Disease in Plants
PREFACE
It has often been represented to me that the cultivators of plants, among whom are to be included planters and foresters, as well as agriculturists and gardeners of every kind, are more particularly concerned with, and interested in, the maladies themselves of the plants they grow, than in the life-history of the fungi, insects or other organisms to which they are due, or in the physiological processes which are involved; and although it is impossible to really understand any disease unless we also understand the processes by which it is brought about, there is room for sympathy with the point of view of the cultivator. He says, in effect, "I do not want to know all about the biology of the fungus of wheat-rust, or of the phylloxera, nor do I want to learn what experts can tell me about the action of bacteria in soil, or the process of starch-formation in the leaves: I have neither the time nor the means to master these details. What I want is guidance as to what is wrong with my tomatoes, apple trees, chrysanthemums, fir trees, turnips, etc., and what I am to do to set things right." Just so. With the latter part of this cry one must sympathize, much as a doctor does with the wail of the parent who calls him in to cure his sick child—we need not stop to classify or compare the motives of the parent and the cultivator, and perhaps I had done better to select a breeder of sheep with his flock and a veterinary doctor in the illustration, but we will let it pass; and as regards the former part of the cry, I do not know that the plant-doctor can expect the cultivator to be initiated in the aetiology of the disease any more than the physician expects the parent to understand the biology of the typhoid bacillus. That both the cultivator and the parent would be the better for a real knowledge of the disease in either case must be admitted—nay insisted on, provided the knowledge is real—but we have to deal with facts, and it is a fact that the clients of both doctors are impatient of the details of the case.
Now, of course, I am aware that no short cut or "royal road" to science exists, and if a man is going to train up trees or other plants, he ought to know all about them in health and in sickness, in youth and in old age, and he ought to learn everything about the soil they grow in, the air that surrounds them, the enemies that beset them, and all the multifarious relations of these one to another; but when I look at my boy and reflect how much his nurse, his schoolmaster, his tutor, his doctor, and his parents ought to know successively and simultaneously about him in sickness and in health, and about his surroundings, etc., I begin to wonder whether there is not after all something to be said for the cultivator's point of view.
Moreover, the cultivator knows a good deal about his plants which I do not know, and although I should much like to know it, his plea of want of time rings in my ears and the conviction strikes home that one ought to try and meet his views, and tell him something about disease as manifested in plants without insisting on his becoming a professional mycologist, entomologist, agricultural chemist, and philosopher.
Of course, beyond a certain point, it is his lookout how much the information is worth, and its educational value—a very different matter—is sure to suffer from any restrictions imposed on the treatment of the subject; but if the theme of disease in plants, treated from a general point of view—I was about to write "treated in a popular manner," but that is impossible until physiology and mycology are more widely taught—enables him to understand better the questions he puts to himself, and, still more, if it stimulates him to enquire further into the inexhaustible field of science glimpsed at, something may come of it.
The purpose of these essays is to treat the subject of disease in plants with special reference to the patient itself, and to describe the symptoms it exhibits and the course of the malady, with only such references to the agents which induce or cause disease as are necessary to an intelligent understanding of the subject, and of the kind of treatment called for. Consequently I have avoided any unnecessary classification or elaborate descriptions of parasitic fungi or insects, histological details of the tissues of plants, chemical and physical details regarding the soil, and even matters purely physiological as far as possible. Several admirable works on these subjects are already available, and must be referred to for further details.
It is, however, quite out of the question to avoid technicalities, though I have chosen the simpler course wherever it was found feasible, and have tried to so employ the examples selected that the student who wishes to go further into the matters dealt with may turn to special treatises for further information. For one eminently technical section I ought perhaps to apologise, but the temptation to try and set forth, in concrete form and suitable for the purposes of this book, some account of what is known of the most essential and profound factors concerned in the difficult question of the nature of life and death, health and disease, was great. Probably my apology should go further, and apply to what after all must be failure to explore this mystery to the bottom: my only excuse must be that it may stimulate others to go further.
It was an afterthought to add, in Part I., the considerations on the factors which influence the plant regarded as a living machine, so to speak, in order that the student may the better apprehend the point of view taken of the bearings of the matters discussed in Part II.
With regard to references, it seemed a better plan to give, in the form of notes after each chapter, the titles of the principal books and papers on which a student may base a further course of reading, than to overweight the pages of what is, after all, merely an introductory sketch to a huge subject, with detailed quotations from the numerous sources of information made use of. I have freely expressed my own opinions, but the sources for others are, I hope, as freely given. It will, however, be understood that I have not aimed at a complete bibliography, and, particularly, I have only given foreign references where it seemed that adequate treatment of the subject could not be found in English.
My sincere thanks are due to Mr. F. Darwin, F.R.S., who has kindly looked through many of the proofs, and given me the benefit of several suggestions: and to my wife for the very material aid she has afforded me in the preparation of the index.
Cambridge,
November, 1900.
PART I.
SOME FACTORS
CHAPTER I.
THE PLANT AND ITS SURROUNDINGS
The plant the central object of study—soil, climate, atmosphere, etc., are factors of its environment. Agricultural chemistry. The plant a machine. Physiology.
If I were asked to sum up the most important result of the numerous advances made during the past decade in agriculture and forestry, I should reply—the clearer and wider recognition of the fact that the plant itself is the centre of the subject, and not the soil, climate, season, or other factors of its environment. Until comparatively recent times it was the habit of farmers, foresters, planters, and gardeners, all the world over, to look upon the plant as a mere item or as a mysterious if important one in their calculations, and to regard the soil as the chief factor in their studies.
Now all is changing, and the world is gradually awakening more and more to the recognition of the truth that the soil and the clouds and the atmosphere are merely reservoirs of more or less inert materials, from which the living plant draws its supplies, and works them up, by means of energy focussed from the sun, into new plant substance.
In other words, the more far-seeing pioneers of scientific agriculture and forestry, etc., are recognising that agricultural chemistry is not the be-all and end-all of agricultural science; but that, in place of the study of the chemical analyses of dead soil, water, air, and plant-remains, which has so long held sway, largely owing, I think, to the influence of Liebig, the student should have his attention more concentrated on the living plant itself and on the physiological actions which make up its life. He must regard the living plant as a sort of working machine—infinitely more complex than any machine made by man, but a machine nevertheless—the purpose of which is to store up energy from the sun, and so to add to our wealth on this planet, at the expense of the extra-terrestrial universe.
It is not, be it noted, that the new study proposes to ignore or abandon the old studies: modern physiology owes too much to the physics and chemistry on which it is partly based, and