CYCADOFILICALES 



43 



the integument or testa, in striking contrast with the ovules of living 

 gymnosperms, in which the integument and nucellus are free only 

 in the region of the pollen chamber or nucellar beak. A long and 

 strong nucellar beak extends into 

 the micropyle, and within it a pollen 

 chamber of great relative size is 

 developed. The vascular strand 

 entering the chalaza expands into a 

 plate, whose margin is continued 

 as a tracheal mantle investing the 

 nucellus beneath the epidermis 

 and extending to the pollen cham- 

 ber, "the floor of which is paved 

 with tracheids." Oliver sees in this 

 distribution of vascular elements 

 a mechanism for the conduction 

 of water for the use of swimming 

 sperms in the pollen chamber. 

 While the pollen chamber is associ- 

 ated with swimming sperms, there 

 is no evidence that a supply of 

 water is necessary. 



Contrasting Stephanospermum 

 with Lagenostoma, it is evident that 

 the latter approaches the structure 

 of cycadean seeds more nearly 

 than does the former, especially in 

 the lack of freedom of nucellus 

 and integument, and in the greater 

 simplicity of the vascular system. 

 On the other hand, the pollen 

 chamber of Stephanospermum is 

 more cycadean, as well as the testa 

 in the development of the three 



characteristic layers. It should be remembered that Lagenostoma is 

 lower carboniferous, while Stephanospermum is permo-carboniferous, 

 so that historically the former is much the older, but its structure shows 

 it to have been far from the simplest type of seed. In living gymno- 



FiG. 45. — Stephanospermum akeni- 

 oides: partial restoration; m, micropyle; 

 b, beak of nucellus; p, pollen chamber; 

 0, outer (sclerotic), and i, inner layer of 

 integument, which is free from nucellus 

 to base of seed; t, tracheal mantle; a, 

 archegonia; w, megaspore membrane; 

 Xio. — After Oliver (47). 



