196 SCIENCE OF GARDENING. Part II. 



individual cannot pass. Some plants are annuals and last for one season only, springing 

 up suddenly from seed, attaining rapidly to maturity, producing and again 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 under-shrubs, 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. 



903. 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. 



904 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, that strip the forest of its foliage, and the landscape 

 of its verdure. But there are some trees that 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 that retain their verdure throughout the year, and are denominated evergreens, as the holly. 

 The leaves of Doth 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 iUustrate the fall of the leaf by any analogous process in the animal economy, it 

 maybe compared to that of the shedding of the antlers of the stag, or of the hair or feathers of other 

 beasts or birds, which being, like the leaves of plants, distinct and peculiar organs, fall off, and are rege- 

 nerated annually, but do not slough. 



905. The flowers, which, like the leaves, are onlv temporary organs, are for the most part very short- 

 lived : for as the object of their production is merely that of effecting the impregnation of the germs, 

 that object is no sooner obtained than they begin again 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. 



906. 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 detach 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, if not gathered ; but some remain long attached to the parent plant after being 

 fully ripe, as in the case of the fruit of euonymus, and mespilus. But these, though tenacious of their 

 hold, detach themselves at last, as well as all others, and bury themselves in the sod, about 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. 



907. Decay of the permanent organs. Such then is the process and presumptive ra- 

 tionale 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 ter- 

 minal 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 de- 

 composition ; 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." (JDuz/. on Bot. p. 365.) 



Chap. X. 



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



and to Man. 



908. The science of the distribution of plants, Humboldt observes (Essai sur la Geo- 

 graphic des Plantes, &c. 1807), considers vegetables in relation to their local associations in 



