289 



a complete nutrient solution, they at once formed a meristem on their outer 

 edges by a repeated cell-division, while the starch supply decreased. 



The distribution of the sexual organs varies according to the nutritive 

 conditions. Ameristic prothallia bear only antheridia, never archegonia, 

 which are associated with the presence of a meristem. Of especial impor- 

 tance at this point is Prantl's observation that ameristic prothallia of Os- 

 munda, which had borne isolated antheridia, developed abundant archegonia 

 after nitrogen had been supplied; besides the archegonia, antheridia also 

 appeared. 



