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 IGOO 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 sori 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. 



