78 REASONS WHY ORGANS MAY GROW WITHOUT LEAVES. time exclusively in the cells of the green organs, the tubes and vessels of the vegetable structure being mere recipients organized by the matters so elaborated. This power of assimilation is believed to be owing to the high vitality of the cells of the green organs; but in proportion as the subterranean parts become organized their vital force increases, and at last it becomes sufficient to enable them to act independently of the leaves or green parts. If, then, at the time when a subterranean organ is cut off from communication with the leaves, its vitality is sufficiently high, its cells not only absorb water and other matters, as was the case from the beginning, but also decompose and elaborate them, in the same way as the cells of the leaves. The result of that elaboration is increase in bulk, partly arising from the distension of the cells and the consolidation of their contents, partly from the increase of the number of the cells themselves, and also from filling the last formed cells with the matter peculiar to the species. What is required in order to secure increase in bulk is the power of organization; that power depends upon the presence of a sufficient amount of vital force; therefore, when a subterranean body has gained enough vital force, it has gained all the organic capabilities which are necessary for increase of size, or growth, and is able to enlarge even though cut off from communication with green organs. It must not however be inferred that an underground organ will increase as rapidly in the absence of leaves as it will if they are present. On the contrary, in the latter case, it grows by virtue of its own vitality and that of the leaves combined; a double power is brought to bear upon its increase, and at least twice as much food in an organizable condition is presented to it for consumption. All that we are justified in asserting is, that although leaves may be gone, growth will go on-and to a much greater extent than is supposed. If, then, a root-crop is from any accident deprived of its leaves, it is by no means a necessary consequence that the crop is arrested in its growth; on the contrary, provided the defoliation does not occur till towards the end of the season, growth will go on notwithstanding. It is to be observed that, as has already been stated, the capability of plants to bear the action of direct light varies according to their specific nature. One species is organized to suit the atmosphere of a dense wood, into which diffuse light only will penetrate; another is planted by nature on the exposed face of a sunburnt rock, upon which the rays of a shadeless sun are daily striking: in these cases, the light which is necessary to the one would be destructive of the other. The organic difference of such species seems to consist chiefly in the epidermis, which regulates the amount of perspiration. It is therefore to be remarked, that it is not the greatest quantity of light which can be obtained that is most favourable to the healthiness of plants, but the greatest quantity they will bear without injury. If the former were true, the concentrated light of a lens would be better than the strongest ordinary light; but the effect of the concentrated light of a lens is to burn the surface, and the ordinary solar rays produce the same effect upon many plants, probably by exhausting the tissue of its water faster than it can be supplied from the roots. In the course of time, a leaf becomes incapable of performing its functions; its passages and surface are choked up by the deposit of impurities; there is no longer a free communication between its parenchyma and that of the rind, or between its veins and the wood and liber; or the air and its interior. It changes colour, ceases to decompose carbonic acid, absorbs oxygen instead, gets into a morbid condition, and dies: it is then thrown off. This phenomenon, which we call the fall of the leaf, is going on the whole year round, except mid-winter, in some plant or other. Those which lose the whole of their leaves at the approach of winter, and are called deciduous, begin, in fact, to cast their leaves within a few weeks after the commencement of their vernal growth; but the mass of their foliage is not rejected till late in the season. Those, on the other hand, which are named evergreens, part with their leaves much more slowly; retain them in health at the time when the leaves of other plants are perishing; and do not cast them till a new spring has commenced, when other trees are leafing, or even later. In the latter class, the functions of the leaves are going on during all 80 MULTIPLICATION BY LEAVES. the winter, although languidly; they are constantly attracting sap from the earth through the spongelets, and are, therefore, in a state of slow but continual winter growth. It usually happens that the perspiratory organs of these plants are less active than in deciduous species. In general, a leaf is an organ of digestion and respiration, and nothing more; some leaves have, however, the power of forming leaf-buds, if placed in or upon the earth, under suitable circumstances. The Bryophyllum calycinum forms buds at the indentations of its margin; Malaxis paludosa throws off young buds from its edge; Tellima grandiflora occasionally buds at the margins of its leaves; the same thing happens to many Ferns; the five leaflets of a pinnated Rose-leaf yield, under proper culture, five little plants; Cape Ornithogalums often produce bulbs from the edge of their leaves; the same fact has been observed by Mr. Rogers on the broken edge of a Lachenalia leaf; and numerous similar instances might be quoted. CHAPTER VI. ACTION OF FLOWERS. STRUCTURE OF FLOWERS.NAMES OF THEIR PARTS. -TENDENCY OF THE PARTS TO ALTER AND CHANGE INTO EACH OTHER, AND INTO LEAVES. CROSSBREDS. A FLOWER is that part of a plant which is formed for the purpose of reproducing the species by means of seeds. It consists of floral envelopes and sexes. The floral envelopes are: 1, the calyx, which is usually green, and always the most external; and 2, the corolla, which is commonly thin, gaily coloured, more fugitive than the calyx, and placed next within it: each of these consists of leaves, called sepals in the calyx, and petals in the corolla. Both calyx and corolla are usually present; but in some cases only one envelope is formed, as in the Marvel of Peru; and in other cases the flower has no envelopes, as in the Willow. Envelopes are, therefore, not a necessary part of a flower. In the middle of the flower stand the sexes, called stamens and pistil, of which the pistil occupies the centre, and the stamens surround it; except in those cases where the sexes are produced in separate flowers, when each sex is central in its own flower. The stamens consist of a filament and an anther, in the interior of the latter of which is secreted a powdery substance termed pollen. The pistil consists of ovary, style, and stigma, in the interior of the first of which are ovules or young seeds. Although the floral envelopes may be, and often are, absent, G 82 TRANSFORMATIONS OF FLOWERS. wholly or in part, yet the sexes are always present. Consequently the latter are all that is essential to a flower, and no part can be a flower from which they are absent. Notwithstanding the difference in form and office of the parts of a flower, they have evidently a strong tendency in cultivated plants to change into or assume the appearance of each other. In the Poppy, the Garden Anemone, and many others, the stamens change into petals; in the Anemone, the Ranunculus, &c., the pistil changes into petals; in the Primrose, Cowslip, &c., the calyx changes into petals; in the Houseleek, the stamens become pistils; and so on. Hence the origin of double flowers. In a double Barbadoes Lily, described by me in the Transactions of the Horticultural Society, in which the parts were very much confused, the young seeds were borne by the edges of the stamen-like petals (Fig. XV.). In their ordinary state the parts of a flower are extremely unlike leaves, and each has its allotted office, which is not the |