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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 cuneijolium, a fossil species related to the 

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

 bearing sporophylls (Sph); s,s, sporangia; s^, s^, 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 

 {homos porous), 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 



