This system has been wonderfully modified to produce the varied groups of extra-tropical and brilliant forms of exotic orchids, yet it can be traced in all.
SECTION XIV.
DICOTYLEDONOUS, OR EXOGENOUS PLANTS.
In Endogenous plants the seeds have but one lobe, and the growth is invariably from the interior. In the Exogenous class, on the contrary, the seeds have two lobes, and the increase in growth is external: hence the botanical distinction of Exogenous plants. Although the distinctive character of the highest class of vegetables is to have seeds with two lobes, yet the structure and position of the seeds are extremely diversified. Many have horny coats, such as the pips of apples and oranges; or hard ones, as nuts, and the stones of plums and cherries. They are sometimes on the outside of the fruit, as in strawberries, but oftener within it, as in the melon, the pear, and a variety of others. These succulent substances nourish the young seeds for a time, but, when they are matured, the light and heat which ripened the fruits now combine to accelerate their decay and decomposition, in order to set the seeds free.
Whatever be the size or form of the seeds, whether large or microscopic, they invariably contain two seed lobes or primary leaves, consisting of cellular tissue, between which the miniature plant, with its radicle, stem, and terminal bud lies concealed. At the end of the first year there is little difference in the structure of a young woody plant, whether from a one or two lobed seed; the distinction begins the second year. In herbaceous plants, the stem, which is in general annual, is of loose cellular tissue, with separate bundles of fibro-vascular tissue running from the roots upwards, and passing at last into the leaves, where they form the ribs or veins. Many of the higher forms of plants have fleshy underground stems, as instances of which may be named the Corm, as seen in the crocus and colchicum, the Tuber, as in the potato, the Rhizome, as in the fleshy rootstock of Iris florentina, which yields the violet powder of the shops, and indeed most bulbs may be considered as modifications of stems, though they are more strictly analogous to buds. The edible parts of the carrot, turnip, parsnip, and radish are not stems, but highly developed succulent roots, the unusual development of which is a direct result of cultivation.
A young tree, at the end of the first year of its growth, has subterranean roots, with their branches and fibrils, and an aërial stem, often more or less branched, formed of bark, wood, and pith, with a few leaves at its extremity, all exceedingly tender. Every succeeding year a new cylinder of woody fibre and vascular tissue is formed between the wood and the bark, both in the stem and branches. It follows from this manner of growth, that the stem of a tree, consisting of bark, wood, and pith, is formed of a series of cylinders or extremely elongated concentric cones closely united, so that a transverse section exhibits a series of concentric circles or zones from the surface of the bark to the central pith. The structure of the branches is similar, but the number of zones depends upon the age. Since all the tissues that have been described are combined to form the organs of nutrition and reproduction in a full-grown tree, it affords the best general illustration of the organization of the highest class of vegetables.
Every part of a tree or plant, except the top of the stigma and extremity of the roots, is covered by an extremely delicate film of cellulose, closely pressed down upon one, two, or three layers of transparent colourless cellular tissue compressed into a tubular form, which constitutes the cuticle. These flattened cells, which are firmly united to one another by their edges, differ in shape in almost every tribe of plants. In the monocotyledons they are elongated in the direction of the parallel ribs of the leaves; in the highest class they differ little from circular discs, but they have large sinuosities in their edges, which make their junction very irregular. The upper cells of the cuticle are lined with a waxy substance, which protects the plant from damp; and in many cases it contains more or less silex.
This general covering, or cuticle, is perforated by numerous pores, especially on the under-side of the leaves, and the green tender parts of the branches; they are the organs of respiration. These pores, or stomata, are usually formed of two crescent-shaped cells, joined together by their points or horns, so as to leave an open space like a mouth, through which the plant breathes. These, however, are only the guardians: the opening between them leads into a cavity full of air, which is the antechamber to an interior cavity. The valves of the stomata open and shut according to the humidity or dryness of the atmosphere. All plants of both classes, woody or herbaceous, have stomates, except water plants, fungi, fuci, and others of the lowest class. They are generally very abundant on the under-surface of leaves. They are sometimes in vast numbers on both sides of the leaves, and are essential to the life of the plant. According to MM. Payen and Liepner, silex and azote, together with calcareous and alkaline salts, are invariably found in the cells forming the skin of the roots, stems, leaves, fruits, hairs, and spines. The strong cohesion of the skin, together with the presence of these substances, becomes a defence against the wearing effect of the weather, without diminishing the transparency of the tissues.
The bark is divided into three regions, or zones. The external coat, lying immediately under the skin, is formed of one or many layers of cubical or oblong cells, elongated horizontally. They are transparent and colourless at first, but become brown and opaque with the colouring matter of cork as they grow older. On that account it is called the suberous zone, and sometimes acquires great thickness, as in the Quercus Suber, or cork tree.
The green cellular envelope comes next to the corky layer, and consists of prismatic cells and laticiferous tubes, which form an irregular wide-meshed network, elongated in the direction of the axis of the tree, and sometimes constituting the chief thickness of the bark. This zone, as well as the succeeding, increases imperceptibly by new layers added to its interior, while the exterior coats of the bark perish annually. In some trees they are annually cast off in plates and large flakes, as in the Oriental plane, whose stem and branches look as if they had been peeled in autumn.
The liber, which is the third and innermost zone of the bark, generally consists of several layers of cellular tissue, traversed longitudinally by bundles of woody fibre and laticiferous tubes.