372 STRUCTURE AND LIFE HISTORIES 



elongate between the whorls of sporangiophores, so that 

 the shields occur in close contact. It is this cessation of 

 growth, in fact, that produces the cone; otherwise the 

 sporangiophores would occur in whorls distributed at 

 wider intervals along the axis. 



The sporangia arise from a single epidermal cell (euspo- 

 rangiate) on the underside of the shield; there are from 

 five to ten on each shield. 



FIG. 268. Sphenophyllum cuneifolium, a fossil species related to the 

 modern horsetails. Diagram of a longitudinal sectional view of an axis 

 bearing sporophylls (Sph); s,s, sporangia; s l , s l , sporangia in longitudinal 

 section, showing spores. A vascular bundle enters the stalk of each 

 sporangium. Enlarged. (Redrawn from Zeiller.) Cf. Fig. 280. 



The spores (which are green when ripe) are alike in size 

 (ho mosporous) , but they produce two kinds of gameto- 

 phytes, male and female (dioecious). Therefore they 

 must be unlike physiologically. Under certain circum- 

 stances, as already mentioned, (page 359), the female 



