The exposure of wood by a wound results also in another mode of stopping up the vessels and so hindering the access of air, loss of water, etc., for the living cells of the medullary rays and wood-parenchyma grow into the lumina of the larger vessels through the pits, forming thyloses, again a phenomenon met with in heart-wood. In Conifers the stoppage of the lumina is increased by deposition of resin, which also soaks into the cell-walls and the wounded wood becomes semi-translucent owing to the infiltration.
Every living cell in an active condition is irritable, and one of the commonest physiological reactions of growing tissues is that of responding to the touch of a resistant body, as is vividly shown by the movements of the Sensitive plant, Dionaea, etc., and by those of tendrils, growing root tips, etc., on careful observation. We have reason for stating that if a minute insect, too feeble to pierce the cuticle, cling on to one side of the dome-shaped growing point of any shoot, the irritation of contact of its claws, hairs, etc., would at once cause the protoplasm of the delicate cells to respond by some abnormal behaviour; and, as matter of experiment, Darwin showed long ago that if a minute piece of glass or other hard body is kept in contact with one side of the tip of a root, the growth on the side in contact is interfered with. Moreover we know from experiments on heliotropism, thermotropism, etc., that even intangible stimuli such as rays of light, etc., impinging unsymmetrically on these delicate cells cause alterations in their behaviour—e.g. arrest or acceleration of growth.
Perhaps the most remarkable class of stimulations, however, is that due to the presence of the entire protoplasmic body of one organism in the cell of another, each living its own life for the time being, but the protoplasm of the host cell showing clearly, by its abnormal behaviour, that the presence of the foreign protoplasm is affecting its physiology. A simple example is afforded by Zopfs' Pleotrachelus, the amoeboid protoplasmic body of which lives in the hypha of Pilobolus, causing it to swell up like an inflated bladder, in which the parasite then forms its sporangia. The Pleotrachelus does not kill the Pilobolus, but that its protoplasm alters the metabolic physiology of the latter is shown by the hypertrophy of the cells, and by the curious fact that it stimulates the Pilobolus to form its sexual conjugating cells, otherwise rare, an indication of very far-reaching interference with the life-actions of the host.
An equally remarkable example is that of Plasmodiophora, the amoeboid naked protoplasm of which lives and creeps about in the protoplasm of a cell of the root of a turnip, to which it gains access through the root-hairs. It does not kill the cell, but stimulates its protoplasm to increased activity and growth and division, itself dividing also and passing new amoebae into each new daughter-cell of the host. Here the processes of stimulation, hypertrophy and further division are repeated, until hundreds or thousands of the turnip root-cells are infected. The externally visible result is the formation of distorted swellings on the root (Finger and Toe), most of the cells of which are abnormally large and filled with amoeboid Plasmodiophora protoplasm, which finally devours the turnip-protoplasm and itself passes over into spores. Here we have most convincing proof of the stimulation of protoplasm by other protoplasm in direct contact with it; and that the metabolism of the host-cells is profoundly altered is shown not only by the abnormal growth of the cells, but also by the starvation of the rest of the turnip plant as the Plasmodiophora gets the upper hand. We have here, in fact, a local intracellular parasitic disease, gradually invading large tracts of tissue and eventually inducing general disease resulting in death—a state of affairs reminding us of cancer in animals.
Irritation and hypertrophy of cells, however, may be induced by parasites which never bring their protoplasm into direct contact with that of the host. Many Chytridiaceae penetrate the cells of plants, and grow inside them as short tubes, vesicles, etc., the protoplasm of which is separated by their own cell-walls from that of the host-cell; nevertheless hypertrophy and abnormal cell-divisions and secretions are induced, and the effect even extends to neighbouring cells—e.g. Synchytrium—showing that some influence is exerted through cells themselves not directly affected. This latter point need not surprise us now we know that the cells of plant-tissues are connected by fine protoplasmic strands passing through the separating cell-walls.
But the invading plant need not actually enter the cells, and may still stimulate them through both its own and their own cell-walls to abnormal growth. This is well shown by the intercellular mycelium of Exoacus and Exobasidium, and the latter affords an excellent illustration of the far-reaching effects of hyphae on the cells (of Vaccinium) into which they do not penetrate. Not only are the cells stimulated to grow larger and divide oftener than normally, thus producing large gall-like swellings, but the chlorophyll disappears, the cell sap changes colour to red, the numerous compound crystals normally found in the tissues diminish in number and are different in shape, large quantities of starch are stored up, and even the vascular bundles are altered in character. All these changes indicate very profound alterations in the physiological working of the protoplasm of the cells of the host, and yet the fungus has done its work through both its own cell-walls and those of the host.
Even harmless endophytic algae in the intercellular spaces of plants may stimulate the cells in their immediate neighbourhood to increased growth, e.g. Anabaena in the roots of Cycads.
Notes to Chapter XIII.
With reference to cork-healing and wound-fever the student may consult Shattock "On the Reparative processes which occur in Vegetable Tissues," Journal of the Linnean Society, 1882, Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in the Aspen," Journal of Botany, 1883, Vol. XXI., p. 306. Also Richards, "The Respiration of Wounded Plants," Annals of Botany, Vol. X., 1896, p. 531; and "The Evolution of Heat by Wounded Plants," Ann. of Bot., Vol. XI., 1897, p. 29.
For details and figures respecting callus, see Sorauer, Physiol. of Plants, p. 175.