a large number of each coming nearer to the type sown than did the mongrel forms intermingled with them: these reversions and intermediate forms were then rigidly eliminated, and only the deepest coloured and densest red and white forms again sown.
In 1879 these two chosen varieties were constant, so far as concerned those selected from the crossing of female English white with male German red wheat, and the following year proved the constancy of the red variety in the reciprocal cross. In 1886 all four varieties—i.e.the two reds and the two whites of both the crossings—had become constant.
Still more instructive are the results of the cross between the same white English non-bearded wheat and a red German bearded wheat.
The first results of the crossing in 1875 showed the loose ear of the German mother, but was paler in colour; while the influence of the English father was shown by the absence of beard.
From the reversions and mixtures of the mongrels showing reminiscences of the parents in all degrees in 1877, rigid selections and re-sowings were made as before, and Rimpau eventually got four very distinct varieties, two red and two white, a bearded and a beardless form of each, and these were declared fixed and constant in 1879-1882.
Passing over many similar results, and merely noting a very successful variety got from a cross between a very early ripening loose red American wheat and the dense heavy cropping English Square-head—the crossed variety which has proved very suitable for certain light soils and dry climates on the Continent, which demand very rapid ripening, and are therefore of great physiological and technical interest—I must pass on to note the curious result of the successful hybridisation of wheat and rye. This cross has been effected several times, and first in this country according to reports from Edinburgh (1875), New York (1886), and elsewhere, and Rimpau's careful experiments seem to leave no doubt on the matter.
First I must remind you that wheat (Triticum) differs from rye (Secale) in several marked characters, such as the breadth and shape of the glumes, the number of flowers in the spikelet, etc.; and that the cultivated rye differs from cultivated wheats in the characters of the straw, in having long ears, and in its flowering glumes remaining widely divaricated for some days when in flower.
In 1888 Rimpau removed the young stamens from the German wheat referred to, and pollinated the stigmas with pollen from a long-eared rye. Four sound grains were obtained, looking like wheat-grains.
The history of one of these grains was as follows: In 1889 it yielded ears which were peculiarly narrow and long, and its stalks were also much longer than the wheat: the flowers remained exposed, with widely open paleae, for several days, and the grains were very peculiar, though wheat-like.
Fifteen of the best grains were selected, and in 1890 three of the resulting plants proved to be a wheat of the Square-head type and one quite sterile. The others retained the elongated, narrow, brownish-red ears, the flowering glumes again opening wide for some days. This last is a characteristic of rye, but not of wheat.
A long series of natural hybrids of wheat, barley, and oats are also described and discussed by Rimpau, as well as artificial crosses—some very remarkable—of barleys, but they must be passed over here.
Peas rarely become hybridised naturally. According to Darwin, H. Müller, and Focke, the flowers are little visited by insects in our countries, though the mechanism points to their adaptation for pollination by large bees.
Rimpau confirms Darwin, H. Müller, and Ogle as to the self-fertilisation of our cultivated peas. Nevertheless, as is well known, marked varieties have been obtained by artificial crossing by Gärtner, Knight, Laxton, and others, especially in this country.
At the same time experiments show that while it is very easy to obtain artificial hybrids of such plants, and there is no fear of natural inter-crossing, the forms are remarkably unstable as yet. Similarly unsatisfactory results were obtained with beet. As experiments are still going on, however, we may expect to hear more about these and other results.
It is probable, from recent experiments by De Vries, Correns, and others, that a remarkable regularity, expressed by Mendel in the form of a law, obtains in the variations which result from hybridising.
In considering these illustrative cases, it is necessary to thoroughly apprehend that two procedures are involved. In the first place we have the cross-pollination leading to the formation of the hybrid plant by cross-fertilisation. But experience shows that this would lead to very uncertain results if the plant-breeder did not supplement them by the second and extremelyimportant process of rigid selection—i.e.by choosing the best of the progeny and breeding from them apart from the parent-forms, and gradually intensifying, as it were, the variations in certain directions which have been started by the crossing.
It is by selection, careful culture, and repeated selection that so much has been done in obtaining the innumerable new varieties of roses, sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries, melons, tomatoes, early potatoes, etc., brought forward by numerous breeders of plants in all countries, as will readily be understood if reference be made to the work of Hays and Webber in America; Saunders in Canada; Garton, Sutton, Veitch, Bateson, and others in this country.
Nor is it necessary that the new materials for selection to work upon should be started by hybridisation. Grafting, change of conditions, and even variations so vaguely understood that we term them "spontaneous," may supply the starting-points for changes in the characters of plants, so remarkable after intensification by breeding that people find it difficult to believe they can have come from one stock.
Here, however, I must conclude, merely remarking that the above sketch is a mere outline of the subjects modern agriculture and horticulture concern themselves with. There are hundreds of problems connected with the germination of seeds, on which valuable recent work has been done by Klebs, Green, Horace Brown, and others; withthe resistance of seeds and seedlings to high and low temperatures, a subject opened out by Sachs, Kny, De Vries, Krasan, Just, Höhnel, Dewar, Dyer, and others; with the conditions of vegetation which affect the various functions of growth, respiration, assimilation, transpiration, and so forth, on which I cannot even touch in these pages.
Meanwhile I hope I have succeeded in impressing upon you the grand fact that the plant is a living and very complex engine, driven by the radiant energy of the sun, and capable of doing work thereby, and this just as truly as any heat-engine is driven by chemical energy gained by means of the sun's rays, or as a water-mill is driven by power which must be referred to the energy of potential in the head of water placed in position by the sun's work in evaporation. Fundamentally the whole of life and work on our planet is to be referred to the one great source of energy which renders possible the establishment of differences of potential.
This machine, then, doing work in various ways, adapts itself—or goes to the wall—to the conditions of its work among competing organisms or opposing circumstances. Curiously enough, while in some cases it suffers from the competition, in others it is benefited by its life-actions fitting in between those of other organisms, which in their turn supplement it. In other words new types of this engine, capable of doing the work in various ways, are obtainable; someare good types for the conditions afforded, others are bad ones.
Examples of both will occur in the further exposition of the subject.
Man's position in regard to the struggle is that of an intelligent being who steps in at certain stages and protects, fosters, and in every way favours the agricultural plant—the living machine—and sees that every opportunity is given it to do its best work in the best way—from his points of view!
The foundation of any course of reading on hybridisation and selection should be Darwin'sEffects of Cross and Self-Fertilisation in the Vegetable Kingdom, which, with his booksOn the Origin of Species by means of Natural SelectionandThe Variation of Animals and Plants under Domestication, will prepare the student for the long course of reading necessary for a full appreciation of what has been done in this department of science.From the numerous works which followed these I should select Bailey'sSurvival of the Unlike, London, 1896, andEvolution of our Native Fruits, New York, 1898, as especially useful for the reader of this book, to which may also be addedPlant Breeding, New York, 1896, by the same author, as giving numerous facts and practical directions of value. Further, the "Hybrid Conference Report,"Journ. Roy. Hort. Soc., 1900, abounds in facts and information. Rimpau,Landw. Jahrb., vol. xx., 1891, p. 239. The student who wishes to get towards the root of the matter will hardly be able to dispense with Strasburger'sNeue Untersuchungen über die Befruchtungsvorgang bei den Phanerogamen, Jena, 1884. An interesting summary of recent work onXeniaand "double fertilisation" will befound inBull. No. 22, U.S. Dept. of Agric., 1900. See alsoNature, Mar. 15, 1900, p. 470.If he wishes to explore the vast region of controversial literature that opens up from these points, and which is far beyond the purpose of this book, he may consult the literature collected in Kassowitz'Allgemeine Biologie, Wien, 1899, B. II., and the references in the works quoted; also, Strasburger, "The Periodic Reduction of Chromosomes in Living Organisms,"Ann. Bot., viii., 1894, p. 281. For "Mendel's Law," see Correns inBer. d. deutsch. bot. Gesellsch., vol. xviii., 1900, p. 158.
The foundation of any course of reading on hybridisation and selection should be Darwin'sEffects of Cross and Self-Fertilisation in the Vegetable Kingdom, which, with his booksOn the Origin of Species by means of Natural SelectionandThe Variation of Animals and Plants under Domestication, will prepare the student for the long course of reading necessary for a full appreciation of what has been done in this department of science.
From the numerous works which followed these I should select Bailey'sSurvival of the Unlike, London, 1896, andEvolution of our Native Fruits, New York, 1898, as especially useful for the reader of this book, to which may also be addedPlant Breeding, New York, 1896, by the same author, as giving numerous facts and practical directions of value. Further, the "Hybrid Conference Report,"Journ. Roy. Hort. Soc., 1900, abounds in facts and information. Rimpau,Landw. Jahrb., vol. xx., 1891, p. 239. The student who wishes to get towards the root of the matter will hardly be able to dispense with Strasburger'sNeue Untersuchungen über die Befruchtungsvorgang bei den Phanerogamen, Jena, 1884. An interesting summary of recent work onXeniaand "double fertilisation" will befound inBull. No. 22, U.S. Dept. of Agric., 1900. See alsoNature, Mar. 15, 1900, p. 470.
If he wishes to explore the vast region of controversial literature that opens up from these points, and which is far beyond the purpose of this book, he may consult the literature collected in Kassowitz'Allgemeine Biologie, Wien, 1899, B. II., and the references in the works quoted; also, Strasburger, "The Periodic Reduction of Chromosomes in Living Organisms,"Ann. Bot., viii., 1894, p. 281. For "Mendel's Law," see Correns inBer. d. deutsch. bot. Gesellsch., vol. xviii., 1900, p. 158.
History. References in the Bible—Greeks and Romans—Shakespeare—Rouen law—Superstitions—Malpighi and Grew—Hales—Unger—Berkeley—De Bary, etc. Physiology and Biology—Diagnosis—Etiology—Therapeutics. Study of causes.
History. References in the Bible—Greeks and Romans—Shakespeare—Rouen law—Superstitions—Malpighi and Grew—Hales—Unger—Berkeley—De Bary, etc. Physiology and Biology—Diagnosis—Etiology—Therapeutics. Study of causes.
Phytopathology, from Greek words which signify to treat of diseases of plants, comprises what is known of the symptoms, course, and causes of the diseases which threaten the lives of plants, or bring about injuries and abnormalities of structure. As a distinct and systematised branch of botany it is a modern study, the history of which only dates from about 1850, though the subject had been treated more or less disjointedly by several authors during the preceding century, and isolated records of diseased crops, fruit-trees, etc., exist far back in the history of Europe. The existence of mildews and blights on cereals indeedwas observed and recorded by the writers of the older books of the Bible, half a dozen references to such blights being found in the Old Testament, as well as others to blasted fig trees, etc., in the New Testament. Aristotle, about 350B.C., noticed the epidemic nature of wheat-rust. The Greeks and Romans were so well acquainted with such diseases that their philosophers speculated very shrewdly as to causes, while the people dedicated such pests to special gods. As regards the Middle Ages, we know little beyond the fact that blights and mildews existed, but Shakespeare's reference inKing Lear(ActIII., Sc. 4) leaves no doubt as to his acquaintance with mildew in the 17th century, and other authorities bear out the same. Even the law took cognisance of the danger of wheat-rust in 1660 in Rouen (Loverdo). Prior to the 18th century, however, only meagre notes on the subject occur scattered here and there among other matters, and much superstition existed then and later regarding these as other diseases.
Malpighi, in 1679, gave excellent figures of leaves rolled by insects and of numerous galls, the true nature of which he practically discovered by observing the insect piercing the tissues; previous observers—Pliny knew that flies emerge from galls, but thought the latter grew spontaneously—having nothing but superstitions and conjectures to offer. Grew, in 1682, also gave a capital figure and description of a leaf mined by "a small flat insect . . . which neither ranging in breadth nor striking deep into the leaf, eats so much onlyas lies just before it, and so runs scudding along betwixt the skin and the pulp of the leaf, leaving a whitish streak behind it, where the skin is now loose, as the measure of its voyage"—a by no means inadequate description of the injury and its cause.
During the eighteenth century several academic treatises or dissertations dealing with diseases of plants appeared.
But as a rule we only find disjointed notes. Hales (1727-33) discusses the rotting of wounds, canker, and a few other matters, but much had to be done with the microscope ere any substantial progress could be made.
With the nineteenth century, and the founding of the modern theories of nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era started. As the discoveries of the microscopists continued to build up our knowledge of the anatomy of plants and began to elucidate the biology of the fungi and other cryptogams, while the chemists and physiologists laid the foundations of our modern science of plant life, it gradually became possible to tabulate and classify plant diseases, and discuss their symptoms and causes in a more scientific manner. Even in 1833, however, Turpin, and a far better observer, Unger, regarded parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846). We may pass over the various treatises of Wiegmann (1839), Meyen (1841), Raspail(1846), Kühn (1859), and a number of other works of the period, merely referring with emphasis to Berkeley's admirable papers in theGardener's Chronicle(1854) for a summary of what was then known. All these works antedate De Bary'sMorphologie und Physiologie der Pilze, etc.(1866), in which he brought together the results of his researches during the decade, proving the real nature of parasitic diseases and infection as worked out by experiments between 1853 and 1863.
This work put the whole subject of parasitic diseases of plants and animals on a new footing, and paved the way for the modern treatment of plant pathology as elaborated in the treatises of Frank (1880 and 1895), Sorauer (1886), Kirchner (1890), and others, to which the reader is referred for further details. I will merely quote the following passage from Raspail'sHistoire Naturelle de la Santé et de la Maladie, 1846 (vol. ii., p. 176), in illustration of the views entertained by high authorities just prior to De Bary's work: "L'insecte qui produit leserineum,uredo,æcidium,xyloma,puccinia, n'est donc plus pour nous un insecte inconnu, mais unacarus(grise), unaphis(puceron) ou unthrips, qui produit au printemps une déviation, etc."
And this view, that fungi already well known to mycologists were called forth by the punctures of insects, was regarded as not out of harmony with the idea that the fungus itself was an abnormal outgrowth of the tissues of the host.
The proper study of plant pathology presupposes and involves a knowledge of the physiology of plants, of the normal relations of the latter to their environment, and of the biology of those animals and plants (principally insects and fungi) which are parasitic on them. It is of the first importance to understand that a disease is a condition of abnormal physiology, and that the boundary lines between health and ill-health are vague and difficult to define. As with the study of the diseases of man and other animals, so with those of plants, the practice resolves itself into the accurate observation and interpretation of symptoms (Diagnosis) on the one hand, and of causes (Aetiology) on the other, before any conclusions of value can be drawn as to preventive or remedial measures (Therapeutics). In plants, however, symptoms of disease are apt to exhibit themselves in a very general manner, or at any rate it may be that our perceptions of them differentiate symptoms due to very different reactions imperfectly, probably because the organisation of the plant is less specialised than that of animals. The turning yellow and premature falling of leaves, for instance, is a frequent symptom of disease; but it may be due to a long series of different causes of ill-health—e.g.drought, too high or too low a temperature, light of insufficient or of excessive intensity, a superfluity of water at the roots, the presence in the tissues of parasitic fungi, or that of worms or insects at the roots or elsewhere, poisonous gases in the air, soil, etc., and soforth. Consequently the science of plant pathology is much concerned with the direct action of external causes, which are probably less obscure than in the case of animals, though by no means always obvious. Such considerations at any rate seem to account for the fact that most authorities on plant pathology base their classification on the causes of disease, there being few noteworthy exceptions.