Disease in plants

by H. Marshall Ward

Excretions from root-hairs—Osmotic phenomena—Turgescence—Plasmolysis—Control of the protoplasm in absorption, etc. Selective absorption.

We see then that the root-hairs are the active living instruments in absorbing the water (containing small quantities of dissolved substances) of the soil.

If the living root-hairs are so numerous and so active, however, a natural inference is that they must exert some influence on the composition or arrangement of their environment. All the teachings of modern physiology go to show that such a living cell as I have sketched cannot carry on its life, brief though it be—the root-hairs are active for about four or five days—without forming substances of the nature of excreta, and we should expect some of these to pass out to the soil.

Sachs showed, in 1860, that roots growing in contact with polished marble corrode the surface of the mineral, and Nobbe, in 1876, showed that the roots of seedlings reduce potassium permanganate, a fact which Molisch confirmed in 1887. The latter observer also proved that living root-hairs secrete substances which colour a solution of guaiacum blue, oxidise pyrogallic acid and other organic substances, and rendered it probable that they excrete some substance which inverts cane-sugar, and in some cases even small quantities of a diastatic enzyme.

Molisch also confirmed an old observation, that roots excrete carbon-dioxide; and he and Czapek showed that the root-hairs excrete acids more permanent in their nature than carbonic acid, and published a method for showing this by means of a dilute solution, slightly alkaline, of phenolphthalein.

Molisch declared that the substances secreted by root-hairs may even be observed, dissolved in drops which ooze from the surfaces of the root-hairs.

That these root-excretions, and particularly the acids, may be of service in dissolving and rendering more available various constituents of the soil is an obvious suggestion, and it is borne out by Sachs' discovery of the corrosion of marble, and by Molisch's observation that living roots slowly corrode ivory if continuously kept in contact with it.

But a deeper insight into the physiology of these organs was only possible when the meaning of the phenomena of osmosis had been rendered clearer by the researches of Pfeffer and De Vries in 1877.

De Vries showed that the turgescence of the living cell can be diminished, and even reduced to nothing, by placing the cell in contact with solutions of substances which attract water from the cell-sap: as the turgescence diminishes, the cell contracts, owing to the elasticity of the cell-wall, which was previously distended; if the abstraction of water continues, the living protoplasmic membrane lining the cell-wall contracts away from the latter. He then proved that no injury need accrue to the cell by this process of plasmolysis, since the turgescence can be restored by washing out the salt with a more dilute solution, or with pure water; and the cell may go on living and even growing as before. These phenomena can only be produced in cells where the protoplasmic lining is intact and alive.

Pfeffer showed that the whole matter depends on the properties of the living protoplasmic membrane, which, so long as it is alive, has the power of governing the entrance or exit of dissolved substances, but is as a rule freely permeable for water. If, then, substances with a powerful attraction for water are formed in the cell cavity, and of such a nature that the protoplasm does not permit their free diffusion to the exterior, they attract water, and hold it fast, and so set up the condition of hydrostatic pressure known as turgescence, the limit of which depends on the attainment of a state of equilibrium between the elastic reaction of the cell-wall and the distending power of the absorbed water. When this limit is reached, water begins to filter back again through the cell-wall. Numerous researches during the last fifteen years have shown that the sap of such a living cell as the root-hair is charged with substances of various degrees of osmotic power; bodies like sugars, amides, vegetable acids and their salts, being formed by the metabolic activity of the protoplasm and accumulated there. Moreover, we now know that the salts of the vegetable acids in particular are effective, and the researches of Warburg and Palladin in 1886 have placed it beyond reasonable doubt that these acids are continually being developed and destroyed in the living cell during normal growth and respiration, and that great variations as to quantity may be brought about by alterations in the conditions of the environment—e.g. temperature, oxygen, etc.