simple isolated cells form an important part of the circulating fluid; the Blood-Corpuscles-as they are commonly called-to which that fluid owes its colour, are truly referrible to the cell type. The food, after undergoing certain changes in the stomach and alimentary canal, constitutes the fluid called chyle; it is admitted that certain Epithelial cells select and absorb the materials of the chyle, and, becoming turgid with them, subsequently transfer them to minute vessels-the lacteals, which convey them to the blood-vessels. In the stomach and alimentary canal, certain cells are actively engaged in pouring out some peculiar and useful secretion. In the stomach such cells are continually forming new broods, which pass out in great numbers, their contents yielding matters necessary in the process of digestion. There are organs whose function it is to separate matters for some special use, as the milk for the nourishment of the young, or to remove substances whose presence would be injurious if retained. It is unnecessary here to enter into details regarding these various organs; suffice it to say that their essentially important parts belong to the cell type. In short, we find that in the animal body some special modification of the cell is concerned in every important function. Cartilage, bone, muscle, nerve, serve different ends in the animal economy, but the cell is the essential element in each. The formation of an image in the eye is mainly effected by the optical properties of parts having a cellular origin, and the impression is conveyed by another tissue, which, as we have already stated, may be referred to the same general type. In a subsequent section we shall have occasion to allude to the general structure as well as modifications of Teeth. Details respecting their mode of development would be out of place here; it may be sufficient to state that cells perform an essential part in the formation of every tooth. Nails, Hoofs, Horn, are all essentially Epidermic products, and necessarily partake of the nature of that part of the skin, that is, are modifications of the cell. Their importance in the animal economy is too obvious to require discussion, serving as they do to protect delicate parts, and to act as means of defence and offence. Hairs and Feathers, notwithstanding their variety in color and texture, have a common origin in cells. The thick and warm fur of the hare, the smooth and silky coat of the mole, the spines of the hedgehog, the quills of the porcupine, and the coarse hairs-resembling split whalebone-of the elephant and ant-bear, are all mere modifications of the elemental cell, and each has a reference more or less obvious to the habits of the animal. The hairs of the mole are closely set, they stand out perpendicularly; in other words, have no particular shed, and thus present no obstacle to the rapid movements of this burrower when traversing its narrow and intricate subterranean tunnels. The spines of the hedgehog and the sharp quills of the porcupine are respectively admirable means of protection to these otherwise defenceless animals. Feathers, constructed, as we have said, after the same cellular type as hairs, present similar modifications in character, varying with the habits of particular birds. The soft plumage of owls enables them noiselessly to steal on their agile prey. The thick-set feathers and down of divers and other aquatic birds, effectually repel the water and prevent soiling of surface, as well as loss of animal heat. CHAPTER II. THE FORMS OF PLANTS. SECT. I. TRACES OF ORDER IN THE ORGANS OF PLANTS. "WHEN Jupiter," says Herder, "was summoning the creation, which he meditated in ideal form before him, he beckoned, and Flora appeared among the rest. Who can describe her charms, who can image forth her beauty? Whatever the earth showers from her virgin-lap was mingled in her shape, her colour, her drapery." We are to attempt no description of her beauty, which can be appreciated only by those who look upon her charms directly, and not through any representation of them. But we are to attempt to give something like a scientific account of that development and structure, of that disposition of parts and distribution of colours, which mainly contribute to give to the plant its graceful proportions and its loveliness. Our present aim is to show that there are system and design in the progress of the plant, from the time it springs from the seed to the time when it yields seed, and that there are determined types to which all its organs are made to conform themselves.* Botanists describe two modifications in the structure of the seeds of the higher forms of plants. In a pea or * We are to confine our illustrations to flowering plants, partly because the order in these classes of plants is most easily explained, but mainly because the morphology of the lower tribes of plants has not been so fully investigated. bean, we observe that the principal bulk of the seed consists of two large bodies in close contact; they are called seed-lobes, or seed-leaves, and, technically, Cotyledons. FIG. 8.* there appears to be only one cotyledon, and such plants are called Monocotyledons. There is a difference between these two kinds of seeds as regards the process of germination; but it will suffice for our purpose, to state that in both there is a general tendency in one part to fix itself in the soil, while the other tends to raise above it into the air; the former is the root or descending axis, the latter the stem or ascending axis. Mere position in reference to the soil is not, however, an invariable test of the nature of a part whether stem or root; for there are not a few instances in which the true permanent stem is underground, as well as the proper root. But whatever be the position of these organs, we may see in the plant a continuous principal axis, one part of which constitutes the root, and the other the stem. Attached to the latter there are various FIG. 9.t appendages. Fig. 8. Embryo of Pea, shewing the point where the young root arises, r; the young stem or plumule, g; the stalk, t; connected with the cotyledons, c c, which are separated and laid open; f, the depression in which the plumule lay. FIG. 9. Vertical section of grain of oats, shewing the embryo plant at the lower part, consisting of r, the parts whence the roots proceed; g, the young stem; c, the single cotyledon. The covering of the entire grain, o; covering of the seed proper, t; the nourishing matter, or albumen, a. On the ascending axis of the plant, we observe two kinds of appendages, leaves and buds. These last, however, are mere repetitions of the plant; each bud consisting of a short axis, and of lateral organs-the young leaves. The Leaf, therefore, is the only essential typical appendage of the vegetable organism. It requires no minute description here; the most inexperienced observer can recognize it; it belongs to the class of "common things." The study of its many forms lies within the province of the botanist. While this typical appendage varies in outline, its general structure is simple enough. The outer surface has a covering called cuticle or skin; the internal portion, parenchyma, as it is technically called, has ramifying Fig. 10. To shew curved venation of Endogen. |