Disease in plants

by H. Marshall Ward

If, now, we bring a solution of some salt, such as potassium nitrate, which has a powerful attraction for water, on the outside of the living root-hair, the question whether the water remains in the cell, or passes out of it, merely depends on whether the substances inside or that outside have the most powerful attraction on the water in the sap, since the protoplasm allows water to pass freely.

But the protoplasmic lining may affect the whole matter in another way; for it may allow the dissolved salt, or other substance, in the solution outside or inside the cell to pass through it also, or it may take it up and fix it, or break it up or otherwise alter it.

More recent researches, and especially those of Pfeffer, have shown that these diosmotic properties of the living protoplasm are of the utmost importance in the whole matter of absorption of substances from the soil.

Let us suppose the following case. A root-hair, in full vigour, is allowed to bathe freely in a dilute solution of various substances, such as sugar, potassium nitrate, phosphates, sulphates and carbonates of iron, soda, lime, magnesium and others known by experiment to be harmless to its life.

Now it turns out to be by no means a foregone conclusion that all or any of the substances, even though freely soluble in the water, can pass through the protoplasm into the interior of the cell. Some may be allowed easy access, others may only be permitted to pass in small quantities, and others, again, may be absolutely refused access by the delicate living filter, so long as it is vigorously alive. Nor, as proved by numerous experimental cultures since De Saussure's time, is the entrance of a salt, etc., ruled by its indispensability or otherwise in the economy of the plant. And it is important to notice that only experiment can prove the point and determine which substances are absorbed and which refused by the root-hair.

If we now suppose the protoplasm to give rise to powerfully osmotic substances which accumulate in the sap-vacuole, but which are not permitted free egress through the protoplasm (and the formation of such bodies will occur if the protoplasm is actively respiring), the conditions for absorption of water, with or without any dissolved salts, which the protoplasm allows to traverse it, are set up.

But the above supposed case is realised, as Pfeffer showed in 1886, when he found by a series of beautiful experiments that certain aniline dyes can accumulate in living root-hairs, and other living cells, whereas others cannot pass the living protoplasm. After accumulating for some time, the dye may either remain stored there, or may eventually diffuse out.

Pfeffer made another discovery, of equal importance, namely, that under the influence of dilute organic acids, such as citric acid, the permeability of the living protoplasm may be altered, so that it allows substances to pass which could not otherwise have traversed it. De Vries had also shown that the condition of the protoplasm affects its power of retaining the colouring matter in the sap of the Beet: so long as the protoplasm is alive, the crimson sap is retained, even when the cell is plasmolysed, but immediately it begins to die the colour escapes through it. A similar case exists when the chlorophyll-corpuscles retain their colour in living cells known to be charged with acids: so long as the protoplasm is alive and normally active the green bodies are protected.

These, and numerous other experiments of the same kind, prove that the healthy root-hair is a living instrument for taking up dilute solutions out of the soil, and holding them in the sap-cavity for a time. If killed, by frost for instance, it loses these powers.

The researches of the last ten years have also shown that a time comes when the turgid cell, if an isolated one, and if sufficient supplies of water are present, is so tightly distended that the surplus water begins to diffuse out again under the pressure proper to the hydrostatic conditions set up.