Anatomy of Plants

The marginal cells of the plates are frequently produced as teeth and their walls are thick. The centre of the leaf is often occupied by a midrib consisting of several layers of parenchymatous cells elongated in the direction of the long axis of the leaf and poor in chloroplasts. This midrib may be considered a primitive con ductive foliar strand or leaf-bundle. Associated with this con ducting parenchyma are frequently found hydroids like those of the central strand of the stem, and in some cases continued into the cortex of the stem as a leaf-trace bundle (the anatomically demonstrable trace of the leaf in the stem). In several cases the leaf-trace runs vertically downwards for some distance in the outer cortex and ends blindly in a fan-shaped expansion : in others it joins the central hydrom strand so that a connected water conducting system is established between stem and leaf.

Further differentiation of tissues characterizes the highest family of mosses, the Polytrichaceae. In these elongated, living, nucleated cells with a thin lining of protoplasm surround the dead hydrom, and form the leptom, inferred to serve for the conduction of organic substances, since the entire cavity of a leptom cell is sometimes occupied by proteid contents. The ends of each lep tom cell are slightly swollen and fit to the similar swollen ends of the next leptoids in a row. The end walls are usually very thin, and the protoplasm, on artificial contraction, commonly sticks to them though no perforations of the walls have been found. It is considered that these cells are in some measure comparable to the sieve-tubes of higher plants (see below). Associated with the leptoids are similar cells without swollen ends and with thick end walls, while between the hydrom and leptom is a cylinder of cell-layers, known as amylon, which may serve for the tem porary reservation and in the distribution of carbohydrates, since they sometimes contain an abundance of starch. The under ground portion of the stem (rhizome) bears rhizoids and simple scales. The aerial parts bear leaves, each with a simple midrib several cells thick with a strong band of stereom above and be low a bundle of leptom, hydrom and amylon cells which join the central cylinder of the stem. Each midrib bears two wings, one cell thick, while above the midrib is a series of closely set, vertical, longitudinally-running plates of green assimilative cells.

Vascular Plants.

It is, however, in the higher vascular plants (Pteridophyta, e.g., ferns, horse-tails, club-mosses and Phanero gama or flowering plants) that the greatest anatomical complexity is found, for these are the dominant land plant;. Such plants are not exactly comparable with the bryophytes for while the body of the latter bears the sexual organs and is called a gametophyte, and later nourishes a simple spore-bearing body (sporophyte), the vascular plant is a sporophyte which later nourishes a simple and reduced gametophyte. Nevertheless the gametophytic bryo phyte and the sporophytic vascular plant have similar physiologi cal needs and are both fixed to the soil. The chief new feature in

external conformation of the body of a vascular plant compared with that of a bryophyte is the presence of "true" roots, the first formed of which is a downward prolongation of the primary axis of the plant. From this and from various parts of the shoot system other roots originate. The roots of Pteridophytes are structurally simple and uniform compared with those of Phanero gams, but all manifest a primary plan of construction in direct relation to their normally subterranean life and fixative and ab sorptive functions ; they differ from aerial stems in the characters of their surface tissues, in the absence of the green assimilative pigment chlorophyll, in the arrangement of their vascular systems, and in their mode of apical growth. Great variety in stem- and leaf-form and structure is shown by the vascular plants.

But in spite of the many considerable differences of detail between Pteridophytes and Phanerogams, we can trace, alike in root, stem and leaf a threefold division of tissue systems, already indicated among the bryophytes, and expressive of the funda mental conditions of evolution of the bulky body of a land plant. Thus there is (I) a specialization of a surface layer of cells which regulates the immediate relations of the plant with its surround ings, while varying in expression in the subterranean and aerial parts. In the former the surface layer is pre-eminently absorptive and in the latter protective provision at the same time being made for the gaseous interchange of oxygen and carbon dioxide involved in respiration, and for such vital functions as assimilation and transpiration. This surface layer is known as the epidermis. On the other hand we have (2) an internal differentiation of con ductive tissue, prefigured in the bryophytes, and collectively known in the vascular plants as the vascular system. The re maining tissue of the plant-body, which shows varied local dif ferentiation and is concerned with special functions, is known by reason of its peripheral position in relation to the central con ductive tissue as (3) the cortex. But besides absorption, as similation, conduction and protection, provision is made for sup port and the storage of food. Thus locally in the cortex and vas cular system special supporting tissues (stereom) may be de veloped according to the varied needs and the conditions of body differentiation, and living cells of both the cortex and the vascular system may serve for temporary storage of foods and are spe cialized accordingly. The functions of aeration, assimilation and transpiration, by which the bulky tissues of the plant-body ob tain oxygen for breathing and carbon dioxide for food-manu facture, and the removal of excess water is assured, are main tained by an extensive system of intercellular spaces communicat ing with the external air.