Honeydew
The name given to a sweet sticky substance, abundant on the leaves of many plants in summer, especially when the weather is warm and dry. It gives the parts on which it lies the appearance of being wet or varnished. Though it may occur on almost any parts of plants, it is most abundant on the leaves, where it is almost always restricted to the upper surface, covering it uniformly, or in the form of minute spots crowded on the spaces between the veins. It is more abundant on woody plants than on herbs. Various causes have been assigned for its production; the belief was at one time entertained that it fell from the air. Afterwards, it was observed that aphides and certain allied insects secreted a fluid similar in its qualities to Honeydew, and that this secretion became sprinkled over adjoining bodies; and it was suggested that these insects were the producers of the Honeydew. The fact that the substance covers the upper surface of the leaves, while the insects are almost always found on the lower surface, was accounted for by supposing that the secretion fell from the insects on to the leaves beneath. There is no doubt that this is one mode in which the Honeydew is produced; but it has frequently been observed that plants, both in the open air and in houses, have been much covered with the coating, when no insects could be found on the plants, or in their neighbourhood. Sometimes, in such case, there is no appearance of disease in the leaves that bear it; but, at other times, they become discoloured, the green colouring matter, or chlorophyll, is destroyed below the shining patches, and the leaves may fall off early. Under these circumstances, the sweet fluid must be produced in, and exuded from, the cells of the leaves without any extraneous producer. Its production may, then, be regarded as analogous to the secretion of similar fluids in nectaries on various green parts of many healthy plants, e.g., on the stipules of Beans; but, when carried to excess, it becomes a disease. The causes, of the change are still only conjectural; and so also are the means for preventing and curing attacks. Among grasses, Honeydew may be produced in yet a third way, viz., in the development of the Ergot Fungus (Claviceps purpurea), so plentiful, in many years, in the heads of numerous species of grasses. In the early stage, before the Ergot has yet become conspicuous in the ear, the fungus produces a body (Sphacelia segetum) that bears on its surface numerous spores, imbedded in a sticky fluid like Honeydew. When the fungus is very plentiful among grasses, this fluid is correspondingly abundant. It assists in the diffusion of the spores of the fungus, as flies are very fond of it, and visit the diseased plants, suck up the secretion, and carry it away, with spores imbedded in it, to other grasses, where the spores, in their turn, may propagate the species. The Honeydews of different plants probably vary a little in chemical composition; but all contain a considerable quantity of sugars, including Mannite and cane sugar. It is to these that the sweet taste is due. Plants covered with Honeydew, whatever its origin, are very attractive to insects, especially to flies, and, at night, to moths. Several kinds of fungi also find it very favourable for their development: hence, plants covered with it are apt to assume a sooty appearance, due to the growth in them of black or brown fungi (Capnodium, &c.). Particles of soot or dust are also apt to stick to the leaves. Such extraneous substances, along with the secretion itself, hinder the healthy processes in the leaf from being properly carried on, though the stomata, or breathing pores, usually escape being clogged, as they are more abundant on, if not confined to, the lower surface of the leaves, and this is not covered. The abundant exudation of sugars must also weaken the plants; but the effect is seldom dangerous--a fortunate circumstance, in our ignorance of methods of cure.