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.
Fig. 367.
Macrosporangium of
Cycas revoluta.
Fig. 368.
Roentgen photograph of same,
showing female prothallium.
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.