Folsom: morphology of yucca glauca 431 



(fig. ?)Z). In several instances only one was seen and then it was 

 observed to be either between the tgg and definitive nucleus or in 

 process of uniting with the latter ; probably in these cases fusion of 

 the other male nucleus with the €:gg had already taken place. Al- 

 though the latter fusion was not observed, double fertilization is in- 

 dicated by the fact that mitoses in somatic and endosperm cells show 

 respectively twelve and eighteen chromosomes, twice and three 

 times the haploid number. 



Ovule After Fertilization 



When the pollen tube enters the embryo sac, one of the synergids 

 disintegrates (figs. 32, ZZ), while the other persists (fig. 34) until 

 the proembryo is formed. After fertilization, wa,lls are distinctly 

 seen around the antipodal cells, whose nuclei begin to disintegrate 

 before fertilization (fig. 30) and continue to do so afterwards (fig. 

 32 ) , but have not entirely disappeared even when the young embryo 

 is formed. Evidently double fertilization is soon followed by a di- 

 vision of the definitive nucleus, for in the many cases observed (see 

 table I) where the pollen tube had entered the embryo sac and the 

 male nuclei were no longer to be seen, the definite nucleus had 

 divided to begin the formation of free endosperm nuclei (figs. 36, 

 37). Walls appear between these after the formation of the embryo 

 (fig. 39). 



The fertilized egg remains dormant for about four days (fig. 

 34). Then it divides and a proembryo is formed (figs. 36-38), con- 

 sisting finally of a row of from four to eight cells, the basal one 

 usually being larger than any other but sometimes becoming divided 

 by an oblique wall after two or more divisions of the egg. In the 

 most mature ovules obtained, the suspensor consists of a row of 

 four to eight cells and the young embryo is ovoid with two rather 

 flat surfaces parallel to the two broad faces of the seed,' one of these 

 surfaces containing a slight depression (figs. 39, 40, 42, 43). 



Comparisons with Observations on Other Species of Yucca 



The somatic number of chromosomes is twelve and they are all 

 uniform in size, while in Yucca aloifolia L., Y. Draconis Torr., and 

 F. guatemalensis L., Miiller reports ten large and many small ones.*' 

 In Yucca filamcntosa L. the tapetal cells are like the megaspores 



