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).
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.
590. Sporangia of isoetes.—If we pull off some of the leaves of the plant we see that they are somewhat spoon-shaped as in [fig. 337]. In the inner surface of the expanded base we note a circular depression which seems to be of a different texture from the other portions of the leaf. This is a sporangium. Beside the spores on the inside of the sporangium, there are strands of sterile tissue which extend across the cavity. This is peculiar to isoetes of all the members of the class of plants to which the ferns belong, but it will be remembered that sterile strands of tissue are found in some of the liverworts in the form of elaters.
Fig. 337.
Base of leaf of isoetes, showing
sporangium with macrospores.
(Isoetes engelmannii.)