Running around the outside of the ripe spore case is a single row of cells (_an._), differing from the others in shape, and having their inner walls thickened. Near the bottom, two (sometimes four) of these cells are wider than the others, and their walls are more strongly thickened. It is at this place (_st._) that the spore case opens. When the ripe sporangium becomes dry, the ring of thickened cells (_an._) contracts more strongly than the others, and acts like a spring pulling the sporangium open and shaking out the spores, which germinate readily under favorable conditions, and form after a time the sexual plants (prothallia). 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.