IQ20] 



KAXDA— VERBENA 



59 



megaspore mother cell and its nucleus with a prominent nucleolus 

 continue to increase in size (fig. 19). Two divisions then occur 

 which result in the typical formation of a row of four megaspores 

 (figs. 20, 21) ; this takes place when the ovule is about at the stage 

 represented in fig. 16. The innermost of the four megaspores is 

 the largest, and is destined to develop into the embryo sac (fig. 22). 



Successive stages in the development of this basal megaspore. 

 accompanied by the destruction of the other three megaspores, are 

 shown in figs. 22-25. The nucellus, consisting of a single layer of 

 cells, surrounds the row of megaspores 

 (fig. 21). It eventually becomes so dis- 

 tended by the enormous expansion of the 

 developing embryo sac that it ruptures, and 

 the ruptured nucellus is then carried down- 

 ward as a cap on the growing embryo sac, 

 as was previously described by Mottier 

 (14) in Arisaema, Caldwell (i) in Lemna, 

 and JMerrell (13) in Silphium. In the 

 next stage (fig. 26) the embryo sac lies free 

 in the space between the funiculus and the 

 integument, and the yellowish-brown rem- 

 nants of the nucellus are observable cap- 

 ping the micropylar end of the sac. 



The phenomena of the enlargement of 

 the sac, the division of its nuclei, and the 

 destruction of the cells of the nucellus do 

 not occur simultaneously, but these pro- 

 cesses take place at different rates. The 27 

 development of the megaspore and the Tig. 27. -V.angustif alia: 



r . . . „ 11 mature pistil with mature 



late 01 the nucellus are exactly the same 



i 



c> 



t — 



- 



When the embry 



Silph 



embryo sac; X35. 



times 



the 



shape, and always constricted just above the egg apparatus. The 



seems 



The nucleus of the egg is 

 several times larger than the nuclei of the synergids and contains 



