264 SCIENCE OF AGRICULTURE. Part II. 



faraiUar 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 

 oy 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 short- 

 lived; 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 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 ; but some remain long attached to the parent plant after being fully ripe, as in 

 the case of the fruit of J?u6nyraus and ilfespilus. 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 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 decom- 

 position ; 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 jHants, Humboldt observes {Essai sur la GSo- 

 graphic 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 inte- 

 rior 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 develops 

 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 distribu- 

 tion ; and the subject of distribution may be also considered relatively to the systematic 

 divisions of vegetables, their arithmetical proportions, and economical applications. 



