194 MORPHOLOGY 
a more or less expanded terminal sterile portion (figs. 440, 441). The 
sporangia are borne on the abaxial surface of the sporophyll, varying 
in number from 1000 or more in Cycas (fig. 440) to very few in Zamia 
(fig. 441). They may cover the whole face of the sporophyll, or may 
occur only on the two flanks. Usually they are in definite son of two to 
six sporangia, and often they are more or less united at base. 
FIG. 438. Cross section of a stem of Zamia, showing the thick cortex, thin vascular 
cylinder, large pith, and curving leaf traces (girdles) ; the vascular cylinder is seen to 
consist of vascular bundles (xylem next the pith, phloem next the cortex) separated by 
pith rays; partly diagrammatic. 
The sporangia are eusporangiate in development, but the initial cell 
or cells are hypodermal (under the epidermis), and not superficial as 
among the pteridophytes. The initials usually form a hypodermal plate 
of four cells which divide periclinally into two plates, the outer giving 
rise to the four to seven wall layers (overlaid by the epidermis), the inner 
giving rise to the mass of sporogenous tissue. The tapetum is organized 
from the peripheral layer of sporogenous cells. The output of spores 
per sporangium varies from 500 in Zamia to 26,000 in Encephalartos. 
The abaxial distribution of sporangia, the sori, the large output, the 
dehiscence, all resemble ferns of the Marattia type. 
