1428 A TEXTBOOK OF THEORETICAL BOTANY 



Interpretation of the Embryo Sac 



The morphological interpretation of the embryo sac and its contained 

 structures in the light of comparison with other groups of seed plants, has 

 proved difficult. There are no really close comparisons possible with existing 

 Gymnosperms, still less with the remoter Pteridosperms. At the same time 

 the fundamental similarity which runs through all groups of the Angio- 

 sperms, combined with abundant variation in detail, reasonably suggests a 

 considerable antiquity and is probably traceable to the still obscure origin 

 and isolation of the Angiosperms as a class, in the late Mesozoic period. 



Speculation has not unnaturally been stimulated by the absence of 

 precise information and various schools of thought on the subject have found 

 expression and supporters. The simplest view is the most non-committal, 

 namely the acceptance of a general homology with the female gameto- 

 phyte of the Gymnosperms, while rejecting any detailed comparison of 

 components. This may be said to be Chamberlain's opinion, based on the 

 belief that the Angiospermic embryo sac has become so specialized that its 

 detailed homologies are obliterated. 



A second view is that all the nuclei of the embryo sac are potentially, 

 and were originally, gametes, and that their differentiation is secondary 

 and peculiar to the Angiosperms, so that, again, no comparisons are possible. 

 The idea that all the nuclei are potentially fertilizable invites an analogy, 

 though scarcely a homology, with the condition in the young embryo sacs 

 of Welwitschia and Gnetum and thus suggests a linkage between the Gnetales 

 and the early Angiosperms. According to one interpretation (Lotsy), the 

 micropylar cells are all gametic and equivalent to reduced archegonia, as in 

 Gnetum, and the antipodals are comparable to the degenerating gametic 

 nuclei in the same genus. Another view (Karsten) considers the antipodals 

 as a remnant of the cellular prothallus at the base of the Gnetum embryo sac, 

 and the polar nuclei as equivalent to free prothallus nuclei. 



The Gnetalean theory in one form or another has found many sup- 

 porters, partly no doubt because it harmonizes with theories of the Gneta- 

 lean origin of the Angiosperms, based on floral morphology and some vege- 

 tative similarities. That the Gnetales, especially Gnetu?n, do show close 

 approaches to the Angiosperms, is undeniable, though the similarities may 

 be due, as many think, to parallelism in evolution rather than to genetic con- 

 nection. The embryo sac comparisons do not greatly strengthen the latter 

 view. They draw into consideration, as intermediate stages, such sixteen- 

 nucleate sacs as those of Peperomia and Euphorbia. It is attractive to sup- 

 pose that these multinucleate sacs might be primitive and the eight-nucleate 

 state a later reduction, but the idea is open to the very strong objection that 

 these sacs are tetrasporic in origin, which it is wellnigh impossible to accept 

 as a primitive condition. 



The theory propounded by Strasburger in 1879 treated the antipodals, 

 the polar nuclei and the synergidae as prothallial cells, remnants of the female 

 prothallus of Gymnosperms. The oosphere alone represented the vestige 



