662. One-celled plants. The cell, as heretofore described, is endowed with a life within itself. It can imbibe fluids, nourish itself, and reproduce others like itself. It may, therefore, and actually does in some cases, exist alone as a plant! Many species of the Confervoids and Diatomes are plants consisting of a single cell - the simplest possible form of vegetation.

663. Plants many-celled. With a few such exceptions, vegetation consists of a combination of cells united in a definite manner and form.

Such combinations arc called tissues, which we may describe under four general names or types:

I. Cellular tissue (Parenchyma): II. Fibrous tissue (Pleurenchyma):

III. Vascular tissue (Trachenchyma):

IV. Laticiferous tissue (Cienchyma).

664. Parenchyma, composed of spheroidal cells, is the most com. mon form of tissue, no plant being without it, and many, especially of the lower orders, being entirely composed of it. Numerous varieties occur according to the forms of the cells and their closeness of contact, intermediate between the following extremes, 1, when there are copious intercellular spaces, the cells slightly touching, and being (a) rounded, or (b) lobed, or (c) stellate; 2, when the cells are crowded, leaving no intercellular space and being (d) prismatic, or (e) polyhedral, or (f) irregular.

665. Examples of these tissues are found (a) in the pulp of fruits, in newly-formed pith, and in all young growths; (b) in the lower stratum of leaf-tissue; (c) in the pith of rushes and other aquatic plants; (d) in the herbaceous stems of Monocotyledons; (e) everywhere, but well observed in full-formed pith; (f) abundant in all the soft, fleshy parts of plants.

666. Pleurenchyma is composed of elongated cells cohering by their sides in such a way that end overreaches end, forming a continuous fibre. Two varieties are noticed (a) wood-fibre, with cells of moderate length, remarkable for its firmness, the main constituent of the steins and trunks of the higher plants; (b) liber, with very long attenuated cells, the substance of the inner layers of bark, remarkable for its tenacity, especially in flax, hemp, linden.

667. The pitted cells (§ 650) constitute a singular variety of wood-fiber, common in pines, firs, etc. That mysterious double ring which encircles each pit, is projected, the inner by the pit itself, which is an aperture in the secondary layer, the outer by a lens-shaped intercellular cavity right opposite outside. (570).

668. Trachenchyma is a tissue of vessels or tubes rather than cells. The vessels are extended lengthwise, and composed each of a row of cells joined end to end, and fused into one by the absorption of the contiguous walls. This tissue varies according to the character of the constituent cells, which are (a) spiral, or (b) annular, or (c) sclariform, or (d) reticulated.

579, Longitudinal section of Thuja (Red Cedar)

579, Longitudinal section of Thuja (Red Cedar). a, Medullary rays.

669. Such cells, with their tapering ends, form vessels with oblique joints. When porous cells (653) with their truncated ends unite they form right-jointed vessels resembling strings of beads, called dotted or vascular ducts. These are usually quite large, and characteristic of the woody layers of all exogenous plants. (470.)

670. The different varieties of trachenchyma are assigned to different regions and offices, (a) to the earliest formed part of the wood, the petioles and veins of leaves, petals of flowers, etc.; (b) to similar parts, but later formed, most abundant in ferns and Equisetaceae; (c) in the woody bundles of the Endogens and in the succulent parts of plants in general; (d) most abundant in ferns, club-mosses.

671. Cienchyma is a system of milk-vessels - vessels secreting the latex or peculiar juice of the plant, white, yellow, red, turbid, containing opium, gamboge, caoutchouc, resin, etc. It occurs in the petioles and veins; in the parenchyma of roots, in the liber especially; sometimes simple, generally branched and netted in a complicated manner, as well seen in the poppy, celandine, blood-root, gum-elastic tree, etc.

672. Their nature. These vessels are probably mere open spaces between the cells at first, subsequently acquiring a lining membrane which never exhibits pores or spiral markings. But there are also true

673. Intercellular passages filled with air and admitting its free circulation in all directions through the parenchyma. These are necessarily very irregular, and they communicate with the external air through the stomata. (§ 678.)

674. Import of the cell. Thus the cell appears to be the type of every form of tissue, the material of which the vegetable fabric is built. and the laboratory where the work is performed.

675. Elevation in rank is marked by the increasing complication of the tissues. The basis of the structure of all plants is parenchyma. In the lowest tribes no other tissue is ever added, this alone performing all the functions. Higher in the scale, as in mosses, a few central bundles of wood tissue are added, as if to strengthen the stem. Still higher, as in ferns, etc., we begin to find vessels (trachenchyma) of the simpler sort, for the freer circulation of the fluids, together with the strengthening pleurenchyma. Lastly, in the highest plants, Phaenogamia, the true spiral vessels appear, filled with air, cienchyma with secretions, and all the tissues in their appropriate functions.

Vessels of Cienchyma; 530, from Dandelion; 5S1, from the Celandine

Vessels of Cienchyma; 530, from Dandelion; 5S1, from the Celandine.