THE ENDOSPERM 



253 



the formation of a vertical wall, but since most of the cytoplasm of 

 the embryo sac is aggregated in its micropylar part and the chalazal 

 is occupied by a large vacuole, the wall ends blindly, reaching down 

 to the upper margin of the vacuole only. The next wall also, which 

 is vertical but at right angles to the first, ends similarly and results 



D 



E 



Fig. 143. Development of endosperm in Hypericum acutum. A, embryo sac 

 showing fertilized egg and endosperm nuclei in division. B, embryo sac with 

 four endosperm nuclei. C, same, all four nuclei dividing. D, eight-nucleate stage 

 of endosperm ; two of the nuclei are at upper end and one at lower end of embryo 

 sac. E, 16-nucleate stage showing four nuclei at upper end, one at chalazal, and 

 the rest irregularly distributed in middle. F,G, chalazal end of embryo sac showing 

 two and four endosperm nuclei respectively. H, micropylar part of embryo sac, 

 showing two-celled embryo surrounded by mass of 16 endosperm nuclei; endosperm 

 nucleus on right belongs to central portion. /, chalazal part of embryo sac of same 

 age, showing mass of eight endosperm nuclei. (After Stenar, 19S8.) 



in the formation of four cells, all open toward the base. In the fol- 

 lowing divisions some of the daughter nuclei remain free and pass 

 into the cytoplasmic layer lining the vacuole, while others divide 

 with the accompaniment of walls. Eventually, therefore, the upper 

 part of the embryo sac shows Cellular endosperm and the lower 

 shows Nuclear endosperm. 



