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 uber 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 Ka.s.sowitz' _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--Superst.i.tions--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 G.o.ds. 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 superst.i.tion 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 superst.i.tions 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 cla.s.sify 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 pa.s.s over the various treatises of Wiegmann (1839), Meyen (1841), Raspail (1846), Kuhn (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 pa.s.sage from Raspail's _Histoire Naturelle de la Sante et de la Maladie_, 1846 (vol. ii., p. 176), in ill.u.s.tration of the views entertained by high authorities just prior to De Bary's work: "L'insecte qui produit les _erineum_, _uredo_, _aecidium_, _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 deviation, 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 (princ.i.p.ally 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 cla.s.sification on the causes of disease, there being few noteworthy exceptions.

NOTES TO CHAPTER IX.

The bibliography here quoted will be found in Berkeley, "Vegetable Pathology," _Gardener's Chronicle_, 1854, p. 4; Plowright, _British Uredineae and Ustilagineae_, 1889; Eriksson and Henning, _Die Getreideroste_, Stockholm, 1896; De Bary, _Comparative Morphology and Biology of the Fungi_, etc., 1887; Frank, _Die Krankheiten der Pflanzen_, 1895-96, and scattered in the works referred to in them and in the text.

CHAPTER X.

HEALTH AND DISEASE.

_Variation--Disease--Comparison to a top. Health--Extinction of species--Natural demise. Examples of complex interactions in health--Interference, and tendencies to ill-health._

When we come to enquire into the causes of disease, it appears at first an obvious and easy plan to subdivide them into groups of factors which interfere with the normal physiology of the plant. Scientific experience shows, however, that the easy and the obvious are here, as elsewhere in nature, only apparent, for disease, like health, is an extremely complex phenomenon, involving many reactions and interactions between the plant and its environment. If we agree that a living plant in a state of health is not a fixed and unaltering thing, but is ever varying and undergoing adaptive changes as its life works out its labyrinthine course through the vicissitudes of the also ever-varying environment, then we cannot escape the conviction that a diseased plant, so long as it lives, is also varying in response to the environment. The princ.i.p.al difference between the two cases is, that whereas the normal healthy plant varies more or less regularly and rhythmically about a mean, the diseased one is tending to vary too suddenly or too far in some particular directions from the mean; the healthy plant may, for our present purposes, be roughly likened to a properly balanced top spinning regularly and well, whereas the diseased one is lurching here, or wobbling there, to the great danger of its stability. For we must recognise at the outset that disease is but variation in directions dangerous to the life of the plant. Health consists in variation also, but not in such dangerous grooves. That the pa.s.sage from health to disease is gradual and ill-defined in many cases will readily be seen.

In fact we cannot completely define disease. Mere abnormality of form, colour, size, etc., is not necessarily a sign of disease, in the usual sense of the word, otherwise the striking variations of our cultivated plants would suggest gloomy thoughts indeed, whereas we have reason to believe that many cultivated varieties are more healthy--in the sense of resisting dangerous exigencies of the environment--than the stocks they came from. Strictly speaking, no two buds on a fruit-tree are alike, and the shoots they produce vary in position, exposure, number, and vigour of leaves, and so forth. The minute variations here referred to are not seen by the ordinary observer, but those who bud, graft and multiply by cuttings on a large scale know that such bud-variations are important, quite apart from more extensive "sports" which occasionally occur.

On the other hand, we have reason to believe that many species have died out gradually as the environment altered. These plants died because they did not vary sufficiently, or did not vary in the right directions; they became diseased with respect to the then prevailing conditions of normal physiology or health.

Disease, therefore, may be said to be variation of functions in directions, or to extents, which threaten the life of the plant, the normal in all cases being the state of the plant characteristic of the species.

Even now, however, we have not obtained a complete definition, because, since all plants die sooner or later, we have not excluded the natural demise of the individual or its parts, and no one would call the autumnal fall of leaves, or the withering of an annual after flowering, death from disease. Clearly then the idea of disease implies danger of premature death, and probably this is as near as we shall get to a satisfactory definition. Since this matter is of primary importance for our present theme, I will add the following instances for consideration.

A plant in perfect health and in the fullest exercise of all its functions, has its roots in a soil which is suitably warmed and aerated, contains the right quant.i.ties of water which dissolve just the proper proportions of all the essential mineral salts, but nothing poisonous, while the soil itself has a texture such that the roots and root-hairs can extend and do their utmost in absorbing.

The leaves above are exposed to just the right intensity of light, in air which is not too dry, and is of suitable temperature and composition, containing no poisonous exhalations, etc.; and as the foliage is gently moved by the breeze, it manufactures carbohydrates at the optimum rate in the chlorophyll, and the so-called "elaborated sap"

containing the dissolved organic food-supplies is prepared in the tissues in maximum quant.i.ties and of just the right degrees of concentration and quality for use in the buds, stem, roots, etc., for which it is destined as they draw on the supplies.

Between these a.s.similating organs, the leaves, and the absorbing roots, we have in the stem the wood, with its vessels adapted in quant.i.ty and calibre to convey the water containing dissolved salts from the absorbing roots to the leaves (to say nothing of other parts) and, separated from this wood by the cambium, we find the sieve-tubes and cortical tissues in suitable quant.i.ty conveying the "elaborated sap"--the solutions of organic food-materials from the leaves down to the roots, up to the buds, and elsewhere. Joining these cortical and wood tissues are adapted series of medullary rays which, apart from other connections, bring about the necessary interchanges of water and "elaborated sap" with the cambium, the formative tissue which has to be fed and served by them, and which by its growth supplies new vessels and sieve-tubes, etc., to carry the continually increasing quant.i.ties of water and food substances as the roots and leaves increase in number and area, and thus enables this ideally correlated system to go on working at maximum energy.

Now suppose the same plant with its roots in an unsuitable soil--too dry or too poor in mineral supplies, for instance--the transpiring leaves above cannot obtain sufficient water and salts to supply their needs, but we will suppose hypothetically that they still a.s.similate under the same ideal conditions as before. The supplies now coming to the cambium are diminished, since the want of water and minerals compels the leaves to put aside any excess of carbohydrates (_e.g._ as stored starch-grains), and the plastic materials which do pa.s.s to the cambium so deficient in water cannot be directly utilised, and a starvation period sets in. Consequently the cambium forms less wood, and this will contain fewer and smaller vessels, and so reduce the conducting pa.s.sages: fewer sieve-tubes also are constructed, and the paths of the water current and food supplies narrowed, which of course reacts on the tissues everywhere. The reserve substances may slowly be dissolved and distributed, however, and considerable quant.i.ties be pa.s.sed in course of time into the roots, which, as opportunity offers, gradually employ them in making new roots, and if the disturbance has not gone too far and the conditions do not become unfavourable, an increased root-supply may by its larger absorbing area gradually establish the former state of equilibrium of functions. But this at the expense of the plant, which is smaller, has fewer leaves and narrower water channels, etc., than a plant not thus checked, and it may take a long time to make up for the loss of time and stature thus incurred. Indeed if the plant is an annual no recovery at all may occur, the reserves pa.s.sing into fruit and seeds instead of slowly supplying the roots as described.

If it be asked, can such a condition of affairs as that described really occur, we have only to think of a transplanted specimen with its roots maimed and put into unsuitable soil, or of plants in the open with feeding roots gnawed by an insect, etc., or of a tree hitherto in equilibrium with its fellows in a plantation suddenly set free by thinning and so forth.

Now take the case where the roots are maintaining their maximum functional activity, but the leaves--owing to want of light, too much moisture or too low a temperature of the air--are functionally depressed. Here we get a state of over-saturation with water set up, the tissues are turgid to bursting point, what supplies do traverse the sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and the cambium again forms less wood, but the lumina of the vessels are larger and the lignification less complete. Growth in length is excessive, but more leaves are formed, though they are apt to be abnormally thin and may be small. Little or no reserves are stored anywhere, and the watery tissues contain dangerously diffusible substances which may render them an easy prey to parasitic fungi. Here again, however, if the disturbance of equilibrium has not gone too far, and if the season permits, the new leaves may come into full activity and the situation be saved by transpiration and a.s.similation gradually increasing and restoring the equilibrium. But, as before, the plant has suffered, and shows the effect in its weak shoots, r.e.t.a.r.ded flowering, and other ways.

Such plight as is here described may actually be attained in greenhouses where over-watering is the fault, and even in the open it is not uncommon in rainy summers, or in plantations where dominant trees get the upper hand and partially shade more slowly growing species, or in fields where rank gra.s.s is allowed to overwhelm crops of lower stature.

Now it will be evident that either of these typical cases of temporary disturbance of functional equilibrium may be carried too far: in the first case the plant may wilt and wither, in the second it may rupture and rot, to take these eventualities only. And yet it is difficult to call these indispositions diseases: they are rather examples of extreme departures from the normal standard of health, just on the borderland between health and disease. A step further, as it were, and disease supervenes: certain tissues die from want of water, and a necrotic area is formed, or the cortex bursts and a wound is formed in another way, or some fungus gets a hold, and so on. These abnormal states are particularly apt to predispose the plant to disease--insects revel in such semi-wilted leaves and shoots crammed with reserves, and fungi in the water-logged leaves of the second case, while a cold dry wind is peculiarly fatal to such tissues.

NOTES TO CHAPTER X.

The reader may consult Hartig, _Diseases of Trees_, Eng. ed., 1894, Introduction; Sorauer, _Pflanzen Krankheiten_, pp. 1-12, and Frank, _Die Krankheiten der Pflanzen_, B. 1, p. 5, for definitions of disease.

CHAPTER XI.

CAUSES OF DISEASE.

_A. External causes--I. Non-living environment: soil, atmosphere, temperature--II. Living environment: plants, animals--Complex interactions--Predisposing causes--No one factor works alone--Tangled problems of natural selection involved. B. So-called internal causes._