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 t s 1 , 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 
(homosporous), 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 
