

- 





i 



1920] EDWARDS— HEDYOSMUM 417 



consists of 7 or 8 layers of cells between the walls of the embryo sac 

 and the outer surface of the nucellus. 



The nucellus in the mature seed has been completely absorbed 

 by the swelling endosperm, and there is left only the crushed 

 remains of its cells against the two integuments, the inner of 

 which is considerably thickened in the micropylar region (fig. 42). 



Embryo sac 



Archesporium and tapetum .— Of the group of hypodermal 

 cells seen in the young ovule at the beginning of its development, 

 two axial cells become distinguishable, owing to their larger size 

 and larger nuclei, at the time the outer integument is clearly 

 evident (fig. 26). These two cells are the parietal or tapetal cell 

 above and the definitive archesporial or megaspore mother cell 

 toward the chalaza (fig. 26). The tapetal cell divides early to 

 form the 4 or 5 layers of parietal tissue found in the young seed 

 above the sac (figs. 27-29). The cells of the tapetum thus formed 

 do not change appreciably until crushed by the growth of the 



m 



The definitive archesporial cell is now considerably elongated 

 and contains a large nucleus (fig. 28). This divides in the usual 

 way to form 3 or more (frequently 4) potential megaspores, as is 

 evident from the condition seen in figs. 29 and 30. The chalazal 

 megaspore of the group is the one which becomes the functional 

 megaspore and develops into the embryo sac, while the 2 or 3 

 micropylar ones degenerate and are later absorbed (figs. 29-31). 

 The functional megaspore increases steadily in size during the degen- 

 eration of the non-functional megaspores, and then evidently 

 begins to divide in the usual way, giving rise to 2, 4, and 8-nucleate 

 stages. The first division of the 



mother 



within 



definite 



divisions (figs. 31, 32). 



equent 



The mature embryo sac is of the 7-nucleate type common to 



most 



mbry 



greatly in size, and the egg apparatus and endosperm nucleus are 



