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. In respect to the irritable movements referred to see Darwin, _The Power of Movements in Plants_, 1880, chapter III. The recent work of Nawaschin, _Beobachtungen ueber den feineren Bau u. Umwandlungen von Plasmodiophora_, Flora, Vol. LXXXVI., 1899, p. 404, should be read for details and literature concerning "Finger and Toe." CHAPTER XIV.