The roots of the sporophyte arise in large numbers, the youngest being always nearest the growing point of the stem or larger roots ([Fig. 67], C). The growing roots are pointed at the end which is also light-colored, the older parts becoming dark brown. A cross-section of the older portions shows a dark-brown ground tissue with a central, light-colored, circular, fibro-vascular bundle ([Fig. 68], F). Growing from its outer surface are numerous brown root hairs (r).
When magnified the walls of all the outer cells (epidermis and ground tissue) are found to be dark-colored but not very thick, and the cells are usually filled with starch. There is a bundle sheath of much-flattened cells separating the fibro-vascular bundle from the ground tissue. The bundle ([Fig. 68], G) shows a band of tracheary tissue in the centre surrounded by colorless cells, all about alike.
All of the organs of the fern grow from a definite apical cell, but it is difficult to study except in the root.
Selecting a fresh, pretty large root, a series of thin longitudinal sections should be made either holding the root directly in the fingers or placing it between pieces of pith. In order to avoid drying of the sections, as is indeed true in cutting any delicate tissue, it is a good plan to wet the blade of the razor. If the section has passed through the apex, it will show the structure shown in [Figure 68], D. The apical cell (a) is large and distinct, irregularly triangular in outline. It is really a triangular pyramid (tetrahedron) with the base upward, which is shown by making a series of cross-sections through the root tip, and comparing them with the longitudinal sections. The cross-section of the apical cell (Fig. L) appears also triangular, showing all its faces to be triangles. Regular series of segments are cut off in succession from each of the four faces of the apical cell. These segments undergo regular divisions also, so that very early a differentiation of the tissues is evident, and the three tissue systems (epidermal, ground, and fibro-vascular) may be traced almost to the apex of the root (68, D). From the outer series of segments is derived the peculiar structure (root cap) covering the delicate growing point and protecting it from injury.
The apices of the stem and leaves, being otherwise protected, develop segments only from the sides of the apical cell, the outer face never having segments cut off from it.
CHAPTER XIII.
CLASSIFICATION OF THE PTERIDOPHYTES.
There are three well-marked classes of the Pteridophytes: the ferns (Filicinæ); horse-tails (Equisetinæ); and the club mosses (Lycopodinæ).
Class I.—Ferns (Filicinæ).
The ferns constitute by far the greater number of pteridophytes, and their general structure corresponds with that of the maiden-hair fern described. There are three orders, of which two, the true ferns (Filices) and the adder-tongues (Ophioglossaceæ), are represented in the United States. A third order, intermediate in some respects between these two, and called the ringless ferns (Marattiaceæ), has no representatives within our territory.
The classification is at present based largely upon the characters of the sporophyte, the sexual plants being still very imperfectly known in many forms.
The adder-tongues (Ophioglossaceæ) are mostly plants of rather small size, ranging from about ten to fifty centimetres in height. There are two genera in the United States, the true adder-tongues (Ophioglossum) and the grape ferns (Botrychium). They send up but one leaf each year, and this in fruiting specimens ([Fig. 70], A) is divided into two portions, the spore bearing (x) and the green vegetative part. In Botrychium the leaves are more or less deeply divided, and the sporangia distinct ([Fig. 71], B). In Ophioglossum the sterile division of the leaf is usually smooth and undivided, and the spore-bearing division forms a sort of spike, and the sporangia are much less distinct. The sporangia in both differ essentially from those of the true ferns in not being derived from a single epidermal cell, but are developed in part from the ground tissue of the leaf.