Disease in Plants. Ward Harry Marshall

those of Frank, Winogradsky, Hellriegel, and Stahl, have brought to light a series of relationships between certain of these soil-organisms and the higher plants which place the matter of soil-biology in quite new lights.

      On the one hand it has been discovered that groups of bacteria are the active agents in bringing about the destruction of organic nitrogenous matter with the formation of ammonia, in oxidising this ammonia to nitrous and to nitric acids, which combine with bases in the soil to form the corresponding salts; while, on the other hand, other forms can decompose the nitrates and reduce them to nitrites, or set free ammonia or even nitrogen from them. Moreover, there are certain species which can fix the free nitrogen of the atmosphere, and start the cycle of up-building of this inert element into the complex higher compounds we term organic. It is impossible to over-estimate the importance of these processes of nitrification and denitrification going on in the soil about the root-hairs of the higher plants.

      But, in addition to this circulation of nitrogen in the soil, it turns out that the life-actions of bacteria, and not mere chemical decompositions, are largely responsible for the circulation of carbon, of iron, of sulphur and other elements formed from the decomposition—also by bacterial and fungal agency—of animal and vegetable remains in the soil.

      Even more startling are the biological relations in the soil between the absorbing roots of the higher plants and some of these bacteria and fungi, for it has now been established beyond all doubt that certain fungi enter the living roots and there flourish not as mere destructive parasites, but as messmates not only tolerated by the plant, but even indispensable to its welfare. It is probable that nearly half the plants of our fields, moors, and forests entertain such fungi in their root-tissues. The curious, and long-known nodules on the roots of leguminous plants—peas, beans, clover, etc.—are filled with bacteria which enable these plants to avail themselves of the free nitrogen of the air, and so enrich the soil with nitrogenous substances.

      The roots of most forest trees, orchids, and plants of the moorlands, meadows and marshes are similarly occupied by fungi, which in some way convey salts—probably especially phosphates and potassium compounds—to the plant in return for the small tax of organic carbon-compounds it exacts from the latter. In some cases at any rate, as Bernard has lately shown, the very existence of the plant depends on its seedling roots obtaining this advantageous attachment and co-operation (symbiosis) of the fungus immediately on germination.

      These remarks must suffice to illustrate this part of my subject, and to emphasise the statement that the question whether a given plant can be grown in a given soil, is by no means one of simply the physical and chemical constitution of the latter. The plant will have to run the gauntlet of a long series of vicissitudes brought about by the presence or absence, relative proportions and vigour, and specific nature of the organisms in the soil at its roots, and it is easy to see that many cases of disease may be due to the absence of advantageous bacteria or fungi, or to circumstances which disfavour their life, as well as to the predominance of competing organisms.

      It will now be evident that the old points of view must be abandoned, and with them, especially, the widely prevalent notion that chemical analyses of the plant and soil can explain the real problems of agriculture.

      It was of course an enormous advance in the science when, thanks to the splendid labours of the chemists, at the end of the last century and the beginning of this, we obtained that preliminary knowledge of the constitution of the air, and of the composition of the water, acids and salts, etc., which plants require for their food-materials and life-processes. Much was gained by De Saussure's establishment of the fact of oxygen respiration, though we now understand by the term something very different from, and much more complex than, what he understood by it, as, also, much had been gained by the previously acquired knowledge of the gas-exchanges in carbon-assimilation: nor must we forget the services of those who proved, by laborious analyses, continued for long periods, what chemical compounds are found in the tissues of plants, and in the soils at their roots and the atmosphere which surrounded them. We must also remember many other contributions which have been furnished, and are still being furnished by the chemist; and I for one hope that his labours will continue to go hand in hand with those of the physiologist.

      But, when all due honour is paid to the scientific chemist, it must still be allowed that his problems are different from the real problems of agriculture. To take one set of instances alone. The chemist can analyse a given soil or a given manure, and can even go a long way towards making them, but his analyses do not tell us what conditions are necessary in order that their ingredients may be presented to the roots so as to be absorbed and become built up into the plant. Chemistry told us that carbon was fixed from the air, but physiological experiments determined how this meant the synthesis of certain definite carbohydrates—this, too, in the face of the powerful authority of the chemist Liebig, who supposed that the vegetable acids were the results of the assimilation of carbon. Wolff, De Saussure, and other chemists have done yeoman service in showing that different plants, growing in the same soil, contain different proportions of mineral substances; but it was by means of water-cultures, and other physiological researches, such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch on root-hairs, that the puzzling question of selective absorption, by means of the living root-hairs, came into the arena of our knowledge.

      In every case—and, as already said, I am not undervaluing the work done—the chemist has left us only on the threshold of the real problem. He has stood outside the factory in which the real work we want to know about is being carried on, and has told us of so many tons of this material being carried in at the gates, and of so many tons of that coming out; he has even burnt down the factory, and all its contents and machinery, and has then told us how many tons of the various materials were there at the time; but this is not what we want, valuable as the information is, and still more will be. What we want, and what we expect to obtain, is more information regarding what is done with the materials in the factory: what machinery they are put into, and how they are put in: what stages they go through, and how the stages follow one another: what wear and tear has to be endured, and how we can step in and stop the working of the machine for our own benefit at the best possible time.