THE AXGIOSPERMAE 1429 



of an archegonium. The impetus towards the formation of prothallus tissue 

 was supposed to become exhausted, and only renewed after the stimulus of 

 fertilization. In this view the formation of endosperm was looked upon as a 

 continuation of the interrupted prothallus formation. This theory was put 

 forward before the polyploidy of the endosperm cells was understood and it 

 is certainly difficult to regard a tissue which is normally triploid, and may 

 sometimes be highly polyploid, as being prothallial or in any way an exten- 

 sion of the haploid cells of the embryo sac. Even the antipodal cells in the 

 Fritillaria type of embr^'o sac are triploid, which is equally against their 

 being prothallus cells. This theory does not explain why the antipodal cells 

 should be, in so many cases, arranged like a duplicate of the micropylar 

 cell group, a point of some importance. 



Schiirhoif put forward a fourth theor\- in 19 19. He considered that 

 there is one archegonium, reduced to two cells, the oosphere and one of the 

 synergidae, the synergid representing the ventral canal cell of the arche- 

 gonium. The second synergid and the antipodals represent prothallial cells 

 and the two polar nuclei are free prothallial nuclei. This initial prothallus 

 formation is then completed by the growth of the endosperm, in the same 

 way that Strasburger supposed. This hypothesis encounters the formidable 

 difficulty that the synergidae are, at least in the vast majority of plants, 

 sister nuclei and Schiirhoff has not been able to produce any instance in 

 which the oosphere and the synergid are sister nuclei, as they should be 

 according to his theory. He rejects a derivation of the Angiosperms from 

 Gymnosperms and considers that both have originated separately from 

 archegoniate ancestors, which implies an independent evolution of the seed 

 habit in each line. 



In 1928 Schiirhoff tried to meet the objections to his first theory by pro- 

 posing a second, namely that one synergid is the ventral canal cell of the 

 oosphere and the other is the ventral canal cell of the upper polar nucleus, 

 the latter being a potential oosphere. There are thus two archegonia, of two 

 cells apiece, and four chalazal prothallial cells. This again involves denying 

 that the synergidae are sister cells, and the author argues that neither from 

 the nuclear divisions nor from the cell wall formation can any proof be 

 derived that they are. Equally, in this case, no proof can be forthcoming 

 for his own view and we are left to balance probabilities. 



The last theory we shall consider is that put forward by Porsch in 1907, 

 which was anticipated in part by Marshall Ward, as long ago as 1880. He 

 postulated that the embryo sac was a prothallus reduced to two fused arche- 

 gonia, which have retained the essentials of archegonial structure, although 

 the vegetative prothallus has disappeared. Analogies for this can be found 

 in the male gametophytes of some Gymnosperms and even among Pteri- 

 dophyta. Each polar group of four nuclei represents one archegonium, con- 

 sisting of two neck cells, namely the synergidae, one oosphere, and one 

 ventral canal cell, which has become the free polar nucleus. Thus the 

 similar organization of the antipodals and the egg-apparatus is explained by 

 their homology, although the antipodal archegonium is normallv sterile. 



