=628. Female prothallium of cycas.=—In figs. 367, 368, are shown mature ovules, or macrosporangia, of cycas. In 368, which is a roentgen-ray photograph of 367, the oval prothallium can be seen. So in cycas, as in selaginella, the female prothallium is developed entirely inside of the macrosporangium, and derives the nutriment for its growth from the cycas plant, which is the sporophyte. Archegonia are developed in this internal mass of cells. This aids us in determining that it is the prothallium. In cycas it is also called endosperm, just as in the pines. [Illustration: Fig. 367. Macrosporangium of Cycas revoluta.] [Illustration: Fig. 368. Roentgen photograph of same, showing female prothallium.] [Illustration: Fig. 369. A sporophyll (stamen) of cycas; sporangia in groups on the under side. _b_, group of sporangia; _c_, open sporangia. (From Warming.)] =629.= If we cut open one of the mature ovules, we can see the endosperm (prothallium) as a whitish mass of tissue. Immediately surrounding it at maturity is a thin, papery tissue, the remains of the nucellus (macrosporangium), and outside of this are the coats of the ovule, an outer fleshy one and an inner stony one. =630. Microspores, or pollen, of cycas.=—The cycas plant illustrated in the frontispiece is a female plant. Male plants also exist which have small leaves in the center that bear only microsporangia. These leaves, while they resemble the ordinary leaves, are smaller and correspond to the stamens. Upon the under side, as shown in fig. 369, the microsporangia are borne in groups of three or four, and these contain the microspores, or pollen grains. The arrangement of these microsporangia on the under side of the cycas leaves bears a strong resemblance to the arrangement of the sporangia on the under side of the leaves of some ferns. =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. [Illustration: Fig. 370. Zamia integrifolia, showing thick stem, fern-like leaves, and cone of male flowers.] [Illustration: 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.