Notes on Chapter VI
In addition to the works referred to in the last chapter, the student should consult Pfeffer's Physiology, 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.
CHAPTER VII.
THE BIOLOGY OF SOIL
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 to the 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 they become 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 that Aspergillus 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 completely oxidised 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 have brought to light a long list of forms, comprising yeasts, such as Hansen's Saccharomyces 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 question of 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 relations in 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 brought about 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 other contributions 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 left us 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 the peculiar 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.
Notes to Chapter VII
An admirable short account of soil in its relation to root-hairs is given in Sachs' Lectures, XV.; but for a more exhaustive treatment 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.
CHAPTER VIII.
HYBRIDISATION AND SELECTION
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, and so 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 in position 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 over again 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 the other, 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 the so-called Square-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.