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,1 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 1 See note at end of chapter.
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.
All these, and numerous other results of enquiry, have led to the conclusions that we must regard living protoplasm as a complex made up of very large molecular units, each containing atom - groupings of the elements named; and, partly on account of the large number of atoms they contain, and partly due to the vibrations of absorbed heat, these units must be extremely labile. Moreover, they are linked up into an invisible and intricate meshwork, bathed in a watery liquid held in the interstices somewhat as water is held in a sponge. In this imbibed liquid are dissolved the substances, consisting of the same elements, which are to serve as food, and which are to be taken up into the molecular framework and built up into the structure of new molecular units - or, as they may be shortly termed, molecules of protoplasm: in the bathing liquid are also dispersed the fragments - again containing the elements named - which have resulted from the breaking asunder of some of the complex protoplasm molecules, and which are partly destined to be used up again, partly to be burnt off in respiration, and partly to be put aside as metabolic products such as reserves, secretions, permanent structure, etc. Among the elements carried into this liquid and dissolved in it the free oxygen of the air also plays an important part.
As new molecules are formed, by mutual combinations of the food-materials selected by molecular attractions, they are taken up into the protoplasmic framework, and built in between those already in existence, thus distending the whole, and we say that the protoplasm Assimilates food-materials and Grows. When distended beyond a given degree, or disturbed in various other ways, the molecular framework breaks, and some of the molecules are shattered, and as they fall to pieces certain of their constituent parts containing carbon and hydrogen forcibly combine at the moment of liberation with the oxygen in the fluid around and are burnt off in the form of carbon-dioxide and water, heat being of course evolved. This is the fundamental process of Respiration.
It is probably the alternation of these processes of Assimilation - the building up into the protoplasmic structure of new complex labile molecules - and Destruction - the shattering of such molecules with redistribution, oxidation, etc., of their fragments - which constitute the fundamental process of life. Different authorities attempt to explain the details of these processes in various ways, but there is practical agreement on the one point, that life consists in the alternate building up of new protoplasm from the food-materials - Assimilation - and the breaking down of the molecular complexes to simpler ones - Disintegration, or Dis-assimilation, as we may call it. During the periods when assimilation prevails, and the protoplasm increases in mass, we recognise Growth, and since this is usually associated with the vigorous imbibition of water, owing to the powerful osmotic attractions for that liquid exhibited by some of the products, and with consequent further stretching of the invisible molecular plexus, the growth may be so evident in increased size, that we are accustomed to look upon the visible increase in volume alone as growth; but it is essential to understand that growth of the protoplasm is always proceeding during life, even when as many older molecules are being shattered and dispersed as new ones are being formed by assimilation, and when, therefore, no visible permanent enlargement occurs. Similarly, during periods when disintegration of the molecules prevails, we must not assume that the assimilation of new molecules is not occurring and that growth is not proceeding. The two processes are always going on during the active life of the protoplasm: in fact life consists in the play of these processes, as already said.