to be attempted by general culture, and particular attention to hinder the propagation of the insects or vermin, whether oviparous or otherwise, by destroying their embryo progeny.

1710. Sometimes the disease is occasioned by an extravasation of juices which coagulate on the surface of the stalk, so as to form a sort of crust, investing it as a sheath, and preventing its farther expansion. 1711. Sometimes the disease arises from want of an adequate supply of nourishment as derived from the soil, in which case the lower part of the plant is the best supplied, while the upper part of it is starved. Hence the top shoots decrease in size every succeeding year, because a sufficient supply of sap cannot be obtained to give them their proper developement. This is analogous to the phenomena of animal life, when the action of the heart is too feeble to propel the blood through the whole of the system: for then the extremities are always the first to suffer. And perhaps it may account also for the fact, that in bad soils, and unfavourable seasons, when the ear of barley is not wholly perfected, yet a few of the lower grains are always completely developed. (Smith's Introduction, p. 279.)

1712. Contortion. The leaves of plants are often injured by means of the puncture of insects, so as to induce a sort of disease which discovers itself in the contortion or convolution of the margin, or wrinkled appearance of the surface. The leaves of the apricot, peach, and nectarine, are extremely liable to be thus affected in the months of June and July. The leaves of the apple are affected by the A`phis lanígera; those of the larch by another woolly aphis (A. larício); those of the hawthorn by a species of Tenthrèdo, &c. (See Major's Treatise on the Insects prevalent in Fruit Trees and Garden Produce.)

1713. The leaf which has been punctured soon begins to assume a rough and wrinkled figure, and a reddish and scrofulous appearance, particularly on the upper surface. The margins roll inwards on the under side, and enclose the eggs which are scattered irregularly on the surface, giving it a blackish and granular appearance, but without materially injuring its health. In the vine, the substance deposited on the leaf is whitish, giving the under surface a sort of a frosted appearance, but not occasioning the red and scrofulous aspect of the upper surface of the leaf of the nectarine. In the poplar, the eggs when first deposited resemble a number of small and hoary vesicles containing a sort of clear and colourless fluid. The leaf then becomes reflected and conduplicated, enclosing the eggs, and exhibiting a few reddish protuberances on the upper surface. The embryo is nourished by this fluid; and the hoariness is converted into a fine cottony down, which for some time envelopes the young fly. The leaf of the lime tree in particular, when fully expanded, is liable to attacks from insects; and hence the gnawed appearance it so often displays, The injury seems to be occasioned by some species of puceron depositing its eggs in the parenchyma, generally about the angles that branch off from the midrib. A sort of down is produced, at first green, and afterwards hoary; sometimes in patches, and sometimes pervading the whole leaf; as in the case of the vine. Under this covering the egg is hatched; and then the young insect gnaws and injures the leaf, leaving a hole or scar of a burnt or singed appearance. Sometimes the upper surface of the leaf is covered with clusters of wart-like substances somewhat subulate and acute. They seem to be occasioned by means of punctures made in the under surface, on which a number of openings are discoverable, penetrating into the warts, which are hollow and villous within. The disease admits of palliation by watering frequently over the leaves; and by removing such as are the most contorted and covered by larvæ.

1714. Consumption. From barren or improper soil, unfavourable climes, careless planting, or exhaustion from too frequent flowering, it often happens that disease is induced which terminates in a gradual decline and wasting away of the plant, till at length it is wholly dried up. Sometimes it is also occasioned by excessive drought, or by dust lodging on the leaves, or by fumes issuing from neighbouring manufactories, or by the attacks of insects.

1715. There is a consumptive affection frequently attacking the pine tree (Willdenow, Princ. Bot. p. 351.), which affects the alburnum and inner bark chiefly, and seems to proceed from long-continued drought, or from frost suddenly succeeding mild or warm weather, or from heavy winds. The leaves assume a tinge of yellow, bordering upon red. A great number of small drops of resin, of a putrid odour, exude from the middle of the boughs. The bark exfoliates, and the alburnum presents a livid appearance: the tree swarms with insects Dypterygia pinástri Steph.), and the disease is incurable, inducing inevitably the total decay and death of the individual. The preventive is obviously good culture, so as to maintain vigorous health: palliatives may be employed, according to the apparent cause of the disease.

SECT. III. Natural Decay.

1716. Although a plant should not suffer from the influence of accidental injury, or from disease, still there will come a time when its several organs will begin to experience the approaches of a natural decline insensibly stealing upon it, end at last inducing death. The duration of vegetable existence is very different in different species. Yet in the vegetable, as well as in the animal kingdom, there is a term or limit set, beyond which the individual cannot pass. Some plants are annuals, and last for one season only, springing up suddenly from seed, attaining rapidly to maturity, producing and sowing their seeds, and afterwards immediately perishing. Such is the character of the various species of corn, as exemplified in oats, wheat, and barley. Some plants continue to live for a period of two years, and are therefore called biennials, springing up the first year from seed, and producing roots and leaves, but no fruit; and in the second year producing both flower and fruit, as exemplified in the carrot, parsnep, and caraway. Other plants are perennials, that is, lasting for many years; of which some are called undershrubs, and die down to the root every year; others are called shrubs, and are permanent both by the root and stem, but do not attain to a great height or great age; others are called trees, and are not only permanent by both root and stem, but attain to a great size, and live to a great age. But even of plants that are woody and perennial, there are parts which perish annually, or which are at least annually separated from the individual; namely, the leaves, flowers, and fruit, leaving nothing behind but the bare caudex, which submits in its turn to the ravages of time, and ultimately to death.

1717. The decay of the temporary organs, which takes place annually, is a phenomenon

familiar to every body, and comprehends the fall of the leaf, the fall of the flower, and the fall of the fruit.

1718. The fall of the leaf, or annual defoliation of the plant, commences for the most part with the colds of autumn, and is accelerated by the frosts of winter, which strip the forest of its foliage, and the landscape of its verdure. But there are some trees which retain their leaves throughout the whole of the winter, though changed to a dull and dusky brown, and may be called ever-clothed trees, as the beech: and there are others which retain their verdure throughout the year, and are denominated evergreens, as the holly. The leaves of both sorts ultimately fall in the spring. Sir J. E. Smith considers that leaves are thrown off by a process similar to that of the sloughing of diseased parts in the animal economy; and Keith observes, that if it is necessary to illustrate the fall of the leaf by any analogous process in the animal economy, it may be compared to the shedding of the antlers of the stag, or of the hair of beasts or feathers of birds, which being, like the leaves of plants, distinct and peculiar organs, fall off, and are regenerated annually, but do not slough. According to Professor Vaucher every leaf consists of a distinct system of fibres, having only a temporary continuity with the shoot, kept up by an adhesive substance, probably formed by a portion of the parenchyma interposed between the two systems of fibres. While this parenchyma is under the influence of vegetable action the adhesion is maintained; when this action ceases the union is dissolved and the leaf falls.

1719. The flowers, which, like the leaves, are only temporary organs, are for the most part very shortlived; for as the object of their production is merely to effect the impregnation of the germs, that object is no sooner attained than they begin to give indications of decay, and speedily fall from the plant; so that the most beautiful part of the vegetable is also the most transient.

1720. The fruit, which begins to appear conspicuous when the flower falls, expands and increases in volume, and, assuming a peculiar hue as it ripens, ultimately detaches itself from the parent plant, and drops into the soil. But it does not in all cases détach itself in the same manner: thus, in the bean and pea the seed-vessel opens and lets the seeds fall out, while in the apple, pear, and cherry, the fruit falls entire, enclosing the seed, which escapes when the pericarp decays. Most fruits fall soon after ripening, as the cherry and apricot; but some remain long attached to the parent plant after being fully ripe, as in the case of the fruit of Euonymus and Méspilus. But these, as well as all others, though tenacious of their hold, detach themselves at last, and bury themselves in the soil, to give birth to a new individual in the germination of the seed. The fall of the flower and fruit is accounted for in the same manner as that of the leaf.

1721. Decay of the permanent organs. Such, then, is the process and presumptive rationale of the decay and detachment of the temporary organs of the plant. But there is also a period beyond which even the permanent organs themselves can no longer carry on the process of vegetation. Plants are affected by the infirmities of old age as well as animals, and are found to exhibit also similar symptoms of approaching dissolution. The root refuses to imbibe the nourishment afforded by the soil, or if it does imbibe a portion, it is but feebly propelled, and partially distributed, through the tubes of the alburnum ; the elaboration of the sap is now effected with difficulty as well as the assimilation of the proper juice, the descent of which is almost totally obstructed; the bark becomes thick and woody, and covered with moss or lichens; the shoot becomes stunted and diminutive; and the fruits palpably degenerate, both in quantity and quality. The smaller or terminal branches fade and decay the first, and then the larger branches also, together with the trunk and root; the vital principle gradually declines without any chance of recovery, and is at last totally extinguished. "When life is extinguished, nature hastens the decomposition; the surface of the tree is overrun with lichens and mosses, which attract and retain the moisture; the empty pores imbibe it; and putrefaction speedily follows. Then come the tribes of fungi, which flourish on decaying wood, and accelerate its corruption; beetles and caterpillars take up their abode under the bark, and bore innumerable holes in the timber; and woodpeckers in search of insects pierce it more deeply, and excavate large hollows, in which they place their nests. Frost, rain, and heat assist, and the whole mass crumbles away, and dissolves into a rich mould." (Dial. on Bot. p. 365.)

CHAP. VI.

Vegetable Geography and History, or the Distribution of Vegetables relatively to the Earth and to Man.

1722. The science of the distribution of plants, Humboldt observes (Essai sur la Géographie des Plantes, 1807), considers vegetables in relation to their local associations in different climates. It points out the grand features of the immense extent which plants occupy, from the regions of perpetual snow to the bottom of the ocean, and to the interior of the globe, where, in obscure grottoes, cryptogamous plants vegetate, as unknown as the insects which they nourish. The superior limits of vegetation are known, but not the inferior; for every where in the bowels of the earth are germs which develope themselves when they find a space and nourishment suitable for vegetation. On taking a general view of the disposition of vegetables on the surface of the globe, independently of the influence of man, that disposition appears to be determined by two sorts of causes, geographical and physical. The influence of man, or of cultivation, has introduced a third cause, which may be called civil. The different aspects of plants, in different regions, have given rise to what may be called their characteristic or picturesque distribution; and the subject of distribution may be also considered relatively to the systematic divisions of vegetables, their arithmetical proportions, and economical applications.

SECT. I. Geographical Distribution of Vegetables.

1723. The territorial limits to vegetation are determined in general by three causes: 1. By sandy deserts, which seeds cannot pass over either by means of winds or birds, as that of Sahara, in Africa; 2. By seas too vast for the seeds of plants to be drifted from one shore to the other, as in the ocean; while the Mediterranean sea, on the contrary, exhibits the same vegetation on both shores; and, 3. By long and lofty chains of mountains. To these causes are to be attributed the fact that similar climates and soils do not always produce similar plants. Thus in certain parts of North America, which altogether resemble Europe in respect to soil, climate, and elevation, not a single European plant is to be found. The same remark will apply to New Holland, the Cape of Good Hope, Senegal, and other countries, as compared with countries in similar physical circumstances, but geographically different. The separation of Africa and South America, Humboldt considers, must have taken place before the developement of organised beings, since scarcely a single plant of the one country is to be found in a wild state in the other.

SECT. II. Physical Distribution of Vegetables.

1724. The natural circumstances affecting the distribution of plants may be considered in respect to.temperature, elevation, moisture, soil, and light.

1725. Temperature has the most obvious influence on vegetation. Every one knows that the plants of hot countries cannot in general live in such as are cold, and the contrary. The wheat and barley of Europe will not grow within the tropics. The same remark applies to plants of still higher latitudes, such as those within the polar circles, which cannot be made to vegetate in more southern latitudes; nor can the plants of more southern latitudes be made to vegetate there. In this respect, not only the medium temperature of a country ought to be studied, but the temperature of different seasons, and especially of winter. Countries where it never freezes, those where it never freezes so strongly as to stagnate the sap in the stems of plants, and those where it freezes with strength sufficient to penetrate into the cellular tissue, form three classes of regions in which vegetation ought to differ. But this difference is somewhat modified by the effect of vegetable structure, which resists, in different degrees, the action of frost. Thus, in general, trees which lose their leaves during winter resist the cold better than such as retain them; resinous trees, more easily than such as are not so; herbs of which the shoots are annual and the root perennial, better than those where the stems and leaves are persisting; annuals which flower early, and whose seeds drop and germinate before winter, resist cold less easily than such as flower late, and whose seeds lie dormant in the soil till spring. Monocotyledonous trees, which have generally persisting leaves and a trunk without bark, as in palms, are less adapted to resist cold than dicotyledonous trees, which are more favourably organised for this purpose, not only by the nature of their proper juice, but by the disposition of the cortical and alburnous layers, and the habitual carbonisation of the outer bark. Plants of a dry nature resist cold better than such as are watery; all plants resist cold better in dry winters than in moist winters; and an attack of frost always does most injury in a moist country, in a humid season, or when the plant is too copiously supplied with water.

1726. Some plants of firm texture, but natives of warm climates, will endure a frost of a few hours' continuance, as the orange at Genoa, (Humboldt, De Distributione PlantaTum); and the same thing is said of the palm and pine-apple, facts most important for the gardener. Plants of delicate texture, and natives of warm climates, are destroyed by the slightest attack of frost, as the Phaseolus, Nasturtium, &c.

1727. The temperature of spring has a material influence on the life of vegetables; the injurious effects of late frosts are known to every cultivator. In general, vegetation is favoured in cold countries by exposing plants to the direct influence of the sun; but this excitement is injurious in a country subject to frosts late in the season; in such cases, it is better to retard than to accelerate vegetation.

1728. The temperature of summer, as it varies only by the intensity of heat, is not productive of so many injurious accidents as that of spring. Very hot dry summers, however, destroy many delicate plants, and especially those of cold climates. A very early summer is injurious to the germination and progress of seeds; a short summer, to their pening, and the contrary.

1729. Autumn is an important season for vegetation, as it respects the ripening of seeds; hence where that season is cold and humid, annual plants, which naturally flower late, are never abundant, as in the polar regions; the effect is less injurious to perennial plants, which generally flower earlier. Frosts early in autumn are as injurious as those which happen late in spring. The conclusion, from these considerations, obviously is, that temperate climates are more favourable to vegetation than such as are either extremely

upon the whole, to vegetation than the colder, and that nearly in proportion to their distance from the equator. The same plants, however, will grow in the same degree of latitude, throughout all degrees of longitude, and also in correspondent latitudes on different sides of the equator; the same species of plants, as some of the palms and others, being found in Japan, India, Arabia, the West Indies, and part of South America, which are all in nearly the same latitudes; and the same species being also found in Kamschatka, Germany, Great Britain, and the coast of Labrador, which are all also in nearly the same latitudes. (Willdenow, p. 374.)

1730. Rules for determining the temperature of a country. "The fact that a degree of latitude is equal to a degree of Fahrenheit, and that 400 feet of elevation is equal also to a degree of Fahrenheit, is original and curious, and will go far to assist us in determining the clime of any country." (Amer. Quart. Rev. March, 1829. p. 174.)

1731. The most remarkable circumstances respecting the temperature in the three zones are exhibited in the following Table by Humboldt. The temperature is taken according to the centigrade thermometer. The fathom is 6 French feet, or 6.39453 English feet.

1732. Elevation, or the height of the soil above the level of the sea, determines, in a very marked manner, the habitation of plants. The temperature lessens in regular gradation, in the same manner as it does in receding from the equator, and 600 feet of elevation, Humboldt states, are deemed equal to one degree of latitude, and occasion a diminution of temperature equal to 23° of Fahrenheit; 300 feet being nearly equal to half a degree. Mountains 1000 fathoms in height, at 46° of latitude, have the mean temperature of Lapland; mountains of the same height between the tropics enjoy the temperature of Sicily; and the summits of the lofty mountains of the Andes, even where situated almost directly under the equator, are covered with snow as eternal as that of the north pole. The highest land in Scotland where corn has been found to attain maturity in favourable seasons is said to be at the mining ground on Lead Hills. (See General Reports of Scotland, chap. Climate.)

1733. Hence it is that plants of high latitudes live on the mountains of such as are much lower, and thus the plants of Greenland and Lapland are found on the Alps and Pyrenees. At the foot of Mount Ararat, Tournefort met with plants peculiar to Armenia; above these he met with plants which are found also in France; at a still greater height he found himself surrounded with such as grow in Sweden; and at the summit with such as vegetate in the polar regions. This accounts for the great variety of plants which are

often found in a Flora of no great extent; and it may be laid down as a botanical axiom, that the more diversified the surface of the country, the richer will its Flora be, at least in the same latitudes. It accounts, also, in some cases, for the want of correspondence between plants of different countries, though placed in the same latitudes; because the mountains or ridges of mountains, which may be found in the one and not in the other, will produce the greatest possible difference in the character of their Floras. To this cause may generally be ascribed the diversity which often actually exists between plants growing in the same latitudes, as between those of the north-west and north-east coasts of North America, and also between those of the south-west and south-east coasts; the former being more mountainous, the latter more flat. Sometimes the same sort of difference takes place between the plants of an island and those of the neighbouring continent; that is, if the one is mountainous and the other flat; but if they are alike in their geographical delineation, then they are generally alike in their vegetable productions.

1734. Cold and lofty situations are the favourite habitations of most cryptogamic plants of the terrestrial class, especially the fungi, algæ, and mosses; as also of plants of the class Tetradynamia, and of the Umbelliferous and Syngenesious tribes; whereas trees and shrubs, ferns, parasitic plants, lilies, and aromatic plants, are most abundant in warm climates: but this is not to be understood merely of geographical climates, because, as we have seen, the physical climate depends upon altitude; in consequence of which, combined with the ridges and directions of the mountains, America and Asia are much colder in the same degrees of northern latitude than Europe. American plants, vegetating at forty-two degrees of northern latitude, will vegetate very well at fifty-two degrees in Europe; the same, or nearly so, may be said of Asia; which, in the former case, is perhaps owing to the immense tracts of woods and marshes covering the surface, and in the latter, to the more elevated and mountainous situation of the country affecting the degree of temperature. So, also, Africa is much hotter under the tropics than America; because in the latter, the temperature is lowered by immense chains of mountains traversing the equatorial regions, while in the former it is increased by means of the hot and burning sands which cover the greater part of its surface.

1735. Elevation influences the habits of plants in various ways: by exposing them to the wind; by causing them to be watered by a very fresh and pure water from the melting of adjoining snow; and to be covered in winter by a thick layer of snow, which protects them from severe frosts. Hence many alpine plants become frozen during winter in the plains, and in gardens which are naturally warmer than their proper stations. In great elevations, the diminution of the density of the air may also have some influence on vegetation. The rarity of the atmosphere admits a more free passage for the rays of light, which, being in consequence more active, ought to produce a more active vegetation. Experience seems to prove this on high mountains; and the same effect is produced in high latitudes by the length of the day. On the other hand, vegetables require to absorb a certain quantity of oxygen gas from the air during the night; and as they find less of that in the rarefied air of the mountains, they ought to be proportionably feeble and languishing. According to experiments made by Theodore de Saussure, plants which grow best in the high Alps are those which require to absorb least oxygen during the night; and, in this point of view, the shortness of the nights near the poles corresponds. These causes, however, are obviously very weak, compared to the powerful action of temper

ature.

1736. Great anomalies are found in the comparative height at which the same plant will grow in different circumstances. In countries situated under the equator, the two sides of the mountain are of the same temperature, which is solely determined by elevation; but in countries distant from it, the warmest side is that towards the south, and the zones of plants, instead of forming lines parallel to the horizon, incline towards the north. The reason, in both cases, is sufficiently obvious. In the temperate zone we find the same plants frequently on low and elevated situations, but this is never the case between the tropics.

1737. Altitude influences the habits of aquatics: thus some aquatics float always on the surface of the water, as Lémna, while others are either partially or wholly immersed. Such aquatics as grow in the depths of the sea are not influenced by climate; but such as are near the surface are influenced by climate, and have their habitations affected by it. 1738. The moisture, or mode of watering, natural to vegetables, is a circumstance which has a powerful influence on the facility with which plants grow in any given soil. The quantity of water absolutely necessary for the nourishment of plants, varies according to their tissue: some are immersed, others float on its surface; some grow on the margin of waters, with their roots always moistened or soaked in it; others, again, live in soil slightly humid or almost dry. Vegetables which resist extreme drought most easily are, 1. Trees and herbs with deep roots; because they penetrate to, and derive sufficient