into Europe. It has been brought, however, even in its milky state, by being confined from the action of the air. If the milky juice is exposed to the air, an elastic pellicle is formed on the surface. If it is confined in a vessel containing oxygen gas, the pellicle is formed sooner. If oxymuriatic acid is poured into the milky juice, the caoutchouc precipitates immediately. This renders it probable that the formation of the caoutchouc is owing to the absorption of oxygen. Caoutchouc, when pure, is of a white colour, with out taste and without smell. The black colour of the caoutchouc of commerce is owing to the method of drying the different layers upon the moulds on which they are spread. They are dried by being exposed to smoke. The black colour of the caoutchouc, therefore, is owing to the smoke or soot alternating with its different layers. It is soft and pliable like leather, and extremely elastic, so that it may be stretched to a very great length, and still recover its former size. Its specific gravity is 09335. Gough, of Manchester, has made some curious and important experiments on the connection between the temperature of caoutchouc and its elasticity, from which it results that ductility as well as fluidity is owing to latent heat Caoutchouc is not altered by exposure to the air. It is perfectly insoluble in water; but if boiled in water for some time its edges become so soft that they will cement, if pressed and kept for a while close together. It is insoluble in alcohol, but soluble in ether. It is soluble also in volatile oils and in alkaBes And from the action operated upon by acids it is thought to be composed of carbon, hydrogen, oxygen, and azote. It seems to exist in a great variety of plants combined with other ingredients. It may be separated from resins by alcohol. It may be separated from the berries of the mistletoe by means of water, and from other vegetable substances by other processes. It is said to be contained both in opium and in mastic; but from these substances it cannot be extracted in sufficient quantities to make it worth the labour. It is applied to a great many useful purposes both in medicine and the arts, to which, from its great pliability and elasticity, it is uncommonly well adapted. In the countries where it is produced the natives make boots and shoes of it, and often use it by way of candle. 1431 Cork. The substance known by the name of cork is the outer and exfoliated bark of the Quércus Saber or cork tree, a species of oak that grows in great abundance in France, Spain, and Italy: but to prevent its natural exfoliation, which is always irregular, and to disengage it in convenient portions, a logitudinal incision is made in the bark from the root to the top of the stem; and a transverse and circular incision at each extremity. The outer layer, which is cork, is then stripped off, and to flatten and reduce it to sheets it is put into water and loaded with weights. The tree continues to thrive, though it is thus stripped of its cork once in two or three years. Cork is a light, soft, and elastic substance, distinguished by the following properties:- Its colour is a sort of light tan. It is very inflammable, and burns with a bright white flame, leaving a black and bulky charcoal behind. When distilled it yields a small quantity of ammonia. Nitric acid corrodes and dissolves it, changing its colour to yellow; and finally docomposes it, converting it partly into an acid, and partly into a soft substance resembling wax or resin. The acid which is thus formed is denominated the suberic acid, and has been proved by the experiments of Lagrange to be an acid of a peculiar nature. It seems probable that cork exists in the bark of some other trees, as well as that of the Quércus Saber. The bark of the Ulmus suberosa assumes something of the external appearance of cork, which it resembles in its thickness, softness, and elasticity, and in its loose and porous texture, as well as also in its chemical properties. Foureroy seems, indeed, to regard the epidermis of all trees whatever to be a sort of cork, but does not say on what grounds his opinion is founded 142 Woody fibre. The principal body of the root, stem, and branches of trees, is designated by the appellation of wood; but the term is too general for the purpose of analytical distinction, as the part designated by it often includes the greater part of the substances that have been already enumerated. It remains, therefore, to be ascertained whether there exists in the plant any individual substance different from those already described, and constituting more immediately the fabric of the wood. If a piece of wood is well dried and digested, first in water and then in alcohol, or such other solvent as shall produce no violent effects upon the insoluble parts; and if the digestion is continued till the liquid is no longer coloured, and dissolves no more of the substance of the plant, there remains behind a sort of vegetable skeleton, which constitutes the basis of the wood, and which has been denominated woody fibre. It is composed of bundles of longitudinal threads, which are divisible into others still smaller. It is somewhat transparent. It is without taste and smell, and is not altered by exposure to the atmosphere. It is insoluble in water and alcohol; but the fixed alkalies decompose it with the assistance of heat. When heated in the open air it blackens without melting or frothing, and exhales a thick smoke and pungent odour, leaving a charcoal that retains the form of the original mass. When distilled in a retort it yields an empyreumatic oil, carburetted hydrogen gas, carbonic acid, and a portion of ammonia, according to Fourcroy, indicating the presence of nitrogen as constituting one of its elementary principles; and yet this ingredient does not appear in the result of the later analysis of Gay Lussac and Thenard, which is, carbon, 52-53; oxygen, 41 78, hydrogen, 569; total 100. 1493. Charcoal. When wood is burnt with a smothered flame, the volatile parts are driven off by the heat, and there remains behind a substance exhibiting the exact form, and even the several layers of the original mass. This process is denominated charring, and the substance obtained charcoal, As it is the woody fibre alone which resists the action of heat, while the other parts of the plants are dissipated, it is plain that charcoal must be the residuum of woody fibre, and that the quantity of the one must depend upon the quantity of the other, if they are not rather to be considered as the same. Charcoal may be ob tained from almost all parts of the plants, whether solid or fluid. It often escapes, however, during combustion, under the form of carbonic acid, of which it constitutes one of the eleinents. From a variety of experiments made on different plants and on their different parts, it appears that the green parts contain a greater proportion of charcoal than the rest; but this proportion is found to diminish in autumn, when the green parts begin to be deprived of their glutinous and extractive juice. The wood contains more charcoal than the alburnum, the bark more than both; but this last result is not constant in all plants; because the bark is not a homogeneous substance, the outer parts being affected by the air and the inner parts not. The wood of the Quercus Robur, separated from the alburnum, yielded from 100 parts of its dred substance 1975 of charcoal; the alburnum, 175; the bark, 26; leaves gathered in May, 80; in September, 26. But the quantity of charcoal differs also in different plants, as well as in different parts of the same. According to the experiments of Mushet, 100 parts of the following trees afforded as follows: 1434. The properties of charcoal are insolubility in water, of which, however, it absorbs a portion when newly made, as also of atmospheric air. It is incapable of putrefaction. It is not altered by the most violent heat that can be applied, if all air and moisture are excluded; but when heated to about 800 it beras in atmospheric air or oxygen gas, and if pure, without leaving any residuum. It is regarded by chemists as being a triple compound, of which the ingredients are carbon, hydrogen, and oxygen. Charcoal is of great utility both to the chemist and artist as a fuel for heating furnaces, as well as for a variety of other purposes. It is an excellent filter for purifying water. It is a very good tooth-powder; and is also an indispensable ingredient in the important manufacture of gunpowder. 1495. The sap. If the branch of a vine is cut asunder early in the spring, before the leaves have begun to expand, a clear and colourless fluid will issue from the wound, which gardeners denominate the tears of the vine. It is merely, however, the ascending sap, and may be procured from almost any other plant by the same or similar means, and at the same season; but particularly from the maple, birch, and walnut tree, by means of boring a hole in the trunk. It issues chiefly from the porous and mixed tubes of the alburnum; though sometimes it does not flow freely till the bore is carried to the centre. A small branch of a vine has been known to yield from twelve to sixteen ounces, in the space of twenty-four hours, A maple tree of moderate size yields about 200 pints in a season, as has been already stated; and a birch tree has been known to yield, in the course of the bleeding season, a quantity equal to its own weight. In the sap of Fagus sylvatica Vauquelin found the following ingredients:- Water, acetate of lime with excess of acid, acetate of potass, gallic acid, tannin, mucous and extractive matter, and acetate of alumina. In 1039 parts of the sap of the Ulmus campestris he found 1027 parts of water and volatile matter, 9-240 of acetate of potass, 1060 of vegetable matter, 0796 of carbonate of lime, besides some slight indications of the presence of sulphuric and muriatic acids; and at a later period of the season he found the vegetable matter increased, and the carbonate of lime and acetate of potass diminished. From the above experiments therefore, as well as from those of other chemists, it is plain that the sap consists of a great variety of ingredients, differing in different species of plants; though there is too little known concerning it to warrant the deduction of any general conclusions, as the number of plants whose sap has been hitherto analysed is but very limited. It is the grand and principal source of vegetable aliinent, and may be regarded as being somewhat analogous to the blood of animals. It is not made use of by man, at least in its natural state: but there are trees, such as the birch, whose sap may be manufactured into a very pleasant wine; and it is well known that the sap of the American maple tree yields a considerable quantity of sugar. 1496. The proper juice. When the sap has received its last degree of elaboration from the different or gans through which it has to pass, it is converted into a peculiar fluid, called the proper juice. This fluid may be distinguished from the sap by means of its colour, which is generally green, as in periwinkle; or red, as in logwood; or white, as in spurge; or yellow, as in celandine; from the last two of which it may readily be obtained by breaking the stem asunder, as it will then exude from the fracture. Its principal seat is in the bark, where it occupies the simple tubes; but sometimes it is situated between the bark and wood, as in the juniper tree; or in the leaf, as in the greater parts of herbs, or it is diffused throughout the whole plant, as in the fir and hemlock; in which case, either the proper juice mixes with the sap, or the vessels containing it have ramifications so fine as to be altogether imperceptible. It is not, however, the same in all plants, nor even in the different parts of the same plant. In the cherry tree it is mucila ginous; in the pine it is resinous; in spurge and celandine it is caustic, though resembling in appearance an emulsion. In many plants the proper juice of the bark is different from that of the flower; and the proper juice of the fruit different from both. Its appearance under the microscope, according to Senebier, is that of an assemblage of small globules connected by small and prism-shaped substances placed between them. If this juice could be obtained in a state of purity, its analysis would throw a considerable degree of light upon the subject of vegetation; but it seems impracticable to extract it without a mixture of sap. Senebier analysed the milky juice of Euphorbia Cyparissias, of which, though its pungency was so great as to occasion an inflammation of the eyes to the person employed to procure it, he had obtained a small quantity considerably pure. It mixed readily with water, to which it communicated its colour. When left exposed to the air, a slight precipitation ensued; and, when allowed to evaporate, a thin and opaque crust remained behind. Alcohol coagulated it into small globules. Ether dissolved it entirely, as did also oil of turpentine. Sulphuric acid changed its colour to black; nitric acid to green. The most accurate experiments on the subject are those of Chaptal. When oxymuriatic acid was poured into the peculiar juice of Euphorbia, a very copious white precipitate fell down, which, when washed and dried, had the appearance of starch, and was not altered by keeping. Alcohol, aided by heat, dissolved two thirds of it, which the addition of water again precipitated. They had all the properties of resin. The remaining third part possessed the properties of woody fibre. The same experiment was tried on the juice of a variety of other plants, and the result uniformly was that oxymuriatic acid precipitated from them woody fibre. In 1497. The virtues of plants have generally been thought to reside in their proper juices, and the opinion seems indeed to be well founded. It is at least proved by experiment in the poppy, spurge, and fig. The juice of the first is narcotic, of the last two corrosive. The diuretic and balsamic virtues of the fir reside in its turpentine, and the purgative property of jalap in its resin. If sugar is obtained from the sap of the sugar-cane and maple, it is only because it has been mixed with a quantity of proper juice. The bark certainly contains it in greatest abundance, as may be exemplified in cinnamon and quinquina. But the peach tree furnishes an exception to this rule: its flowers are purgative, and the whole plant aromatic; but its gum is without any distinguished virtues. Malpighi regarded the proper juice as the principle of nourishment, and compared it to the blood of animals; but this analogy does not hold very closely. The sap is perhaps more analogous to the blood, from which the proper juice is rather a secretion. one respect, however, the analogy holds good, that is, with regard to extravasated blood and peculiar Juices. If the blood escapes from the vessels it forms neither flesh nor bones, but tumours; and if the proper juices escape from the vessels containing them, they form neither wood nor bark, but a lump or deposit of inspissated fluid. To the sap or to the proper juice, or rather to a mixture of both, we must refer such substances as are obtained from plants under the name of expressed juices, because it is evident that they can come from no other source. In this state they are generally obtained in the first instance, whether with a view to their use in medicine or their application to the arts. It is the business of the chemist or artist to separate and purify them afterwards, according to the peculiar object he may happen to have in view, and the use to which he purposes to apply them. They contain, like the sap, acetate of potass or of lime, and assume a deeper shade of colour when exposed to the fire or air. The oxymuriatic acid precipitates from them a coloured and flaky substance as from the sap, and they yield by evaporation a quantity of extract; but they differ from the sap in exhibiting no traces of tannin or gallic acid, and but rarely of the saccharine principle. 1498. Ashes. When vegetables are burnt in the open air the greatest part of their substance is evapo rated during the process of combustion; but ultimately there remains a portion which is altogether incombustible, and incapable of being volatilised by the action of fire. This residuum is known by the name of ashes. Herbaceous plants, after being dried, yield more ashes than woody plants; the leaves more than the branches; and the branches more than the trunk. The alburnum yields also more ashes than the wood; and putrefied vegetables yield more ashes than the same vegetables in a fresh state, if the putrefaction has not taken place in a current of water. The result of Saussure's experiments on 1000 parts of different plants was as follows: 1490. The analysis of the ashes of plants, with a view to the discovery of the ingredients of which they 1500 Alkalies. The alkalies are a peculiar class of substances, distinguished by a caustic taste and 11. The utility of the alkalies, as obtained from vegetables, is of the utmost importance in the arts, 1502. Earths. The only earths which have hitherto been found in plants are the following: lime, silica, magnesia, and alumina. Line is by far the most abundant earth. It is generally combined with a portion of phosphoric, carbonic, or sulphuric acid, forming phosphates, or carbonates, or sulphates of lime. The phosphate of lime is, next to the alkaline salt, the most abundant ingredient in the ashes of green herbaceous plants whose parts are all in a state of vegetation. The leaf of a tree, bursting from the bud, contains in its ashes greater portion of earthy phosphate than at any other period: 100 parts of the ashes of the leaves of the oak, gathered in May, furnished 24 parts of earthy phosphate; in September, only 18-25. In annual plants the proportion of earthy phosphate diminishes from the period of their germination to that of their flowering Plants of the bean, before flowering, gave 14:5 parts of earthy phosphate; in flower, only 135. Carbonate of lime is, next to phosphate of lime, the most abundant of the earthy salts that are found in Vegetables. But if the leaves of plants are washed in water the proportion of carbonate is augmented. This is owing to the subtraction of their alkaline salts and phosphates in a greater proportion than their e. In green herbaceous plants whose parts are in a state of increase, there is but little carbonate of me; but the ashes of the bark of trees contain an enormous quantity of carbonate of lime, and much more than the alburnum, as do also the ashes of the wood. The ashes of most seeds contain no carbonate of time; but they abound in phosphate of potass. Hence the ashes of plants, at the period of the maturity of the fruit, yields less carbonate of lime than at any previous period. 1504 Silica is not found to exist in a great proportion in the ashes of vegetables, unless they have been previously deprived of their salts and phosphates by washing; but, when the plants are washed in water, the proportion of their silica augments. The ashes of the leaves of the hazel, gathered in May, yielded 25 parts of silica in 100. The same leaves, washed, yielded four parts in 100. Young plants, and leaves barting from the bud, contain but little of silica in their ashes; but the proportion of silica augments as the parts are developed. Perhaps this is owing to the diminution of the alkaline salts. The ashes of Some stalks of wheat gathered a month before the time of flowering, and having some of the radical leaves withered, contained 12 parts of silica and 65 of alkaline salts in 100. when more of their leaves were withered, the ashes contained 32 parts of silica and 54 of alkaline salts. At the period of their flowering, and Seeds divested of their external covering, contain less silica than the stem furnished with its leaves; and it is somewhat remarkable that there are trees of which the bark, alburnum, and wood contain scarcely ays, and the leaves a great deal, particularly in autumn. This is a phenomenon that seems inexpli cable. The greater part of the grasses contain a very considerable proportion of silica, as do also the plants of the genus Equisetum. Sir H. Davy has discovered that it forms a part of the epidermis of these plants, and in some of them the principal part. From 100 parts of the epidermis of the following plants the proportions of silica were, in bonnet cane, 90; bamboo, 714; common reed, 481; stalks of corn, 665. Owing to the silica contained in the epidermis, the plants in which it is found are sometimes used to give a polish to the surface of subtances where smoothness is required. The Dutch rush (Equisetum hyemale), a plant of this kind, is used to polish even brass. 136 Magacata does not exist so abundantly in the vegetable kingdom, as the two preceding earths. It has been found, however, in several of the marine plants, particularly the Fuci; but Salsola Soda contains more of magnesia than any other plant yet examined. According to Vauquelin, 100 parts of it contain 17 929 of Magnesia. 1506. Alumina has been detected in several plants, but never except in very small quantities. 1507. Metallic oxides. Among the substances found in the ashes of vegetables, we must class also metals. They occur, however, only in small quantities, and are not to be detected except by the most delicate experiments. The metals hitherto discovered in plants are iron, manganese, and perhaps gold. Of these iron is by far the most common. It occurs in the state of an oxide; and the ashes of hard and woody plants, such as the oak, are said to contain nearly one twelfth of their own weight of this oxide. The ashes of Salsola contain also a considerable quantity. The oxide of manganese was first detected in the ashes of vegetables by Scheele, and afterwards found by Proust in the ashes of the pine, calendula, vine, green oak, and fig tree. Beccher, Kunckel, and Sage, together with some other chemists, contend also for the existence of gold in the ashes of certain plants; but the very minute portion which they found, seems more likely to have proceeded from the lead employed in the process, than from the ashes of the plant. It has been observed by Saussure, that the proportion of the oxides of iron and of manganese augments in the ashes of plants as their vegetation advances. The leaves of trees furnish more of these principles in autumn than in spring, as do those of annual plants. Seeds contain metals in less abundance than the stem; and if plants are washed in water, the proportions of their metallic oxides are augmented. 1508. Such are the principal ingredients that enter into the vegetable composition. They are indeed numerous, though some of them, such as the metallic oxides, occur in such small proportions as to render it doubtful whether they are in reality vegetable productions or not. The same thing may be said of some of the other ingredients that have been found in the ashes of plants, which it is probable have been absorbed ready formed by the root, and deposited unaltered, so that they can scarcely be at all regarded as being the genuine products of vegetation. 1509. Other substances. Besides the substances above enumerated, there are also several others which have been supposed to constitute distinct and peculiar genera of vegetable productions, and which might have been introduced under such a character; such as the mucus, jelly, sarcocol, asparagin, inulin, and ulmin, of Dr. Thomson, as described in his well known System of Chemistry; but as there seems to be some difference of opinion among chemists with regard to them, and a belief entertained that they are but varieties of one or other of the foregoing ingredients, it is sufficient for the purposes of this work to have merely mentioned their names. Several other substances, of a distinct and peculiar character, have been suspected to exist in vegetable productions: such as the febrifuge principle of Seguin, as discovering itself in Peruvian bark; the principle of causticity or acridity of Senebier, as discovering itself in the roots of Ranunculus bulbosus, Scilla marítima, Bryonia álba, and Arum maculatum, in the leaves of Digitális pur. pàrea, in the bark of Daphne Mezèreon, and in the juice of the spurges: to which may be added the fluid exuded from the sting of the common nettle, the poisons inherent in some plants, and the medical virtues inherent in others; together with such peculiar principles as may be presumed to exist in such regions of the vegetable kingdom as remain yet unexplored. The important discoveries which have already resulted from the chemical analysis of vegetable substances encourage the hope that further discoveries will be the result of further experiment; and, from the zeal and ability of such chemists as are now directing their attention to the subject, every thing is to be expected. 1510. A very few constituent and uncompounded elements include all the compound ingredients of vegetables. The most essential of such compounds consist of carbon, oxygen, and hydrogen; a small proportion of nitrogen is said to be found only in cruciform plants. The remaining elementary principles which plants have been found to contain, although they may be necessary in the vegetable economy, yet they are by no means principles of the first importance, as occurring only in small proportions, and being dependent in a great measure on soil and situation; whereas the elements of carbon, oxygen, and hydrogen form as it were the very essence of the vegetable subject, and constitute by their modifications the peculiar character of the properties of the plant. This is conspicuously exemplified in the result of the investigations of Gay Lussac, and Thenard, who have deduced from a series of the most minute and delicate experiments the three following propositions, which they have dignified by the name of Laws of Vegetable Nature (Traité de Chem. Element., tom. iii. chap. iii.): — 1st, Vegetable substances are always acid, when the oxygen they contain is to the hydrogen in a greater proportion than in water; 2dly, Vegetable substances are always resinous, or oily, or spirituous, when the oxygen they contain is to the hydrogen in a smaller proportion than in water; 3dly, Vegetable substances are neither acid nor resinous, but saccharine, or mucilaginous, or analogous to woody fibre or starch, when the oxygen and hydrogen they contain are in the same proportion as in water. (See Dr. Thomson's System of Chemistry.) CHAP. IV. Functions of Vegetables. 1511. The life, growth, and propagation of plants necessarily involve the several following topics: germination, nutriment, digestion, growth and developement of parts, anomalies of vegetable developement, sexuality of vegetables, impregnation of the vegetable germen, changes consequent upon impregnation, propagation and dispersion of the species, causes limiting the dispersion of the species, evidence and character of vegetable vitality. SECT. 1. Germination of the Seed. 1512. Germination is that act or operation of the vegetative principle, by which the embryo is extricated from its envelopes, and converted into a plant. This is universally the first part of the process of vegetation; for it may be regarded as an indubitable fact, that all plants spring originally from seed. The conditions necessary to germination relate either to the internal state of the seed itself, or to the circumstances in which it is placed with regard to surrounding substances. 1513 The first condition necessary to germination is, that the seed must have reached maturity. Unripe seeds seldom germinate, because their parts are not yet prepared to form the chemical combinations on which germination depends. There are some seeds, however, whose germination is said to commence in the very seed-vessel, even before the fruit is ripe, and while it is yet attached to the parent plant. Such are those of the Tangekolli of Adanson, and Agave vivípara of East Florida, as well as those of the Camus Nelumbo of Sir J. E. Smith, or sacred bean of India; to which may be added the seeds of the com tuta garden radish, pea, lemon, &c. But these are examples of rare occurrence; though it is sometimes Decessary to sow or plant the seed almost as soon as it is fully ripe, as in the case of the coffee-bean; which will not germinate unless it is sown within five or six weeks after it has been gathered. Most seeds, however, if guarded from external injury, will retain their germinating faculty for a period of many years. This has been proved by the experiment of sowing seeds which have been long so kept; as well as by the deep ploughing up of fields which have been long left without cultivation. A field which was thus ploughed up, near Dunkeld, in Scotland, after a period of forty years' rest, yielded a considerable blade of black oats without sowing. This could have only been by the plough's bringing up to the surface seeds which had been formerly too deeply lodged for germination. 1514 The second condition is, that the seeds sown must be defended from the action of the rays of light. This has no doubt been long known to be a necessary condition of germination, if we regard the practice of the harrowing or raking in of the grains or seeds sown by the farmer or gardener as being founded upon it. 1515 A third condition necessary to germination is the access of heat. No seed has ever been known to germinate at or below the freezing point. Hence seeds do not germinate in winter, even though lodged in their proper soil: but the vital principle is not necessarily destroyed in consequence of this exposure; for the seed will germinate still, on the return of spring, when the ground has been again thawed, and the temperature raised to the proper degree. This degree varies considerably in dif ferent species of seeds, as is obvious from observing the times of their germination, whether in the same climate or in different ones: for if seeds, which naturally sow themselves, germinate in different climates at the same period, or in the same climate at different periods, the temperature necessary to their germi nation must of consequence be different. Now these cases are constantly occurring and presenting them. selves to our notice; and have also been made the subject of particular observation. Adanson found that seeds which will germinate in the space of twelve hours in an ordinary degree of heat, may be made to germinate in the space of three hours by exposing them to a greater degree of heat; and that seeds transported from the climate of Paris to that of Senegal, have their periods of germination accelerated from one to three days. (Familles des Plantes, vol. i. p. 84.) Upon the same principle, secds transported from a warmer to a colder climate, have their periods of germination protracted till the temperature of the latter is raised to that of the former. This is well exemplified in the case of green-house and hot-house plants, from which it is also obvious that the temperature must not be raised beyond a certain degree, otherwise the vital principle is totally destroyed. 1516 A fourth condition necessary to germination is the access of moisture. Seeds will not germinate if they are kept perfectly dry. Water, therefore, or some liquid equivalent to it, is essential to germination. Hence rain is always acceptable to the farmer or gardener, immediately after he has sown his seeds; and, if no rain falls, recourse must be had, if possible, to artificial watering. But the quantity of water applied is not a matter of indifference. There may be too little or there may be too much. If there be too little, the seed dies for want of moisture; if there be too much, it then rots. The case is not the samne, however, with all seeds. Some can bear but little moisture, though others will germinate even when partially immersed; as was proved by an experiment of Du Hamel's, at least in the case of peas, which he placed metely upon a piece of wet sponge, so as to immerse them by nearly the one half, and which germinated as if placed in the soil. But this was found to be the most they could bear; for when totally immersed in the water they rotted. There are some seeds, however, which will germinate even when wholly submersed. The seeds of aquatics must of necessity germinate under water; and peas have been known to do so under certain conditions. 1517. A fifth condition necessary to germination is the access of atmospheric air. Seeds will not germi nate if placed in a vacuum. Ray introduced some grains of lettuce-seed into the receiver of an air-pump, which he then exhausted. The seeds did not germinate. But they germinated upon the readmission of the air, which is thus proved by consequence to be necessary to their germination. Achard proved that no sred will germinate in nitrogen gas, or carbonic acid gas, or hydrogen gas, except when mixed with a certain proportion of oxygen gas; and hence concluded that oxygen gas is necessary to the germination of all seeds, and the only constituent part of the atmospheric air which is absolutely necessary. Hum. beldt sound that the process of germination is accelerated by means of previously steeping the seed in water impregnated with oxymuriatic acid. Cress-seed treated in this manner germinated in the space of three hours, though its ordinary period of germination is not less than thirty-two hours. 1518. The period necessary to complete the process of germination is not the same in all seeds, even when all the necessary conditions have been furnished. Some species require a shorter, and others a longer period. The grasses are among the number of those plants whose seeds are of the most rapid germination; then perhaps cruciform plants; then leguminous plants; then labiate plants; then umbelliferous plants; and in the last order rosaceous plants, whose seeds germinate the slowest. The following table indicates the periods of the germination of a considerable variety of seeds, as observed by Adanson: 1519. Physical phenomena. When a seed is committed to the soil under the conditions which have been just specified, the first infallible symptom of germination is to be deduced |