Cook : Potamogeton lucens 213 



in e the sac has increased somewhat in size, the partition-wall has 

 disappeared, the large tube-nucleus has reached its maximum in 

 both size and activity and the embryo is about the same age as in 

 figure 22 ; in/ the sac is very much enlarged and the new cham- 

 ber at the chalazal end of the sac has developed, the embryo at 

 this time being about the same age as \n figure 24.. 



The formation of a wall across the embryo-sac separating the 

 two daughter-nuclei formed by the first division of the primary 

 endosperm-nucleus has been observed in many plants. It occurs 

 in both dicotyledonous and monocotyledonous families but further 

 studies will probably show marked differences in its behavior in 

 these two main groups of angiosperms. 



The embryo-sac of Alisma Plantago (now known as Alisma 

 Plantago-aquatica) as described by SchafTner (12) corresponds in 

 most respects to that of Potamogeton lucens, although he did not 

 observe the formation of a wall after the division of the primary 

 nucleus. However, in his later work on Sagittaria variabilis (13), 

 (now known as Sagittaria latifolia) he describes a sac which cor- 

 responds remarkably well to that of ft lucens, with the exception 

 that the antipodals are more persistent and that the nucleus of the 

 nucellar tube has a tendency to fragment.* It is possible that 

 further study of Alisma Plantago-aquatica will also show the forma- 

 tion of a wall across the sac. My own studies on Nymphaeaceae 

 show the same formation of a wall across the sac separating the 

 two daughter-nuclei of the primary endosperm-nucleus, the 

 daughter-nucleus in the synergid end of the sac giving rise to the 

 endosperm and the daughter-nucleus in the antipodal end passing 

 into a tube or saccular structure (differing somewhat in different 

 genera) formed by disintegration of the nucellar tissue. The 

 behavior of this nucleus of the nucellar tube of the genus Nym- 

 phaea was more nearly like that of Potamogeton lucens than that 

 of any other genus. In fact the general appearance, history, and 

 behavior of these structures in P. lucens and in JVymp/iaca are strik- 

 ingly similar. Johnson (10) c 

 runts cernuus, which is very similar to that described by me for 



*Schaffner's studies on^. variabilis were verified by my own studies on S. tana- 

 folia with the exception that I found the antipodals of SJancifolia not so persistent as 

 ■n 6". variabilis. 



