binations effected in the cells and vessels of plants and the inferior animals, man derives his corporeal frame, and is, in fact, as far as that portion of his nature is concerned, part and parcel of the earth he moves on; the first step, therefore, in this extraordinary metamorphosis well deserves a careful examination.

VEGETABLE PHYSIOLOGY.

CHAPTER I.

STRUCTURE AND PROPERTIES OF VEGETABLE TISSUE.

1. VEGETABLE structure "chemically composed of oxygen, hydrogen, and carbon, to which nitrogen is always superadded,"*-consists, in common with that of all organized beings, of

1. The matter which forms the actual substance of the plant itself.

2. One or more liquids, either contained in, or secreted by, its organs.

3. Other substances, more or less solid, deposited during the passage of those liquids through the different portions of the body.

The researches of modern investigators, aided by the improved powers of the microscope, have shown that the solid structure of plants consists of, Cellular Tissue, Vessels, Fibres, and Skin.

2. Cellular Tissue (contextus cellulosus), is a membranous tissue, very similar, in arrangement and form, to a honeycomb, being composed of detached cells, as its name denotes, which are closed, and adhere more or less nearly together; it is found universally in all plants; and many of the lower tribes, such as lichens, mosses, &c., are entirely formed of it. It surrounds the vascular parts so that

* Lindley's Elements of Botany, p. 1.

in the vegetable as in the animal conformation, no vessel is ever exposed and bare. The diameter of the cells, or vesicles, which is perhaps their more correct appellation, varies considerably, from the thirtieth to the three-thousandth of an inch; their shape also is much diversified, but the normal form appears to be round, and it is probable, indeed almost certain, that the variety depends on the pressure of one part of the plant on another during its growth. The vesicles seem to originate from a point, called by modern writers a cytoblast,* which sometimes continues visible after they have reached maturity. The property of uniting firmly together, possessed by the cells which compose this tissue, forms a very important part of the history of vegetation, for it is to these adhesions in the cellular tissue, that all the seams in the various organs of a plant are owing. The term parenchyma is applied to the cellular tissue, considered as a mass, to distinguish it from those parts which abound in vessels. Cellular tissue is self-productive, one cell not only having the power of generating another on its surface," but cells. frequently produce others, generally in a definite number, within their own cavities, on the complete development of which, the parent cell generally perishes or is re-absorbed.

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3. Vessels, or Vascular Tissue. This term is applied to tubes, nearly or quite cylindrical, which are observed in the greater number of plants. They are now usually distinguished as Spiral Vessels and Ducts.

A. Spiral Vessels, or Tracheæ, resemble a ribbon which has been rolled round a cylinder, and

* Probably from xùrog a cavity or hollow point, Braçog a branch or sprout.

which by its spiral convolutions forms a continuous tube. These vessels are very apparent in the young shoots of plants, particularly those which can be readily broken without tearing, such as the rose, &c. They are formed in the medullary sheath (27) in the nervures of leaves, &c.; but are usually wanting in wood and bark, and are never seen in any of the lower tribes of plants. Their diameter varies from the three-hundredth to the three-thousandth of an inch.

B. Ducts are transparent tubes, the sides of which are marked with rings, bars, or transverse streaks. They differ essentially from Trachea by being inelastic, and incapable of unrolling. They are found in the wood of phænogamous plants, and of Ferns and Lycopodiæ.

4. "The office of all the ducts is the same-that of conveying fluid. It is only in the true spiral vessel that we find air."* And even here at certain

* Carpenter's Elem. Veg. Phys., p. 66.

"The functions of the Ducts' have not been accurately determined. It is probable that they act as spiral vessels when young; but it is certain that they become filled with fluid as soon as their spires are separated." (Lindley's Elements of Botany, p. 6.)

"There are some large Ducts which appear to have originated from cells, which have been placed together end to end, and whose partitions have been so broken down as to form one continuous tube. These are the largest vessels (if they may be truly so considered) in the whole vegetable fabric, and are of the class called 'dotted ducts :'-through them the sap principally rises." See Dr. Carpenter's Vegetable Physiology- 84, et seq.-The line of demarkation between the form of the true spiral vessel, and some of the ducts, is sometimes difficult to find; in some vessels there are obscure traces of spiral form, interrupted in places, and covered by membrane." In Ferns (which have no true spiral vessels), we find Ducts, which very closely approach the spiral vessel in character, having an unbroken coil of spiral fibre throughout their whole extent; but besides the important difference that these ducts are long, continuous tubes, they are

periods of the existence of a plant, fluid has also been found by recent observers; though if a branch be cut asunder whilst in a soft state, no juice is ever seen to issue from the orifice of a spiral vessel; and though, as the lymph is found to ascend in the stalks of mosses, &c., which do not possess these vessels, we may probably conclude that they are not requisite to the transmission of fluid, though occasionally so employed.

The Laticiferous Tissue consists of very delicate and anastomosing tubes, principally occurring in the young bark, and on the under sides of young leaves. They convey the fluid called Latex, or proper juice; which constitutes the nourishment of the young organs, and in which a curious oscillation of globules is visible in the bright sunshine, with a powerful microscope.*

5. Fibres and Layers. When a branch of a vascular plant is cut transversely, a certain number of points are observed, which are of a more compact character than the rest of the structure. If the branch be divided lengthwise, we shall perceive that these points are the ends of so many longitudinal threads, which will separate from the rest of the tissue more readily than they will themselves break. These threads are called fibres. With a microscope we can see that each fibre is composed of bundles of vessels, bound up and intermixed with cellular tissue. If we macerate the branch in water, after some time the fibres separate of themselves, as in the case of hemp, flax, &c. This separation in reality disorganizes the vegetable struc

further distinguished by the brittleness of the spire, which snaps when we attempt to unrol it." Ibid., § 82.

* For a further account of this and other local circulations, see Appendix A.