Plants take their food directly from the soil by myriads of mouths situated in their roots. It is in a state of the finest possible division and solution, and consequently needs no tedious mastication and digestion. It goes by one set of vessels through the branches to the leaves, which constitute their lungs. Here it is exposed to the action of the air, its color is changed, oxygen and water are emitted by day, and a small quantity of carbonic acid gas by night. The purified sap or lymph returns by another set of vessels situated along the bark, ministering to the support and growth of the whole plant, each leaf, flower, fruit, seed and structure, selecting with unerring instinct, what is adapted to its peculiar organization. Thus the two sustain a mutual relation to each other. Plants emit oxygen which is essential to the existence of animals, while carbonic acid gas is nearly as important to healthy vegetation. Vegetables have the principle of vitality which works in a mysterious manner peculiar to itself. Alone it eludes our knowledge, but when acting on matter and blended with it, we see its wonderful influence and power. From the simple acorn or seed it builds up the sturdy oak and graceful elm. It holds in abeyance all the laws of chemical decomposition. It may long delay the onward march of death, but when it ceases to act, all organic matter is resolved into its constituent parts. Then the very air and heat, that once were the sources of vigor and growth of plants, become the swift agents of their destruction. They return to dust from which they originated.

It should then be our endeavor to ascertain the conditions on which vitality depends and the various causes which impair it, that we may promote the one, and remove the other, and thus we may both prevent and cure the diseases of vegetation.

A correct knowledge of these conditions forms the basis of all systematic vegetable pathology. We proceed to enumerate some of the most important. All vegetables with scarcely an exception, contain a considerable quantity of mineral ingredients which, dissolved by water, they absorb by their roots.

By interrogating analytical chemistry, we learn the quantity and kinds of mineral matter the different plants require, and also what missing or insuffi cient ingredients to supply to the soil to enable us to raise any desired crop. Thus an average crop of wheat grown on an acre of ground needs only 30 pounds of potash, while turnips on the same surface must be furnished with 230 pounds, and potatoes still more. This is but one instance of a single element required. If then, any particular plant has not supplied to its roots enough of the mineral substances it needs, or if water enough is not furnished to dissolve its solid food and to form a part of its own composition, it sickens

and dies.

Besides, plants require carbon and other inorganic clements which once constituted living vegetable structure that has decayed and been converted into mould or loam, to yield suitable food for another generation. They also need nitrogen, which as it respects our cultivated crops must have formed part of an animal organization. They must receive it in the form of ammonia. If an insufficient quantity of these elements is not supplied, disease and decay result.

All the different elements that compose the food must gradually be added to the sip that is constantly contributing to the structure of the plant which has the ability to manufacture, from the circulating fluid, those albuminous, starchy and oily compounds adapted to its wants. If this power of assimilation, or changing them to its own substance by digestion is impaired, disease and death are induced.

The sap must flow in constant round from roots to leaves, there absorb carbonic acid gas, evolve oxygen, and then proceed onward to nourish all parts of the plant. If these functions of circulation and respiration be much impaired, fatal consequences must result.

Heat is another condition of the vitality of plants, varying in amount in different kinds. Wheat cannot mature at a lower temperature than 45 deg., potatoes at not less than 52 deg., barley at not less than 59 deg., and Indian

corn is not certain to ripen in the northern part of this State. If the temperature around a plant becomes much less than its constitution requires, it eventually perishes.

Under the most favorable circumstances, vitality in plants is limited in duration. In annuals it continues about six months; in biennials it lingers in the root through one winter, but cannot survive a second, while in the oak it endures for centuries. Wheat in Scotland requires six months to mature its grain, but in Venezuela only three. Vital action in deciduous trees and shrubs is nearly dormant every winter as it is in some animals.

A disease in a plant may be defined an alteration of one or more of its functions, which tends to interfere with the due play and performance of some of these conditions of vitality.

The causes of disease may be grouped in two classes, predisposing and exciting. Of the individual plants exposed to the ravages of any particular disease, some will be attacked, while others may escape. In the one there must be a tendency to be affected which does not exist in the other. This tendency is termed a predisposition, while the agents which may call it into action are denominated exciting causes.

Among predisposing causes may be mentioned hereditary tendency. As certain families among men transmit certain peculiarities of form and feature from generation to generation, and are liable to certain constitutional diseases, so many kinds of plants inherit all the peculiarities of structure and constitution of their parents with a predisposition to the same maladies. It is owing to this law of nature that the different varieties of cultivated crops continue unchanged. It is not the discase but the hereditary proneness to it that is transmitted. This may long remain latent, but favoring circumstances will develop it. This predisposition is manifested in such affections as the potato disease, and disorders of assimilation, particularly in cereal crops; hence the importance of using only good seed of healthy origin.

A plant that has once had a disease is more liable to it again. An excessive amount of sap and nutriment, and much exposure to heat is another predisposing cause. On the other hand, a deficiency and deterioration of the required food from want of soluble salts of the necessary mineral substances combined, with too little heat and light during a cloudy wet season, occasion a state of debility, and powerfully predispose to mildew, ergot, smut, and various other chronic affections.

Miasm or malaria, which produces such deleterious effects upon mankind, is believed to contribute to the nourishment of plants. It certainly has no bad influence upon them, for malaria causes essentially diseases of the nervous system which vegetables do not possess, though some have a remarkable sensibility. Dr. Cartwright states that an aquatic plant (Jussieua grandiflora,) which grows abundantly in the stagnant waters in the southern part of Louisiana, completely prevents the miasmatic diseases peculiar to that region. It derives its sustenance wholly from water, making the foulest sweet and pure by consuming the products of vegetable decomposition as fast as they are formed.

Contagion and epidemic influences of the air are efficient exciting causes. A disease is said to be contagious which is capable of producing by contact, by inoculation or through the medium of the atmosphere the same malady in other plants, propagating itself from its source in rapid succession to the surrounding plants, and gradually extending from one field to another; while a disease excited by some peculiar condition of the air which suddenly prevails simultaneously throughout a more or less extensive range of country, and differs in character, progress and disappearance from the ordinary complaints of the region, is called an epidemic. It may also be contagious, of which we have instances in the Asiatic cholera and potato disease.

In respect to the action and influence of insects and parasitical fungi as exciting causes, there is a difference of opinion among scientific men. Some

maintain that they are prolific sources of disease, others that they may excite it in a few instances, and others doubt whether they produce any at all, but the weight of evidence preponderates in favor of the conclusion that insects produce a few diseases, aggravate others and convey morbid contagious matter; that fungi or parasitical plants, although they principally affect decaying vegetation, sometimes prey upon debilitated living tissues, which they destroy by their attacks; hence, practically, we may consider them in many cases as exciting, or at least secondary causes, and may more rationally take measures to prevent or destroy them.

The ravages of army worms, grasshoppers, and all the larger insects which eat the growing crops, thereby preventing their growth, are not diseases, but mere accidents, like the depredations of cattle.

The disease of the potato, which, even during the past season has destroyed more than half the crop in Ireland, deserves particular consideration, on account of its wide prevalence, and the injury it has caused to an article of food of very general consumption. It may be defined an epidemic and contagious disease, in which black spots appear on the stems and gradually extend to the tubers, many of which are affected by putrid degeneration. It first appeared in the Islands of the Hebrides, but did not widely spread before 1844. In that, and the two following years, it extended over Ireland and other parts of Great Britain, and prevailed also in the United States, every where making great havoc in a crop which before had been healthy and abundant. It rarely commences before the end of July, and sometimes not until October. In some instances it has not been discovered until the potatoes have been dug and stored; but generally it attacks the plants towards the end of August, often blackening and killing the leaves and stems of the most luxurious vegetation in a few hours. It may affect the foliage and tubers simultaneously, or the tubers may be diseased, while the foliage has a healthy appearance; but usually, the "potato tops" first look faded and sallow, then the dark spots are seen on the leaves and stems, and after an interval of some days, spots of a brownish color begin on the skin of the tuber, and gradually affect its substance, resulting in ulceration, with an offensive watery discharge, and terminating at length in gangrene. As potatoes were mainly propagated from tubers, and the same variety was continued for a long series of years, the idea at first was, that it had grown old, and the disease was the effect of advanced age, and that new varieties just originated from seed might escape; but it was soon ascertained that the new varieties equally fell victims. It was also attributed to changes of temperature, but it was found to prevail in all vicissitudes, in heat and cold, in rain and drouth; only long continued wet weather seen ed to promote its progress and dissemination. Certain kinds of soil are peculiarly liable, such as damp clay, wet land, gardens pampered with guano and animal excrement, and cultivated fields that have long been robbed of valuable ingredients by annual crops of grain and vegetables.

When decay or putrefaction has once been induced in the plant, various parasitical growths fasten upon it. At least six fungi have been noticed, of which particularly the kind called botrytis infestans penetrates the foliage and tubers, contributing to their rapid destruction. They are, however, mere attendants, not primary causes of the disease.

It is well known, when the Asiatic cholera swept so rapidly over the world, that it generally selected for its victims those whose constitutions had been impaired by previous disease, intemperance or debilitating causes; so the potato epidemic has always been most destructive to those plants which have degenerated and acquired a predisposition to it by long cultivation and improper treatment. It has been ascertained by careful investigation, that many soils have been exhausted of those elements essentially necessary to the healthy growth and robust vigor of the potato. Among the mineral substances, it especially needs large supplies of potash, of which ordinary soils possess much less than one part in the hundred; and yet a farmer, who cultivates five acres

of potatoes, each acre yielding about 300 bushels, or eight tons, must be able to extract 1,265 pounds of potash, if he would produce a sound, rightly constituted crop. Of this he may return in the tops 250 pounds, having removed 1,015; while the waste has not been repaired by using guano and sea-weeds which contain none, and barn-yard manure, which possesses but a small proportion. It is easy then to understand, if this scarce commodity is not annually supplied in some manner, an element will soon be withdrawn that is essential to the vitality of this vegetable. As the malady has generally prevailed after the middle of July, it may be avoided by planting early varieties that may mature, be harvested and consumed before it makes its appearance. It has been found by observation that potatoes planted in peaty, mossy or virgin soils, or in recently cleared burnt lands, or in the valleys of granite countries which had not been much subject to culture, were comparatively exempt and furnished much healthy seed to the agriculturist, for all these abound in potash. It is also a remarkable coincidence that Asiatic cholera rarely visited regions in which granite predominated.

If, then, potato blight is mainly occasioned by a deficiency of this alkali, we may hope to diminish or avert it by planting the potato on suitable lands manured with ashes, muck, well composted vegetable substances which contain potash. Even barn yard manure may be used with advantage, if it has undergone the process of nitrification. This occurs whenever it is collected in a heap, mixed with earth rich in vegetable material, and allowed to ferment and decompose during summer, where the air can have free access to it. The nitrogen of the atmosphere enters into new combinations with the fermenting organic matter, nitric acid is produced, and then nitrate of potassa is formed. In a similar way the nitre of commerce, so valuable for manufacturing important compounds, crystalizes rapidly and abundantly in beds on the surface of the ground in hot climates. One great advantage of drainage is, that it renders the soil more permeable to air, and facilitates the production of these valuable nitrates which readily dissolve and are immediately appropriated by the growing crops.

Another disease which formerly prevailed extensively, particularly in Eu̟rope, is called the potato curl. It is propagated by contagion, in consequence of which the plants become rapidly and successively curled and the tubers shrivel. It is attributed to a preternatural thickened condition of the juices in the tuber planted. The sap is in such a state that it cannot circulate freely in the growing foliage. Since greater care has been used in procuring uninfected potatoes for seed, and preserving them unchanged during the winter, it has almost disappeared.

One more epidemic, which has devastated the vineyards of Europe, and may extend to this country, deserves a brief notice. It is the vine disease in which brown spots first appear on the leaves and permeate quite through their tissue, then small white patches of a soft and delicate fungus seize the under side of the leaves, causing rapid defoliation, extend to the fruit stalks, attack the berries, choke up their pores, mechanically confine them with their dense felty substance, arrest their growth, and severely injure the whole vine. These white patches are produced by a species of parasite called Oidium. They are the growth of fungoid vegetation, the plants of which analagous to seeds are so extremely small that they are invisible to the naked eye, and can float in the air scarcely influenced by the force of gravitation, each one capable of producing millions of its kind. When viewed under the microscope, they appear like a vast congregation of little mushroons. This kind of mildew seldom does much mischief when it commences after the fruit begins to ripen. Nor is it apt to attack perfectly healthy vines, but chiefly those whose vital energy has been long impaired by various debilitating causes. Excessive changes of temperature, damp and close weather favor its development. The grape vine is a gross feeder. It requires, with other nutriment, potash in various combinations, as silicate, nitrate, tartrate, soap suds, &c.; ammonia, or nitro

genous matter in the form of manure composted with sods and muck applied in the autumn, or solutions of guano during the period of growth; and lime with its compounds, sulphate and superphosphate. In old vineyards, some of these indispensable constituents have doubtless been taken from the soil, and no equivalent has been returned, and thus the vines were affected with chronic debility, predisposing them to disease and the ravages of parasites.

If these premises are correct, the appropriate treatment would be to furnish the needed fertilizing ingredients, or to plant the vines in new soils. When the disease is appreliended, or appears in graperies, it may be prevented or arrested by sprinkling the inside border with one pound of sulphur to each fifteen feet square, and keeping the house closed until the fumes of sulphur pervade the whole interior."

The kind of mildew which has attacked our grape-vines, appears in the form of a minute white mould, (botrytis,) which affects the upper surface of the leaves, the young and tender branches, and the skins of the berries, arresting their farther growth. This species of fungus is similar to that which affects peas and gooseberries. The best preventive is to maintain a vigorous and healthy action by the proper food, due amount of heat and moisture, and a uniform temperature by well regulated ventilation at the top of the house, excluding bottom air until the grapes begin to color, and to scatter sulphur over the floor every year, about the twentieth of June, and first of August.

The inflammation of plants is a healthy process within moderate limits, and is, in fact, the means of repairing injuries. When a branch is cut from a tree, the sap exudes, its albumen is exposed to the air, soon hardens, and the wound heals; or if another branch is carefully fitted to it, it becomes the medium by which a permanent junction is affected. The cultivator of fruits takes advantage of this in grafting, to make the scion unite with the stock. But sometimes, when a branch is severed in the spring, the bleeding is so profuse that the sap cannot thus coagulate so as to impede the farther exit of fluid. This is illustrated in the grape-vine, but this hemorrhage can generally be arrested by binding over the wound a dry powder composed of three parts of oyster shell lime, and one part of cheese parings. It is only when the inflammation becomes excessive, that it results in disease and mortification. This is the more apt to occur when the plant is in a bad state of health, from deficient food and circulation. A sudden reduction of temperature or alternation of cold nights and hot days, causes an inflammation of the more delicate varieties of fruits which rapidly end in putrefaction. Good culture, drainage and shelter may prevent or mitigate the disease.

An abundant exudation of gum often flows from stone fruit trees, particularly from the trunk and larger branches of the cherry. It is caused by wounds, too rich soil, or difference between grafted tree and stock. When the bark has been injured in spring before the foliage appears, the sap in descending, oozes out of the wound and condenses into gum, or when the soil has been too highly manured, sap is supplied in excess more than the tree can appropriate, and it fills the circulating vessels in the bark to repletion, so that its texture bursts open and gum cxudes freely or when the stock is more vigorous than the scion, it furnishes more circulating fluid to the tree than the returning sap vessels can hold, and the same effect follows. As the cherry tree dislikes a rich soil, root pruning or replanting in poorer land, or care in selecting scions of equal luxuriance with the tree into which they are to be inserted, are the remedies.

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Besides the diseases already enumerated, there are various chronic morbid affections, which occur in the organs of respiration, circulation and assimilalation of which examples of the principal are now to be considered. These occasion more mortality than all others.

Among all the diseases of cultivated plants, mildew from a remote antiquity has been the greatest scourge to the farmer. Scarcely a year has passed when it has not more or less infested all the wheat fields, yet it is only when the