[Illustration: FIG. 69.--_A_, mother cell of the sporangium of the maiden-hair fern, × 300. _B_, young sporangium, surface view, × 150: i, from the side; ii, from above. _C-E_, successive stages in the development of the sporangium seen in optical section, × 150. _F_, nearly ripe sporangium, × 50: i, from in front; ii, from the side. _an._ ring. _st._ point of opening. _G_, group of four spores, × 150. _H_, a single spore, × 300.] By holding a leaflet between two pieces of pith, and using a very sharp razor, cross-sections can be made. Such a section is shown in Fig. 68, _A_. The epidermis (_e_) bounds the upper and lower surfaces, and if a vein (_f.b._) is cut across its structure is found to be like that of the fibro-vascular bundle of the leaf stalk, but much simplified. The ground tissue of the leaf is composed of very loose, thin-walled cells, containing numerous chloroplasts. Between them are large and numerous intercellular spaces, filled with air, and communicating with the breathing pores. These are the principal assimilating cells of the plant; _i.e._ they are principally concerned in the absorption and decomposition of carbonic acid from the atmosphere, and the manufacture of starch. The spore cases, or sporangia (Fig. 69), are at first little papillæ (_A_), arising from the epidermal cells, from which they are early cut off by a cross-wall. In the upper cell several walls next arise, forming a short stalk, composed of three rows of cells, and an upper nearly spherical cell--the sporangium proper. The latter now divides by four walls (_B_, _C_, i-iv), into a central tetrahedral cell, and four outer ones. The central cell, whose contents are much denser than the outer ones, divides again by walls parallel to those first formed, so that the young sporangium now consists of a central cell, surrounded by two outer layers of cells. From the central cell a group of cells is formed by further divisions (_D_), which finally become entirely separated from each other. The outer cells of the spore case divide only by walls, at right angles to their outer surface, so that the wall is never more than two cells thick. Later, the inner of these two layers becomes disorganized, so that the central mass of cells floats free in the cavity of the sporangium, which is now surrounded by but a single layer of cells (_E_). Each of the central cells divides into four spores, precisely as in the bryophytes. The young spores (_G_, _H_) are nearly colorless and are tetrahedral (like a three-sided pyramid) in form. As they ripen, chlorophyll is formed in them, and some oil. The wall becomes differentiated into three layers, the outer opaque and brown, the two inner more delicate and colorless.