I J K 



Fig. 68. Development of embryo sac in Podostemon subidatus (A-F) and Dicrae a 

 elongata (G-K). A, Podostemon, megaspore mother cell. B, dyad cells. C, 

 lower dyad cell enlarging; upper in course of degeneration. D, two-nucleate embryo 

 sac formed from lower dyad cell. E, four-nucleate stage with accompanying wall 

 formation. F, mature embryo sac showing two synergids, egg, and a polar nucleus. 

 G, Dicraea, upper dyad cell degenerating; lower divided into two cells. H-J, upper 

 dyad cell crushed and disorganized; of the other two cells, upper has divided trans- 

 versely and lower has divided vertically. K, embryo sac after fertilization, show- 

 ing degeneration of all the cells except the zygote. (After Magnus, 1913.) 



105 



