Campbell: The embryo-sac of Pandanus 211 



adjoining cells of the nucellus, and the nucleus is somewhat larger. 

 The nucleolus in this case lay in the cytoplasm near the nucleus, 

 but this was doubtless the result of the nucleus having been sec- 

 tioned and the nucleolus accidentally displaced in mounting the 

 section. At this stage the embryo-sac had enlarged very little, 

 being only slightly broader than the sister-cell above it, which in 

 this instance was rather larger than usual, although there is a good 

 deal of difference in different cases in this particular. Figure 4 



F 



shows a slightly more advanced stage. The upper sporogenous 

 cell is smaller and the nucleus less conspicuous than usual. The 

 appearance of the transverse division in the upper cell is probably 

 due to a shrinkage of the protoplasm, and this probably also ac- 

 counts for the very small and poorly defined nucleus. The cyto- 

 plasm of the young embryo-sac is vacuolate, and the conspicuous 

 nucleus with its single nucleolus occupies the center of the cell, 

 A slightly older stage is shown in figure 5. The vacuoles have 

 become more numerous but less definite and the nucleus is notice- 

 ably larger. The sister-cell has divided longitudinally instead of 

 transversely as is the case in most angiosperms. While this longi- 

 tudinal division is rather unusual, it has been observed in a num- 

 ber of other forms, both in monocotyledons and dicotyledons.* 



The young embryo-sac rapidly increases in size and the nucleus 

 divides, one of the daughter nuclei moving to the upper end of the 

 sac, the other to the chalazal end. The two nuclei are quite 

 similar in appearance (fig. 8), and the cytoplasm still occupies the 

 whole cavity of the sac, although there are numerous large vacu- 

 oles. The large vacuoles in the center of the sac finally unite, 

 and by the time the second mitosis is complete (fig. 9) a single 

 large vacuole occupies the greater part of the sac and the cyto- 

 plasm is mainly confined to the ends of the sac, there being only 

 a thin layer lining the lateral walls. ^ 



Up to this point Pandanus agrees exactly with the typical 

 angiosperms, but the subsequent history of the embryo-sac is de- 

 cidedly different. The two nuclei at the micropylar end of the 

 sac undergo no further division, and in the oldest stages that were 

 found, two nuclei only, unchanged except for an increase in size, 



^ Coulter, J. M., & Chamberlain, C. J. Morphology of angiosperms, pages 

 75. 76. 1903. 



