Disease in Plants

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.

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 extremely important 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; with the 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; some are 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!

Notes To Chapter VIII

The foundation of any course of reading on hybridisation and selection should be Darwin's Effects of Cross and Self-Fertilisation in the Vegetable Kingdom, which, with his books On the Origin of Species by means of Natural Selection and The 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's Survival of the Unlike, London, 1896, and Evolution of our Native Fruits, New York, 1898, as especially useful for the reader of this book, to which may also be added Plant 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's Neue Untersuchungen über die Befruchtungsvorgang bei den Phanerogamen, Jena, 1884. An interesting summary of recent work on Xenia and "double fertilisation" will be found in Bull. No. 22, U.S. Dept. of Agric., 1900. See also Nature, 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 in Ber. d. deutsch. bot. Gesellsch., vol. xviii., 1900, p. 158.

PART II.

DISEASE IN PLANTS

CHAPTER IX.

PHYTOPATHOLOGY. DERIVATION AND MEANING

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 indeed was 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 350 B.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 in King Lear (Act III., 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 only as 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 the Gardener's Chronicle (1854) for a summary of what was then known. All these works antedate De Bary's Morphologie 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's Histoire 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 les erineum, uredo, æcidium, xyloma, puccinia, n'est donc plus pour nous un insecte inconnu, mais un acarus (grise), un aphis (puceron) ou un thrips, 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 so forth. 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.