OTHER FORMS OF ALTERNATION. 18;} 



detachment of gemmse, and like it connected with tlie more 

 diffused vitality of the lower organisms, owing to wliich the 

 spore has sufficient intrinsic plastic energy to develupe its 

 derivative cellular mass, even when severed from that con- 

 tinuity with the parent stock, which seems absolutely essen- 

 tial to the maturation of the ovule. 



The second diiferential character, on the other hand — 

 that of the great development of the cellular mass during 

 the germination of the spore, so as to form an external pro- 

 thallium — is not a constant one ; for, though it is well 

 marked in the ferns and Equisetacea3, it is absent in the 

 allied cryptogamic orders of L^^copodiace^e and Khizocarpea*. 

 In these orders the general course of phenomena is very 

 similar, only the cellular mass is never so much devebjped 

 as to rupture the spore -coat and appear externally, but is 

 limited to a stratum lying immediately beneath that por- 

 tion of the investing membrane of the spore which is per- 

 forated by a sort of micropyle. Hence the term endothal- 

 loid has been applied to these spores, and that of escothal- 

 hid to those of the true ferns.* Neither ao-ain is the ab- 

 sence of a prothallial structure universal among the Pha- 

 nerogamia, for in the ovule of the Coniferas a peculiar mass 

 of cellular tissue of a very analogous kind — the albuminous 

 body — is formed within the nucleus, but, like the prothal- 

 lium of the Ehizocarp, it never protrudes externally. The 

 ovule of the Coniferae being naked, like the spore of the 

 Cryptogamia at the time of impregnation, the additional 

 protection of a prothallial structure may possibly be re- 

 quired as a substitute for the germen, which encloses the 

 ovules in the higher Plianero2;amia till the embryo is fully 

 formed. 



A similar inconstancy attaches to the third point of dif- 

 ference — the multiplication of archegonia in the prothallium 



* Jennerin Ed. New PliO. Jour., III., 279 (April, 185G). 



