are among the phenomena which have been attributed to instinct. Keith has endeavoured (Lin. Trans. xi. p. 11.) to establish the doctrine of the existence and agency of an instinctive principle in the plant, upon the ground of the direction invariably assumed by the radicle and plumelet respectively, in the germination of the seed.

1670. Definition of the plant. But if vegetables are living beings endowed with sensation and instinct, or any thing approaching to it, so as to give them a resemblance to animals, how are we certainly to distinguish the plant from the animal? At the extremes of the two kingdoms the distinction is easy; the more perfect animals can never be mistaken for plants, nor the more perfect plants for animals; but at the mean, where the two kingdoms may be supposed to unite, the shades of discrimination are so very faint or evanescent that of some individual productions it is almost impossible to say to which of the kingdoms they belong. Hence it is that substances which have at one time been classed among plants, have at another time been classed among animals; and there are substances to be met with whose place has not yet been satisfactorily determined. Of these may be mentioned the genus Corallìna (fig. 199.), which Linnæus placed among

199

animals, but which Gartner places among plants. Linnæus, Bonnet, Hedwig, Mirbel, and Link, have each given particular definitions. According to Link, a plant is a compound organic body, deriving nourishment from the soil in which it grows. According to Keith, a vegetable is an organised and living substance springing from a seed or gem, which it again produces; and effecting the developement of its parts by means of the intro-susception and assimilation of unorganised substances which it derives from the atmosphere or the soil in which it grows. The definition of the animal is the counterpart an animal is an organised and living being proceeding from an egg or embryo, which it again produces, and effecting the developement of its parts by means of the intro-susception of organised substances or their products. For all practical purposes, perhaps plants may be distinguished from animals with sufficient accuracy by means of the trial of burning; as animal substances in a state of ignition exhale a strong and phosphoric odour, which vegetable substances do not.

CHAP. V.

Vegetable Pathology, or the Diseases and Casualties of Vegetable Life.

1671. As plants are, like animals, organised and living beings, they are, like animals, also liable to such accidental injuries and disorders as may affect the health and vigour, or occasion the death, of the individual. These are wounds, accidents, diseases, and natural decay.

SECT. I. Wounds and Accidents.

1672. A wound is a forcible separation of the solid parts of the plant effected by means of some external cause, intentional or accidental.

1673. Incisions are sometimes necessary to the health of the tree, in the same manner, perhaps, as bleeding is necessary to the health of the animal. The trunks of the plum and cherry tree seldom expand freely till a longitudinal incision has been made in the bark; and hence this operation is often practised by gardeners. If the incision affects the epidermis only, it heals up without leaving any scar; if it pene trates into the interior of the bark, it heals up only by means of leaving a scar; if it penetrates into the wood, the wound in the wood itself never heals up completely, but new wood and bark are formed above it as before.

1674. Boring is an operation by which trees are often wounded for the purpose of making them part with their sap in the season of their bleeding, particularly the birch tree and American maple. A horizontal, or rather slanting, hole is bored in them with a wimble, so as penetrate an inch or two into the wood; from this the sap flows copiously; and though a number of holes are often bored in the same trunk, the health of the tree is not very materially affected. For trees will continue to thrive, though

subjected to this operation for many successive years; and the hole, if not very large, will close up again Like the deep incision, not by the union of the broken fibres of the wood, but by the formation of new bark and wood projecting beyond the edge of the orifice, and finally shutting it up altogether.

1675. Gerdling is an operation to which trees in North America are often subjected, when the farmer wishes to clear his land of timber. It consists in making parallel and horizontal incisions with an axe into the trunk of a tree, and carrying them quite round the stem, so as to penetrate through the alburnum, and then to scoop out the intervening portion. If this operation is performed early in the spring, and before the commencement of the bleeding season, the tree rarely survives it; though some trees that are pecularly tenacious of life, such as Acer saccharinum and Nýssa integrifolia, have been known to survive it a considerable length of time.

1676. Fracture. If a tree is bent so as to fracture part only of the cortical and woody fibres, and the stem or branch but small, the parts will again unite by being put back into their natural position, and well propped up. Especially cure may be expected to succeed if the fracture happens in the spring; but it will not succeed if the fracture is accompanied with contusion, or if the stem or branch is large; and even where it succeeds the woody fibres do not contribute to the union, but the granular and herbaceous substance only, which exudes from between the wood and liber, insinuating itself into all interstices, and finally becoming indurated into wood.

1677. Pruning. Wounds are necessarily inflicted by the gardener or forester in pruning or lopping off the superfluous branches; but this is seldom attended with any bad effects to the health of the tree, if done by a skilful practitioner: indeed, no further art is required, merely for the protection of the tree, beyond that of cutting the branch through in a sloping direction, so as to prevent the rain from lodging. In this case the wound soon closes up by the induration of the exposed surface of the section, and by the protru sion of a granular substance, forming a sort of circular lip between the wood and bark; and hence the branch is never elongated by the growth of the same vessels that have been cut, but by the protrusion of new buds near the point of section.

1678 Grafting. In the operation of grafting there is a wound both of the stock and graft, which are united, not by the immediate adhesion of the surfaces of the two sections, but by means of a granular and herbaceous substance exuding from between the wood and bark, and insinuating itself as a sort of cement into all open spaces: new wood is finally formed within it, and the union is complete.

1679 Felling is the operation of cutting down trees close to the ground, which certain species will survive, if the stump be protected from the injuries of animals, and the root fresh and vigorous. In this case the fibres of the wood are never again regenerated, but a lip is formed as in the case of pruning; and buds, which spring up into new shoots, are protruded near the section; so that from the old shoot, ten, twelve, or even twenty, new stems may issue, according to its size and vigour. The stools of the oak and ash tree will furnish good examples; but there are some trees, such as the pine and fir tribe, which never send out any shoots after the operation of felling. The frankincense pine is said to be an exception; but any specimens we have seen do not incline us to the belief of such an anomaly.

1680. If buds are destroyed in the course of the winter, or in the early part of the spring, many plants will again generate new buds, which will develope their parts as the others would have done, except that they never contain blossom or fruit. Du Hamel thought these buds sprang from preorganised germs, which he conceived to be dispersed throughout the whole of the plant; but Knight thinks he has discovered the true source of the regeneration of buds, in the proper juice that is lodged in the alburnum. Buds thus regenerated never contain or produce either flower or fruit; perhaps because the fruit-bud requires more time to develope its parts, or a peculiar and higher degree of elaboration; and that this hasty production is only the effect of a great effort of the vital principle for the preservation of the individual, and one of those wonderful resources to which nature always knows how to resort when the vital principle is in danger. But though such buds do not produce flowers directly, as in the case of plants which bear their blossoms on last year's wood; yet they often produce young shoots which produce blossons and fruit the same season, as in the case of cutting down an old vine or pruning the rose.

1681. Sometimes the leaves of a tree are destroyed partially or totally as soon as they are protruded from the bud, whether by the depredations of caterpillars or other insects, or by the browsing of cattle. But if the injury is done early in the spring, new leaves will be again protruded without subsequent shoots. Some trees will bear to be stripped even more than once in a season, as is the case with the mulberry tree, which is cultivated in the south of France and Italy for the purpose of feeding the silkworm; but if it be stripped more than once in the season, it requires now and then a year's rest.

162. The decortication of a tree, or the stripping it of its bark, may be either intentional or accidental, partial or total. If it is partial, and affects the epidermis only, then it is again regenerated, as in the case of slight incision, without leaving any scar. But if the epidermis of the petal, leaf, or fruit is destroyed, it is not again regenerated, nor is the wound healed up, except by means of a scar. Such is the case also with all decortications that penetrate deeper than the epidermis, particularly if the wound is not protected from the action of the air. If the decortication reaches to the wood, then new bark issues from between the bark and wood, and spreads till it covers the wound. But the result is not the same when the wound is covered from the air. In the season of the flowing of the sap Du Hamel detached a ring of bark of three or four inches in breadth, from the trunks of several young elm trees, taking care to defend the decorticated part from the action of the air, by surrounding it with a tube of glass cemented above and below to the trunk. After a few days the tubes became cloudy within, par ticularly when it was hot; but when the air became cool, the cloud condensed and fell in drops to the At last there began to appear, as if exuding from between the bark and wood of the upper part of the wound, a sort of rough scurfy substance; and on the surface of the wood, as if exuding from be tween the longitudinal fibres of the alburnum, a number of gelatinous drops. They were not connected with the scurfy substance at the top, but seemed to arise from small slips of the liber that had not been completely detached. Their first appearance was that of small reddish spots changing by degrees into white, and finally into a sort of grey, and extending in size till they at last united and formed longitudinal ridges, which constituted a new bark.

bottom.

103. Abortion or failure in the produce of flowers, fruits, or of perfect seeds, is generally the effect of accidental injuries, either directly to the flower or fruit, by which they are rubbed off or devoured by insects; or to the leaves by insects; or to the roots by exposure to the air or cutting off so much of them as essentially to lessen their power of drawing up nourishment. Other causes will readily suggest themselves; and one of the commonest, as to seeds and fruits, is want of sufficient impregnation.

1684 Premature flowering or fruiting is sometimes brought on by insects, but more generally by checks produced by cold, or injuries from excessive heat, or long-continued drought. Fruit is often ripened prematurely by the puncture of insects; and a pine-apple plant of almost any age may be thrown into fruit by an hour or two's exposure to a frosty atmosphere in winter, or by scorching the roots in an overhot tan-bed at any season.

SECT. II. Diseases.

1685. Diseases are corrupt affections of the vegetable body, arising from a vitiated state of its juices, and tending to injure the habitual health either of the whole or part of the plant. The diseases which occur the most frequently among vegetables are the following: - Blight, smut, mildew, honey-dew, dropsy, flux of juices, gangrene, etiolation, suffocation, contortion, consumption.

1686. Blight. Much has been written on the nature of blight; and in proportion as words have been multiplied on the subject, the difficulties attending its elucidation have increased.

1687. The blight, or blast, was well known to the ancient Greeks, who were, however, totally ignoran of its cause, regarding it merely as a blast from heaven, indicating the wrath of their offended deities, and utterly incapable of prevention or cure. It was known also to the Romans under the denomination of rubigo, who regarded it in the same light as the Greeks, and even believed it to be under the direction of a particular deity, Rubigus, whom they solemnly invoked that blight might be kept from corn and trees. It is still well known from its effects to every one having the least knowledge of husbandry or gardening; but it has been very differently accounted for: and, perhaps, there is no one cause that will account for all the different cases of blight, or disease going by the name of blight; though they have been supposed to have all the same origin. If we take the term in its most general acceptation, it will include at least four distinct species,-blight originating in cold and frosty winds; blight originating in a sort of sultry and pestilential vapour; blight originating from want of nourishment; and blight originating in the immoderate propagation of a sort of small and parasitical fungus,

1688. Blight originating in cold and frosty winds, is often occasioned by the cold and casterly winds of spring, which nip and destroy the tender shoots of the plant, by stopping the current of the juices. The leaves which are thus deprived of their due nourishment wither and fall, and the juices which are now stopped in their passage swell and burst the vessels, and become the food of innumerable little insects which soon after make their appearance. Hence they are often mistaken for the cause of the disease itself; the farmer supposing they are wafted to him on the east wind, while they are only generated in the extravasated juices as forming a proper nidus for their eggs. Their multiplication will no doubt contribute to the spread of the disorder, as they always breed fast where they find plenty of food. But a similar disease is often occasioned by the early frost of spring. If the weather is prematurely mild, the blossom is prematurely protruded, which, though it is viewed by the unexperienced with delight, yet it is viewed by the judicious with fear. For it very often happens that this premature blossom is totally destroyed by subsequent frosts, as well as both the leaves and shoots, which consequently wither and fall, and injure if they do not actually kill the plant. This evil is also often augmented by the unskilful gardener, even in attempting to prevent it; that is, by matting up his trees too closely, or by keeping them covered in the course of the day, and thus rendering the shoots so tender that they can scarcely fail to be.destroyed by the next frost.

1689. Blight, originating in sultry and pestilential vapour, generally happens in the summer, when the grain has attained to its full growth, and when there are no cold winds or frosts to occasion it. Such was the blight that used to damage the vineyards of ancient Italy, and which is yet found to damage our hopplantations and wheat-crops. The Romans observed that it generally happened after short but heavy showers occurring about noon, and followed by clear sunshine, about the season of the ripening of the grapes, and that the middle of the vineyard suffered the most. This corresponds pretty nearly to what is in this country called the fire-blast among hops, which has been observed to take place most com monly about the end of July, when there has been rain with a hot gleam of sunshine immediately after; the middle of the hop-ground is also the most affected, whether the blight is general or partial, and is almost always the point in which it originates. In a particular case which was minutely observed, the damage happened a little before noon, and the blight ran in a line forming a right angle with the sunbeams at that time of the day. There was but little wind, which was, however, in the line of the blight. (Hale's Body of Husbandry.) Wheat is also affected with a similar sort of blight, and about the same season of the year, which totally destroys the crop. In the summer of 1809, a field of wheat, on rather a light and sandy soil, came up with every appearance of health, and also into ear with a fair prospect of ripening well. About the beginning of July it was considered as exceeding any thing expected from such a soil. A week afterwards a portion of the crop on the east side of the field, to the extent of several acres, was totally destroyed; being shrunk and shrivelled up to less than one half the size of what it had for merly been, and so withered and blasted as not to appear to belong to the same field. The rest of the field produced a fair crop.

1690. Blight from want of nourishment may happen to all plants, wild or cultivated; but it is most commonly met with in corn fields, in very dry seasons, in those thin gravelly surfaces which do not sufficiently retain the moisture. In such spots the plants are thrown prematurely into blossom, and the ear or seed-pod ripens before it is filled. In England the farmers call this the white blight.

1691. Blight, originating in Fúngi, attacks the leaves or stems both of herbaceous and woody plants, such as Euphorbia Cyparissias, Berberis vulgàris, and Rhamnus catharticus; but more generally grasses, and particularly our most useful grains, wheat, barley, and oats. It always appears in the least ventilated parts of a field, and has generally been preceded by cold, moist weather, which, happening in the warm month of July, suddenly chills and checks vegetation. It generally assumes the appearance of a rustylooking powder, that soils the finger when touched. In March, 1807, some blades of wheat attacked with this species of blight were examined by Keith; the appearance was that of a number of rusty-looking spots or patches dispersed over the surface of the leaf, exactly like that of the seeds of dorsiferous ferns bursting their indusium. Upon more minute inspection, these patches were found to consist of thousands of small globules collected into groups beneath the epidermis, which they raised up in a sort of blister, and at last burst. Some of the globules seemed as if embedded even in the longitudinal vessels of the blade. They were of a yellowish or rusty brown, and somewhat transparent. But these groups of globules have been ascertained by Sir J. Banks to be patches of a minute fungus, the seeds of which, as they float in the air, enter the pores of the epidermis of the leaf, particularly if the plant is sickly; or they exist in the manure or soil, and enter by the pores of the root. (Sir J. Banks on Blight, 1805.) This fungus has been figured by Sowerby, and by F. Bauer and Grew. It is known among farmers by the name of red rust, and chiefly affects the stalks and leaves. But there is another species of fungus known to the farmer by the name of red gum, which attacks the ear only, and is extremely prejudicial. In the aggregate it consists of groups of minute globules interspersed with transparent fibres. The glo. bules are filled with a fine powder, which explodes when they are put into water. It is very generally accompanied with a maggot of a yellow colour, which preys also upon the grain, and increases the amount of injury.

1692. The only means of preventing or lessening the effect of any of the different varieties of blight mentioned is proper culture. Palliatives are to be found in topical applications, such as flower of sul phur, and where the disease proceeds from, or consists of, innumerable minute insects, it may occasionally be removed. Grisenthwaite conjectures that in many cases in which the blight and mildew attack corncrops, it may be for want of the peculiar food requisite for perfecting the grain; it being known that the fruit or seeds of many plants contain primitive principles not found in the rest of the plant. Thus the grain of wheat contains gluten and phosphate of lime, and where these are wanting in the soil, that is, in the manured earths in which the plant grows, it will be unable to perfect its fruit, which of conse quence becomes more liable to discase. (New Theory of Agr.)

1693. Smut is a disease incidental to cultivated corn, by which the farina of the grain, together with its proper integuments and even part of the husk, is converted into a black soot-like powder. If the injured ear be struck with the finger, the powder will be dispersed like a cloud of black smoke; and if a portion of the powder be wetted by a

drop of water and put under the microscope, it will be found to consist of millions of minute and transparent globules, which seem to be composed of a clear and glairy fluid encompassed by a thin and skinny membrane. This disease does not affect the whole body of the crop, but the smutted ears are sometimes very numerously dispersed throughout it. Some have attributed it to the soil in which the grain is sown, and others have attributed it to the seed itself, alleging that smutted seed will produce a smutted crop: but in all this there seems to be a great deal of doubt. Willdenow regards it as originating in a small fungus, which multiplies and extends till it occupies the whole ear (Princip. of Bot. p. 356.): but F. Bauer of Kew seems to have ascertained it to be merely a morbid swelling of the ear, and not at all connected with the growth of a fungus. (Smith's Introd. p. 282.) It is said to be prevented by steeping the grain, before sowing, in a weak solution of arsenic. But, besides the disease called smut, there is also a disease analogous to it, or a different stage of the same discase, known to the farmer by the name of bags or smut balls, in which the nucleus of the seed only is converted into a black powder, whilst the ovary, as well as the husk, remains sound. The ear is not much altered in its external appearance, and the diseased grain contained in it will even bear the operation of threshing, and consequently mingle with the bulk but it is always readily detected by the experienced buyer, and fatal to the character of the sample. It is said to be prevented as in the case of smut.

1694. Mildew is a thin and whitish coating with which the leaves of vegetables are sometimes covered, occasioning their decay and death, and injuring the health of the plant. It is frequently found on the leaves of Tussilago Fárfara, Hùmulus Lupulus, Corylus avellana, and the white and yellow dead-nettle. It is found also on wheat in the shape of a glutinous exudation, particularly when the days are hot and the nights without dew. J. Robertson (Hort. Trans. v. 178.) considers it as a minute fungus of which different species attack different plants. Sulphur be has found to be a specific cure. In cultivated crops mildew is said to be prevented by manuring with soot; though by some this is denied, and soot, by rendering the crop more luxuriant, is said to be an encourager of mildew, the richest parts of a field being always most infected by it. As it is least common in airy situations, thinning and ventilation may be considered as preventives.

1695. Honey-dew is a sweet and clammy substance which coagulates on the surface of the leaves during hot weather, particularly on the leaves of the oak tree and beech, and is regarded by Curtis as being merely the dung of some species of aphides. This seems to be the opinion of Willdenow also, and it is no doubt possible that it may be the case in some instances or species of the disease: but Sir J. E. Smith contends that it is not always so, or that there are more species of honey-dew than one, regarding it particularly as being an exudation, at least in the case of the beech, whose leaves are, in consequence of an unfavourable wind, apt to become covered with a sweet sort of glutinous coating, similar in flavour to the fluid obtained from the trunk.

1606. It is certain, however, that saccharine exudations are found on the leaves of many plants, though not always distinguished by the name of honey-dew; which should not perhaps be applied except when the exudation occasions disease. But if it is to be applied to all saccharine exudations whatever, then we must include under the appellation of honey-dew, the saccharine exudations observed on the orange tree by De la Hire, together with that of the lime tree which is more glutinous, and of the poplar which is more resinous; as also that of the Cistus créticus, and of the manna which exudes from the ash tree of Italy and larch of France. It is also possible that the exudation or excrement constituting honey-dew may occasionally occur without producing disease; for if it should happen to be washed off soon after by rains or heavy dews, then the leaves will not suffer. Washing is therefore the palliative; judicious culture the preventive.

1697. Dropsy. Plants are also liable to a disease which affects them in a manner similar to that of the dropsy in animals, arising from long-continued rain or too abundant watering. Willdenow describes it as occasioning a preternatural swelling of particular parts, and inducing putrefaction. It is said to take place chiefly in bulbous and tuberous roots, which are often found much swelled after rain. It affects fruit also, which it renders watery and insipid. It prevents the ripening of seeds, and occasions an immoderate production of roots from the stem.

1608. In succulent plants this disease generally appears in consequence of excessive waterings, and is for the most part incurable. The leaves drop, even though plump and green; and the fruit rots before reaching maturity. In this case the absorption seems to be too great in proportion to the transpiration; but the soil when too much manured produces similar effects. Du Hamel planted some elms in a soil that was particularly well manured, and accordingly they pushed with great vigour for some time; but at the end of five or six years they all died suddenly. The bark was found to be detached from the wood, and the cavity filled up with a reddish-coloured water. The symptoms of this disease suggest the palliatives; and the preventive is ever the same― judicious culture.

1699. Flux of juices. Some trees, but particularly the oak and birch, are liable to a great loss of sap, which bursts out spontaneously, owing to its superabundance, or issues from accidental wounds: sometimes it is injurious to the health of the plant, and some

times not.

1700. There is a spontaneous extravasation of the sap of the vine, known by the name of the tears of the vine, which is not always injurious. As it often happens that the root imbibes sap, which the leaves are not yet prepared to throw off, because not yet sufficiently expanded, owing to an inclement season, the

sap which is first carried up, being propelled by that which follows, ultimately forces its way through all obstructions, and exudes from the bud. But this is observed only in cold climates; for in hot climates, where the developement of the leaves is not obstructed by cold, they are ready to elaborate the sap as soon as it reaches them. There is also a spontaneous extravasation of proper juice in some trees, which does not seem in general to be injurious to the individual. Thus the gum which exudes from cherry, plum, peach, and almond trees is seldom detrimental to their health, except when it insinuates itself into the other vessels of the plant and occasions obstructions.

1701. But the exudation of gum is sometimes a disease, and one for which there is seldom any remedy. It is generally the consequence of an unsuitable soil, situation, or climate. Cold raw summers will produce it in the peach, apricot, and more tender sorts of plum and cherry; or grafting these fruits on diseased stocks. Cutting out the part and applying a covering of loam, or tar and charcoal, to exclude the air, are palliatives; but the only effectual method, where it can be practised, is to take up the tree and place it in a suitable soil and situation,

1702. The extravasation and corruption of the ascending or descending juices, have been known to occasion a fissure of the solid parts. Sometimes the fissure is occasioned by means of frost, and forms what is called a double alburnum; that is, first a layer that has been injured by the frost, and then a layer that passes into wood. Sometimes a layer is partially affected, and that is generally owing to a sudden and partial thaw on the south side of the trunk, which may be followed again by a sudden frost. In this case the alburnum is split into clefts or chinks, by means of the expansion of the frozen sap.

1703. Chilblains. But clefts thus occasioned often degenerate into chilblains which discharge a blackish and acrid fluid, to the great detriment of the plant, particularly if the sores are so situated that rain or snow will readily lodge in them and become putrid. The same injury may be occasioned by the bite or puncture of insects while the shoot is yet tender; and as no vegetable ulcer heals up of its own accord, the sooner a cure is attempted the better, as it will, if left to itself, ultimately corrode and destroy the whole plant, bark, wood, and pith. The only palliative is the excision of the part affected, and the application of a coat of grafting wax. (Willdenow, p. 354)

1704. Gangrene. Of this disorder there are two varieties, the dry and the wet. The former is occasioned by means of excessive heat or excessive cold. If by means of cold, it attacks the leaves of young shoots, and causes them to shrink up, converting them from green to black; as also the inner bark, which it blackens in the same manner, so that it is impossible to save the plant except by cutting it to the ground. If by means of heat, the effects are nearly similar, as may oftentimes be seen in gardens, or even in forests, where the foresters are allowed to clear away the moss and withered leaves from the roots. Sometimes the disease is occasioned by the too rapid growth of a particular branch, depriving the one that is next it of its due nourishment, and hence inducing its decay. Sometimes it is occasioned by means of parasitical plants, as in the case of the bulbs of the saffron, which a species of Lycoperdon often attaches itself to and totally corrupts.

1705. Dry gangrene. The harmattan winds of the coast of Africa kill many plants, by means of inducing a sort of gangrene which withers and blackens the leaves, and finally destroys the whole plant. The nopal of Mexico is also subject to a sort of gangrene which begins with a black spot, and extends till the whole leaf or branch rote off, or the plant dies. But plants are sometimes affected with a gangrene by which a part becomes first soft and moist, and then dissolves into foul ichor. This is confined chiefly to the leaves, flowers, and fruit. Sometimes it attacks the roots also, but rarely the stem. It seems to be owing, in many cases, to too wet or too rich a soil; but it may originate in contusion, and may be caught by infection. But the nopal is subject also to a disease called by Thierry la dissolution, considered by Sir J. E. Smith, as distinct from gangrene, and which appears to be Willdenow's dry gangrene. A joint of the nopal, or a whole branch, and sometimes an entire plant, changes in the space of a single hour from a state of apparent health to a state of putrefaction or dissolution. Now its surface is verdant and shining, and in an instant it changes to a yellow, and its brilliancy is gone. If the substance is cut into, the parts are found to have lost all cohesion, and are quite rotten. The attempt at a cure is by speedy amputation below the diseased part. Sometimes the vital principle, collecting and exerting all its energies, makes a stand as it were against the encroaching disease, and throws off the infected part. (Smith's Introduction, p. 276., edit. 6.)`

1706. Etiolation. Plants are sometimes affected by a disease which entirely destroys their verdure, and renders them pale and sickly. This is called etiolation, and may arise merely from want of the agency of light, by which the extrication of oxygen is effected, and the leaf rendered green. Hence it is that plants placed in dark rooms, or between great masses of stone, or in the clefts of rocks, or under the shade of other trees, look always peculiarly pale. But if they are removed from such situations, and exposed to the action of light, they will again recover their green colour. Etiolation may also ensue from the depredations of insects nestling in the radicle, and consuming the food of the plant, thus debilitating the vessels of the leaf so as to render them insusceptible of the action of light. This is said to be often the case with the radicles of Secale cereale; and the same result may also arise from poverty of soil.

1707. Suffocation. Sometimes it happens that the pores of the epidermis are closed up, and transpiration consequently obstructed, by means of some extraneous substance which attaches itself to, and covers, the bark. This obstruction induces disease, and the disease is called suffocation.

1708. Sometimes it is occasioned by the immoderate growth of lichens upon the bark, covering the whole of the plant, as may be often seen in fruit trees, which it is necessary to keep clean by means of scraping off the lichens, at least from the smaller branches. For if the young branches are thus coated, so as that the bark cannot perform its proper functions, the tree will soon begin to languish, and will finally become covered with fungi, inducing or resulting from decay, till it is at last wholly choked up.

1709. But a similar effect is also occasionally produced by insects, in feeding upon the sap or shoot. This may be exemplified in the case of the aphides, which sometimes breed or settle upon the tender shoot in such multitudes as to cover it from the action of the external air altogether. It may be exemplified also in the case of Cóccus hesperidum and A'carus telarius, insects which infest hot-house plants, the latter by spinning a fine and delicate web over the leaf, and thus preventing the access of atmospheric air. Insects are to be removed either by the hand or other mechanical means, or destroyed by excess of some of the elements of their nutrition, as heat, cold, or moisture, where such excess does not prove injurious to the plant; or by a composition, either fluid or otherwise, which shall have the same effects. Prevention is