5° 



MORPHOLOGY OF GYMNOSPERMS 



in obtaining sections of them, showing the internal structure (fig. 50). 

 The cavity of the grain is completely filled (allowing for shrinkage) 

 with a tissue of about twenty cells, arranged in five rows radially 

 disposed about a central axis. In another species of Stephanosper- 

 mum the same observer (47) has described pollen grains containing 

 only two cells in addition to the large tube cell, a structure at once 

 suggesting that found in existing gymnosperms. 



Since the pollen grains of certain undoubted Cordaitales have 

 long been known to contain a tissue similar to that just described 

 for the Cycadofilicales, the inference is strong that this represents a 



general condition of the male 

 gametophyte among paleozoic 

 gymnosperms. Even among 

 existing gymnosperms there 

 are cases of excessive pro- 

 thallial tissue (as among the 

 podocarps and araucarians) 

 and of excessive spermato- 

 genous tissue (as in Micro- 

 cycas); so that the internal 

 tissue of the paleozoic pollen 

 grains may be either mainly 

 prothallial or mainly sper- 

 matogenous; or both tissues 

 may occur in more equal proportion. In our specimens of the pollen 

 grains of Physostoma, the internal tissue is not restricted to one pole of 

 the spore, as is true of all male prothallial tissue among living gymno- 

 sperms, but is just as conspicuous in the middle region of the spore. 

 We are inclined to the belief, therefore, that most of these internal 

 cells are spermafogenous. However this may be, the chief interest 

 lies in the fact that the male gametophyte of the paleozoic gymno- 

 sperms is a more extensive tissue than in most living gymnosperms; 

 and that in all probability it is not restricted to the production of 

 two sperms. That these sperms were discharged into the cavity of 

 the pollen chamber, after its base had broken down to the necks of 

 the archegonia, can scarcely be questioned. There is no evidence of 

 the development of pollen tubes as sperm carriers; and that they did 



50 51 



Fig. 50. — Stephanospermum akenioides: 

 transverse section of pollen grain showing inter- 

 nal walls; X 180. — After Oliver (47). 



Fig. 51. — Physostoma elegans: section of 

 pollen grain showing internal walls; the dotted 

 area is the exospore where it has not been 

 ground away; X480. — After Oliver (85). 



