Disease in plants

by H. Marshall Ward

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.

That numerous chemical rearrangements of the atom-complexes take place outside the protoplasmic molecules—both of those left unemployed in assimilation and of those rejected during the destructive processes—will be readily understood: many of the bye-products found in plants, such as vegetable acids, alkaloids, colouring matters, crystalline bodies, etc., etc., are due to these, so to speak, fortuitous combinations and re-combinations.

The part played by respiration has often been misunderstood. It consists in the burning off of some of the carbon and hydrogen of the shattered protoplasm molecules, by means of the oxygen of the air, which finds its way into the fluids around the protoplasm, and when it is active every act of combustion—which is here an explosion—leads to the shattering of more protoplasm molecules, and consequently to more respiratory combustion of the products. If the supply of oxygen is limited the breaking down of the molecules of protoplasm does not cease, but the carbon and hydrogen which would otherwise have been oxidised are now in part left to form other compounds in the surrounding liquid, and thus incompletely oxidised bodies, such as vegetable acids, alcohols, etc., accumulate. Even in the complete absence of atmospheric oxygen the protoplasm may go on breaking down and accumulating various compounds containing relatively much carbon and hydrogen—so-called intramolecular respiration; but in ordinary plants this process soon comes to an end, because the blocking up of the molecular plexus leads to obstruction and interferes with the normal assimilation and dis-assimilation, and, if prolonged, leads to pathological conditions, and eventually death.

Here, then, we meet with a cause of disease, or of predisposition to disease. The deprivation of oxygen interferes with the normal processes of building up and breaking down of the protoplasmic molecules, and bodies we term poisonous accumulate and may lower the vitality or even bring life to an end.

During normal life other products of the disruption of the protoplasm molecules are nitrogenous bodies, such as proteids, and these we have reason to believe are used up again, acting as the nuclei, so to speak, of the new molecules, and so being built up again with fresh food-materials into the plexus, to be again set free, and again used up, and so on. Others are the carbohydrates, such as cellulose, which pass out of the molecule into an insoluble form, and are accumulated outside the protoplasm in the form of cellulose membranes, and so forth. It is these formed products of metabolism (Metabolites), especially cellulose and bodies which result from its subsequent transformation, which constitute the main permanent mass of the ordinary plant.

We are now in a position to see how another fundamental cause of disease or predisposition to disease exists in the deprivation of the protoplasm of any of the elements needed to supply—in the food-materials—the place of those which have been permanently put aside in the form of cell-walls, or burnt off in respiration, passed out as excretions, or in other ways lost.

It is clear that the indispensability of an element must mean that the protoplasmic molecule cannot be completed without it: the same conclusion is supported by the experimental proof that these elements cannot be replaced by chemically similar elements.

It does not follow, however, that the protoplasm molecule must always have the same number of atoms of these elements, and grouped always in the same atom-complexes before being assimilated; nor that the protoplasm molecule, when once built up, always breaks down in exactly the same way. On the contrary, while the protoplasm of corresponding parts of a daisy and of a rose must contain all the elements named, we must believe that the atom groupings are different in the protoplasm molecule in each case; and though the molecules of the cell-protoplasm, of the nucleus, of the chlorophyll-corpuscles, etc., of one and the same plant must have all these elements, the atom groupings and modes of building up and breaking down may be very different in each case.