rosive sublimate, the leaf rapidly contracts, and the leaflets curl up in an unusual manner, and do not again expand. When put into pure water, the sensibility does not return, but the whole remains stiff and immovable. A little solution of corrosive sublimate being put into a portion of pure water, containing an expanded branch of the plant, gradually caused curling up of the leaves, which then closed and fell. If the solution be very weak, the leaves open on the morrow, and are still sensible, but ultimately contract, twist, and remain stiff till they die. Solutions of arsenic and arseniate of potash produce the same effects.

A leaf of the sensitive plant was in a cold diluted solution of opium: in a few moments it opened out as in water, and, after half an hour, gave the usual signs of contractibility. In six hours it was expanded, and had a natural appearance, but could not be excited to move. The leaflets were flexible at the articulations, and offered a singular contrast to the state of irritation produced by corrosive sublimate. Pure water did not recover the plant. A large branch, similarly situated, expanded its leaves; but in half an hour had lost much of its sensibility: the leaflets, though alive, seemed asleep, and required much stimulating to cause contraction. In one hour the contractions ceased: in two hours the branch was dead.

A leaf placed in prussic acid (Scheele's strength) contracted, then slightly dilated, but was quite insensible, and the articulations were flexible: water did not recover it. If the acid be very weak, the leaflets dilate and appear to live, but are insensible. A drop of the acid placed on two leaflets of a healthy plant gradually causes contraction of the other leaflets, pair by pair. Solutions of opium and corrosive poisons have no effect when applied this way. After some time they dilate, but are insensible to external irritation: the sensibility returns in about half an hour; but the leaflets appear as if benumbed.

The plant exposed to the vapour of prussic acid is affected in the same way: ammonia appears to favour the recovery of the plant.

A cup containing dilute prussic acid was so placed that one or two leaves, or sometimes a branch, of a healthy plant

could be plunged into the liquid, or left to repose on its surface. The leaflets remained fresh and extended, but were almost immediately insensible. Being left in this state for two hours, they were expanded; and no irritation could cause their contraction, though otherwise there was no appearance of an unnatural state. At five o'clock in the evening the leaves were left to themselves. At nine o'clock they were open and insensible. At midnight they were still open, whilst all the rest of the plant, and the neighbouring plants, were depressed, contracted, and in the state of sleep. On the morrow they resumed a little sensibility, but seemed benumbed.

In the same manner M. Macaire has interfered with other plants as to the state of sleep, and observes that prussic acid thoroughly deranges the botanical indications of time of Linnæus.

CHAPTER XIV.

OF THE DISEASES TO WHICH PLANTS ARE SUBJECT.

THE diseases to which plants are subject are many and important: a few arise from mechanical causes, such as bruises or wounds, but the origin of the greater part is almost wholly unknown. It is probable, however, that some of them arise from a derangement of the circulation of the fluids, and an undue absorption of water, by which a brown colour and decay are produced.

Tabes, or gangrene, consists in a general languor of the system: the leaves and stem become flaccid, and the plant withers away; or, if it be succulent, becomes rotten. It is said by Link to arise from exposure to excessive cold, and a too rapid subsequent change to heat.

Anasarca, or dropsy, is a similar disease, peculiar to succulent plants, arising from an excessive introduction of water into the system. It produces rapid rottenness, and can only be stopped by destroying all the parts affected by it, and exposing the individual to a very dry atmosphere.

Scorching, or insolation, is a local disease attributable to exposure to too high a temperature. It is vulgarly supposed to be caused by drops of water collected on the surface of leaves, and destroying them by acting as burning lenses; but, as there is no focus formed when the water-drops lie on the leaves, it is obvious that no burning can be produced by them. It is often, I believe, caused by excessively rapid evaporation. Marcor, or welting, is a variety of this.

Chlorosis, or etiolation, is a kind of constitutional debility. The individual affected is pale, and destitute of a healthy green: the stems are weak, long, and slender; no flowers are produced; and the plant is readily killed. It is supposed to depend upon the accumulation of oxygen, and to be caused by various circumstances. The attacks of insects upon the roots,

by which the motion of the sap is deranged, are often the real cause. A form of this, in which healthy, well-formed leaves, which apparently perform their natural functions, become perfectly white, is common in Camellia reticulata: neither the cause nor the cure of this has been ascertained. Cold is probably sometimes the cause of similar appearances.

Canker, or caries, exhibits itself continually in a brown discolouration of the medulla and parts adjacent, and externally in small brown dead spots, which gradually extend on all sides, until they surround the branch and kill it. These spots are always dry and hard, never containing any fluid. It is this which is so fatal to many of the apple and pear trees of this country. Its cause and mode of cure are equally unknown. Apparently healthy shoots will, if grafted on another stock, carry the disease with them, and, like the gout and scrofula in human constitutions, will sooner or later be sure to break out. The cure of the disease is, therefore, as far as we know, impracticable.

Carcinoma is a disease in which an unusual deposit of cambium takes place between the wood and bark: no wood is formed; but, instead, the cambium becomes putrid, and oozes out through the bark, which thus separates from the alburnum. The cause of this is probably to be sought in the soil. It is a very dangerous disease, and the elm is particularly liable to its attacks. Some fine trees of this kind perished, a few years since, in the avenue at Camberwell called "The Grove." As soon as the bark is separated from the wood, the intervening space is peopled by swarms of Scolytus destructor, and

similar insects.

Extravasation, or gumming, consists in a discharge of thick sap from particular parts of the tree through the bark: the circumjacent surface withers but does not rot, as in Carcinoma.

Alburnitas is when a layer of soft wood is interposed between others of a harder texture: it is supposed to arise from a wet

season.

Galls, or tumid excrescences, are local affections caused by the puncture of insects. They are produced by an excessive deposition of cellular tissue; and are of no consequence to the general health of the individual subject to them.

pre

Albigo, ferrugo, and uredo, commonly called mildew, smut, rust, brand, and other names, are diseases caused by the sence of myriads of minute fungi of the genera Erycibe, Cæoma, Aspergillus, Puccinia, Uredo, and others. They are to plants what intestinal worms are to animals. Whether their presence is due to a languid state of the plant, which is thus rendered unusually susceptible of their attacks; whether the minute particles from which they are generated rise up from the earth through the vessels of the stem along with the sap, or whether they are originally contained in the seed and carried onwards with its growth, manifesting themselves whenever the plant arrives at a suitable state; finally, whether they are produced spontaneously, in consequence of the particular state of the atmosphere and tissue of the plant, are all points hereafter to be determined: nothing certain is known upon the subject at present. A very good account of the smut of barley is given by Mons. Adolphe Brongniart in the Annales des Sciences, vol. xx. p. 171.

Ergot, or clavus, is an excrescence from the seeds of grasses, of a brown or blackish colour; its nature or origin is undetermined. It does not depend upon the presence of parasitical fungi, and it possesses properties wholly foreign to the plant that bears it. That of the rye is frequently used successfully in medicine for the purpose of accelerating parturition.

Spotting, or necrosis, is chiefly found upon the leaves and soft parenchymatous parts of vegetables. It consists of small black spots, below which the substance of the plant decays: in many cases it no doubt arises from wet and cold, as in the cucumber and melon, which are wholly free from it in the warm weather of summer, but which are attacked immediately upon the arrival of the cold dewy nights of August and September.

Melligo and salsugo, by some reckoned diseases, are rather natural exudations of the juices of certain plants. Melligo produces the manna of the ash, the gum ladanum of the Cistus, &c.; salsugo, saline secretions of the same kind.

To these

be added, may

Suffocatio, or choking up, when every part diminishes in size.