OTHER FORMS OF ALTERNATION. 183 



detachment of gemmae, and like it connected with the more 

 diffused vitality of the lower organisms, owing to which the 

 spore has sufficient intrinsic plastic energy to develope 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 differential 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 Equisetacere, it is absent in the 

 allied cryptogamic orders of Lycopodiacese and Rhizocarpese. 

 In these orders the general course of phenomena is very 

 similar, only the cellular mass is never so much developed 

 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 exothal- 

 loid to those of the true ferns.* Neither again is the ab- 

 sence of a prothallial structure universal among the Pha- 

 nerogamia, for in the ovule of the Coniferse 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 Coniferse 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 Phanerogamia 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 Ec\ New Phil. Jour., III., 279 (April, 1856). 



