631. The gingko tree is another very interesting plant belonging to this same group. It is a relic of a genus which flourished in the remote past, and it is interesting also because of the resemblance of the leaves to some of the ferns like adiantum, which suggests that this form of the leaf in gingko has been inherited from some fern-like ancestor.
Fig. 370.
Zamia integrifolia, showing thick stem,
fern-like leaves, and cone of male flowers.
Fig. 371.
Two spermatozoids in end of pollen tube of
cycas. (After drawing by Hirase and Ikeno.)
632. While the resemblance of the leaves of some of the gymnosperms to those of the ferns suggests fern-like ancestors for the members of this group, there is stronger evidence of such ancestry in the fact that a prothallium can well be determined in the ovules. The endosperm with its well-formed archegonia is to be considered a prothallium.
633. Spermatozoids in some gymnosperms.—But within the past two years it has been discovered in gingko, cycas, and zamia, all belonging to this group, that the sperm cells are well-formed spermatozoids. In zamia each one is shaped somewhat like the half of a biconvex lens, and around the convex surface are several coils of cilia. After the pollen tube has grown down through the nucellus, and has reached a depression at the end of the prothallium (endosperm) where the archegonia are formed, the spermatozoids are set free from the pollen tube, swim around in a liquid in this depression, and later fuse with the egg. In gingko and cycas these spermatozoids were first discovered by Ikeno and Hirase in Japan, and later in zamia by Webber in this country. In figs. [371]-[374] the details of the male prothallia and of fertilization are shown.
Fig. 372.
Fertilization in cycas, small spermatozoid
fusing with the larger female nucleus of the
egg. The egg protoplasm fills the archegonium.
(From drawings by Hirase and Ikeno.)
