_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.