Notes to Chapter XXIX.
The student should read Bailey, The Nursery Book, 1896, for details regarding the practice of grafting, and facts in abundance can be obtained from the pages of the Gardeners' Chronicle.
Concerning graft-hybrids and the variations of grafted plants see Jouin, Can Hybrids be obtained by Grafting? and especially Daniel, "La Variation dans la Greffe," in Ann. des Sc. Naturelles, S. VIII., Vol. 8, 1898, p. 1, and the literature there collected. The whole subject is largely controversial, and much work remains to be done.
CHAPTER XXX.
LIFE AND DEATH.
Protoplasm—Hypothesis as to its structure and behaviour—Assimilation—Growth—Respiration—Metabolism—Action of the environment—Nuclear protoplasm—Pollination—Grafting—Parasitism—Graft-hybrids—Life—Death—Variation—Disease.
We have seen that all the essential phenomena of disease concern only the living substance—the protoplasm—of the plant, and that however complex the symptoms of disease may be, the occurrence of discolorations, lesions, hypertrophies, and so forth are all secondary matters subsidiary to the fundamental alterations of structure and function constituting the disease. It remains to see if we can adopt any hypothesis as to the nature of this physical basis of life—the protoplasm—which shall help us to understand still more clearly in what must reside those processes which, so long as they proceed harmoniously and uninterruptedly, constitute life and health, and which when interfered with result in disease and death. The protoplasm of the living plant-cell looks like a slimy translucent mass which has been superficially compared in appearance to well-boiled sago or clear gum. Fifty years of observations and experiments with it have convinced physiologists that it is not a mere solution or emulsion, however, or even a chemical compound in the ordinary sense of the term, although chemical analysis gets little out of it beyond water, proteids, carbohydrates and fats, and traces of certain mineral salts; for living protoplasm does not respond to the laws of physics and mechanics in obeying them, simply as do ordinary solutions and liquids. On the other hand, the most delicate chemical manipulation fails us, because when killed it is no longer protoplasm. Nor does the microscope advance matters far, beyond convincing us that this marvellous material must have a structure far more intimate than anything visible to the highest magnifying powers at our disposal.
Nevertheless, some information is forthcoming from the comparative examination of the protoplasm of numerous different kinds of organisms, for we have learnt that certain ingredients and no others are necessary for its composition—namely, carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, calcium[Note: See [note] at end of chapter.], magnesium, potassium—and it is as a rule of no use trying to foist on to it any substitute for any one of these. Moreover, these chemical elements must be given in certain definite proportions and forms: for instance it is of no use to offer the carbon and sulphur in such a form as carbon disulphide, or the nitrogen and hydrogen in that of hydrocyanic acid, but the carbon must be given to the protoplasm in the form of a carbohydrate or in some similar form, the nitrogen as an ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so forth, and thus water, air, carbohydrates, and the nitrates, sulphates, and phosphates of potassium, calcium, and magnesium become the chief natural sources of the essential ingredients. Again, we have learnt that while there are different forms of protoplasm in the cell, and that these react on each other, and go through cycles of arrangement and rearrangements, the intimate structure must be of that kind termed molecular—beyond the region of vision, just as is the microscopic structure of a crystal; but, while like the latter affording evidence of order and sequence when properly examined, the structural arrangements and changes must be infinitely more complex.
