BOOK SECOND.

CO-ORDINATE SERIES OF FACTS, GIVING INDICATIONS OF COMBINED ORDER AND ADAPTATION THROUGHOUT THE VARIOUS KINGDOMS OF NATURE.

CHAPTER I.

THE MINUTE STRUCTURE OF PLANTS AND ANIMALS.

SECT. I.-ORDER IN THE STRUCTURE OF THE CELL.

WE are to be chiefly occupied in these chapters in displaying the skill to be found in the plant and animal, as built up into their finished forms, with all their harmonious proportions and varied fitnesses. But before inspecting the finished temple, we may take a look at the materials of which it is built, and these we shall find to be like the stone of Solomon's temple, which "was made ready before it was brought thither, so that there was neither hammer nor axe nor any tool of iron heard in the house while it was building."

It has long been admitted among botanists, that the cell is the typical element in the structure of the plant, that the lower forms of plants actually consist of cells separate and independent, and that the higher are built of the same material, compacted into masses of varied texture.

The general structure of the vegetable cell is very simple. On the outside there is a transparent membrane, called cell-wall, enclosing another part which has received various names, as endochrome, or internal utricle. In

the fresh cell the cell-wall and internal utricle are often in such close contact that the presence of the internal layer may be overlooked; but the action of various chemical agents produces shrinking of the inner layer, and thus its presence may be demonstrated. The primary form of the entire cell is stated by some authors to be spherical; the principal modifications in shape are generally regarded as departures from that type. We are inclined to direct attention to the typical structure, rather than to insist on a unity of primary form, since the latter may depend on the original development of the cell. The most usual structure of the animal cell is essentially the same as that of the vegetable cell. The question of this identity, in other words, as to their being referrible to a common type, has been recently examined by Professor Huxley, who has proposed a new and convenient nomenclature. The outer part, or cell-wall, he calls periplast, or periplastic substance, and the contents he calls endoplast. The homologies of the parts of the animal and of the vegetable cell had been the subject of discussion; Professor Huxley has arrived at the general result, that "in all animal tissues, the internal part called nucleus is the homologue of the contents or internal utricle of the

FIG. 1. Section of leaf of Agave, shewing the cell-wall and contents.

+ Sometimes that which corresponds to the internal part is alone present, in some of the lower forms of both animals and plants.

plant, the other elements being invariably modifications of the cell-wall or periplast."

The elements, therefore, of all animals and plants are referrible to a common type.

Animal-cells and plant-cells are microscopical, and this is true whatever be the size of the entire animal or plant. These minute cells bear the same relation to the entire organism as the component materials of a building to the whole fabric.

SECT. II. SPECIAL MODIFICATIONS OF THE CELL.

1. In Plants. Having found traces of unity in the elementary structures of the plant, we proceed to inquire into the relation between modifications of such and the performance of certain functions necessary in the economy of the plant, and essential to its existence and increase.

Modifications of the cell have generally an evident relation to some particular end to be accomplished, as, for example, to increase the density, the tenacity, or the resisting power, or to furnish a passage to the fluids needful to the life of the plant. The stone of a cherry presents an example of cells

specially modified with the view of increasing the power of resistance. This is accomplished by additional layers to the cell-wall, which thus becomes very much increased in thickness. Similar

FIG. 2.*

modifications are met with in certain fruits-nuts, for

FIG. 2. Gritty matter of pear, longitudinal and transverse sections; magnified. The stone of a cherry or peach presents the same structure.

instance, and in the skin of some seeds. We need scarcely add how admirably such tissues are fitted to give protection to the important parts within. The so-called

་་་

FIG. 3.*

wood or woody fibre consists of elongated cells, the walls of which are often thickened by secondary deposits, thus adding great tenacity and durability to the material. The woody part of trees, as well as of smaller plants, consists mainly of this substance; it abounds where firmness, tenacity, and elasticity are needful in the economy of the plant; and in the form of flax, hemp, &c., man turns it to good account for his own purposes.

Again, when free circulation of fluid is necessary, we find a tubular structure provided; the different modifications of vascular tissue, known under the names of ducts, spiral vessels, &c., are examples. The peculiar vessels admitted to

exist in certain plants having a milky sap, are modifications of cells for a special purpose; they are so constituted as to give free passage to fluids by longitudinal as well as lateral channels, the adjoining tubes having usually free communication. It may not be easy to recognise the cell type in such a modification, but its nature becomes patent, if we suppose that a number of cells, of whatever form, are attached in linear series, and that the

FIG. 4.t

*FIG. 8. Wood or woody tissue, composed of spindle-shaped cells.

+ FIG, 4. Milk-vessels from dandelion.

partitions between them become obliterated; it is obvious that a continuous tube or duct will be the result.

The pollen or fecundating matter of the plant, so essential to the continuance of the species, consists of transformed cells, and the first trace of the new plant is also a cell, which is stimulated to full development by the contents of the pollen cell.

These are some of the principal modifications of the vegetable fabric; a general plan prevails, which plan is made to accommodate itself to some particular purpose, whether this be to produce a tough or elastic fibre, a hard structure for defence, or a tube required for the passage of fluids. In the absence of such special and evidently designed adaptations it seems evident that the plants which have been so bountifully disseminated over the surface of our world, would be unavailable for various economic purposes, man could not derive from them food and clothing for his person, nor covering and furniture to his dwelling; nay more, the very existence of many vegetable forms is dependent on the special modifications of their simple elements.

2. In Animals.-The higher endowments of the animal organism imply, in the way of final cause, greater departures from the primitive cell structure, and, accordingly, we meet with a greater number of more widely diverging modifications. The researches of different observers have, however, tended to shew that a common plan regulates the nature of the primary tissues. We may now proceed to inquire into modifications bearing a relation to some necessary end in the economy of the animal.

The thin pellicle which is separated from the skin in consequence of a scald, or the application of a blistering plaster, is called Epidermis; in its different layers we can distinctly trace transitional forms of the typical cell.