=587. Female prothallia.=—The female prothallia are developed from the macrospores. The macrospores when mature have a rough, thick, hard wall. The female prothallium begins to develop inside of the macrospore before it leaves the sporangium. The protoplasm is richer near the wall of the spore and at the upper end. Here the nucleus divides a great many times, and finally cell walls are formed, so that a tissue of considerable extent is formed inside the wall of the spore, which is very different from what takes place in the ferns we have studied. As the prothallium matures the spore is cracked at the point where the three angles meet, as shown in fig. 334. The archegonia are developed in this exposed surface, and several can be seen in the illustration. [Illustration: Fig. 333. Section of mature macrospore of selaginella, showing female prothallium and archegonia. After Pfeffer.] [Illustration: Fig. 334. Mature female prothallium of selaginella, just bursting open the wall of macrospore, exposing archegonia. After Pfeffer.] [Illustration: Fig. 335. Seedling of selaginella still attached to the macrospore. After Campbell.] =588. Embryo.=—After fertilization the egg divides in such a way that a long cell called a suspensor is cut off from the upper side, which elongates and pushes the developing embryo down into the center of the spore, or what is now the female prothallium. Here it derives nourishment from the tissues of the prothallium, and eventually the root and stem emerge, while a process called the “foot” is still attached to the prothallium. When the root takes hold on the soil the embryo becomes free. CHAPTER XXXI. QUILLWORTS (ISOETES). [Illustration: Fig. 336. Isoetes, mature plant, sporophyte stage.] =589.= The quillworts, as they are popularly called, are very curious plants. They grow in wet marshy places. They receive their name from the supposed resemblance of the leaf to a quill. Fig. 336 represents one of these quillworts (Isoetes engelmannii). The leaves are the prominent part of the plant, and they are about all that can be seen except the roots, without removing the leaves. Each leaf, it will be seen, is long and needle-like, except the basal part, which is expanded, not very unlike, in outline, a scale of an onion. These expanded basal portions of the leaves closely overlap each other, and the very short stem is completely covered at all times. Fig. 338 is from a longitudinal section of a quillwort. It shows the form of the leaves from this view (side view), and also the general outline of the short stem, which is triangular. The stem is therefore a very short object.