1902J A MORPHOLOGICAL STUDY OF ASCLEPIADACEAE 405 



probable that this often occurs. It seems that the normal burst- 

 ing of the end of the tube after penetrating the embryo sac might 

 be due to the same cause. 



The polar nuclei have usually not fused when the tube bursts. 



j The antipodal one is somewhat below the middle of the sac, 



while the micropylar one is usually somewhat above it {^fig. 48), 



I Before the polars have fused, one of the male cells unites with the 



antipodal one, and about the same time the other male cell fuses 

 with the egg {fig- 48). The male cells are crescentic in form, 

 reticulated with granules, and not conspicuously unlike in size. 

 In one case a male cell was found fusing with the polars after 

 their union. This does not seem singular, however, when we 

 recall the method of pollination. Insect pollination requires 

 some range in time for the process. Fertilization may then occur 

 at different stages, or the embryo sacs must remain stationary in 

 the proper stage until the male cells arrive. But unfertilized ova- 

 ries show that the embryo sacs pass into the seven-celled stage 

 {figs, 4j, 44, 46)\ that is, the polars unite whether fertilization 

 has taken place or not. Again, the older ovules are nearer the 

 base of the ovarian cavity, while the pollen tubes enter in a 

 mass at the top. The younger ones it seems, in any given 

 ovary, would be fertilized first, with the result that fertilization 

 in different stages of fusion of polars would occur. In fact, the 

 literature of doubleTertilization gives us instances of fertiliza- 

 tion before, during, and after fusion of polars, but not all in the 

 same species. The fusion of a male cell with the antipodal 

 polar is characteristic of A. Cormtti when fertilization occurs 

 before fusion of the polars; but it is not unique in plants, for 

 Guignard (9) reports seeing a case of it in Liliiim Mariagon, He 

 thinks, however, that the male cell goes to the polar nearest it at 

 the time of its issue from the tube. The conditions in A, Cornuti 

 make such a theory improbable, at least for that species. The 

 micropylar polar was in all the cases observed nearer the end of 

 the tube than the antipodal {fig, 48), yet in all cases of fertili- 

 zation before polar fusion the male cell fused with the antipodal 

 polar. If the male cells have independent motion, the question 

 arises as to what impels the male cell in one plant to seek the 



t 



