THEORETICAL CONCLUSIONS 421 



gous with the similar temporary or permanent vacuole seen in 

 Gnetum and other gymnosperms. 



3. The cells in the angiosperm embryo sac are homologous with 

 the peripheral cells in the gametophyte of Gnetum; the egg is a 

 fertile peripheral cell or an arrested archegonium, and the antipodal 

 cells correspond to the lower nutritive part of the gametophyte of 

 Gnetum. 



4. The endosperm of angiosperms is arrested game tophy tic tissue 

 which is stimulated to further development through fusion with a 

 male gamete. 



Both these views, although interesting, leave one point unex- 

 plained. In the angiosperms the endosperm is formed only after 

 the polar nuclei have fused with a male gamete. In Gnetum, on 

 the other hand, there is no such fusion. A further difficulty, al- 

 though less serious, is the presence of the synergids in one group 

 and their absence in the other. They no doubt seem to be unes- 

 sential elements, for embryo sacs without synergids (Plumbagella 

 and Plumbago) seem to function just as satisfactorily as those 

 with them; nevertheless the fact that they are the usual accom- 

 paniments of the angiosperm egg demands an explanation, which is 

 not yet available. 



If the embryo sac of angiosperms were to consist of only an egg 

 and a variable number of free nuclei, irregularly placed at the 

 periphery, we could probably assume its derivation from a condi- 

 tion like that in Gnetum. As it is, however, it seems best to con- 

 clude that while the angiosperms have probably passed through 

 some such stages as are shown by Gnetum, we have no decisive 

 evidence in favor of this view. Considered in this light, therefore, 

 the Gnetalean view, although the most attractive of the three we 

 have considered, can only be regarded as a working hypothesis, 

 useful to stimulate further research, but entirely tentative for the 

 present. Regarding the other two theories, proposed by Porsch 

 and SchurhofT, there is now little to support them. 



Fertilization. In gymnosperms the pollen grains land directly 

 on the nucellus and the pollen tube has to grow only a short dis- 

 tance in order to reach the archegonium. In Larix and Pseudo- 

 tsuga the apical portion of the integument becomes stigmatic, and 

 in Tsupa. Araucaria, and Agathis pollen may germinate even on 



