48 MORPHOLOGY OF GYMNOSPERMS 



rence in an undoubted strobilus of a lycopod made the relationship 

 clear. An integument or testa, growing from the surface of the 

 sporophyll, completely invests the megasporangium except for a 

 slitlike micropyle. To discover an embryo in such a structure, 

 however, would be very unlikely, for it develops continuously into the 

 sporeling. It would seem impossible to avoid the conclusion that a 

 prolonged period of intraseminal rest (as distinct from intraseminal 

 growth) must have been established gradually, and therefore that it 

 may not have been attained by paleozoic seeds in general. 



In comparing the majority of known paleozoic seeds with those of 

 angiosperms, it is evident that the latter have advanced in the recep- 

 tion of pollen by the micropyle, and in the nutrition of the embryo (86) ; 

 but that the general structure of paleozoic seeds is much more com- 

 plex, with their elaborate vascular system, pollen chamber, and often 

 highly differentiated testa. It has been suggested that the seed is 

 "an organ which required to be much more elaborate in the days of 

 spermatozoid fertilization, now only lingering in a few archaic sur- 

 vivals from the past" (86), as among cycads and in Glngko. 



3. The gametophytes 



THE FEM.A.LE GAMETOPHYTE 



The structure of the female gametophyte is largely a matter of 

 inference. In some cases delicate, collapsed tissue has been seen in 

 the embryo sac, but no archegonia, unless Stephanospermum (fig. 45) 

 and Renault's Gnetopsis elliptica (p. 37) belong to the Cycadofili- 

 cales, of which there can be little doubt. The similarity in the 

 general development of the gametophyte of such heterosporous 

 pteridophytes as Selaginella and Isoetes and of cycads suggests that 

 the endosperm of Cycadofilicales was of the same general nature in 

 development and structure. 



The embryo sac enlarges in the usual way, and is remarkable in 

 Physostoma in growing into the nucellar core of tissue which is sur- 

 rounded by the pollen chamber. In other known gymnosperms with 

 pollen chambers this core is either destroyed, resulting in a large 

 pollen chamber, or remains solid (as in Lagenostoma). If the exten- 

 sion of the embryo sac into the nucellar beak of Physostoma should 

 become filled with permanent endosperm tissue, there would be an 



