Slime-flux.—The trunks of trees may sometimes be observed to pour out a slimy fluid from cracks in the bark, or from old wounds, or branch scars. In some cases, e.g. in Oaks, the slime has a beery odour and white colour, and abounds in yeasts and other fungi to the fermentative activity of which the odour and frothiness are due. In other cases the slime is red e.g.—Hornbeam; or brown—e.g. Apple and Elm; or black—e.g. Beech, the colour in such cases being due to the mixture of yeasts, bacteria, and fungi with which these slimes abound. The phenomenon appears to be due to the exudation of large quantities of sap under pressure—root pressure—and is primarily a normal phenomenon comparable to the bleeding of cut trees in spring: the fungi, etc., are doubtless saprophytes, but their activity is concerned with the putrefactive processes going on in the diseased wood, and which may lead to rotting of the timber.
The origin of the wounds in the bark and cortex, and which extend into the wood and other tissues as the putrefactive and fermentative processes increase, appears to be in some cases at least due to lightning.
Resin-flux or Resinosis.—The stems of Pines and other conifers are apt to exude resin from any cut or wound made by insects, or by the gnawing of other animals; but in many cases the flow is due to fungi, e.g. Peridermium, the hyphae of which invade the medullary rays and resin canals and thus open the way to an outflow through cracks in the bark. Agaricus melleus not only invades the resin passages, but stimulates the tree to produce abnormal quantities of resin, which flows down to the collar and roots, and exudes in great abundance at the surface of the soil. Various other plants also exude resin from wounds, and in some cases the flux seems to be increased by degeneration of the tissues, e.g. Copaifera.
Gummosis.—Cherries, Apricots, Acacias, and many other trees are apt to produce abnormal quantities of gum, which flows from any wound or exudes through cracks in the bark. Degeneration of the wood-cells, and especially of the cell-walls of a soft wood formed by abnormal activity of the cambium, points to its origin being due, in some cases at any rate, to a conversion of the cellulose, and fungi are sometimes found in the masses of gum; but beyond the fact that gummosis is a pathological phenomenon we know very little of the disease.
With regard to such gumming, it is significant how frequently pruned trees—Cherries, Oranges, Lemons, Plums, etc.—suffer.
Manna flux.—Certain trees, such as the Manna Ash, species of Tamarisk, etc., yield manna from wounds, and in some cases the latter are due to insects, e.g. Cicada.
The Potato-disease is best known by the pale whitish fringe, giving an almost mealy appearance to the margins of the brown to black patches in damp weather. In dry weather the brown patches shrivel and dry, and as they are apt to be at the edges and tips of the leaflets, these curl up. The young disease spots are yellowish, and the leaves of badly affected plants are apt to be sickly yellow throughout.
This Potato-disease due to Phytophthora must be distinguished from the curling and puckering, with wilting and browning of the leaves and yellow glassy look of the stems, due to the invasion of the vessels by a fungus which lurks in the tubers, and gains access thence to the shoots.
In the disease traceable to Phytophthora the stock remains green and the leaves plump and plane, and only the brown patches slough out in wet or shrivel in dry weather, and are bordered by the pale whitish zone of conidiophores.
In the leaf-curl the yellow and flaccid appearance of all the leaves of a stalk, or even of the plant, is the striking symptom, and the stem soon droops and blackens just above the soil, a white mould appearing also at the black spots. Subsequently black spots appear higher up, and bacteria gain an entrance. The stolons rot, and eventually the roots and the leaves wither. The tubers appear sound, but are small; they are apt to rot in the store, the vascular zones turning brown.
