No. 591] HYBRIDISM IN ANGIOSPERMS 139 



etical contamination. Proceeding to aquatics, in the Po- 

 tamogetonaceae, the monotypic Zannichellia and Zostera 

 have perfectly developed microspores ; while Potamo- 

 geton, with its numerous species, is often distinguished 

 by a large degree of pollen imperfection. Similar state- 

 ments hold in a like sense in regard to members of the 

 Alismaceae, Sparganiaceae, etc. 



Pig. 6. Pollen of Rubus villosus (Blackbriar), showing high degree of 1m- 



The pressure of time compels a summing up of the situ- 

 ation without further references to detailed facts, which 

 will be supplied by publications soon to appear. The gen- 

 eral condition in the Angiosperms in contrast to the Gym- 

 nosperms is a large degree of variability in the species. 

 Where the species are highly inconstant and cause great 

 difficulty to the systematist, as, for example, in the Ona- 

 graceae, Eosaceae, Solanaceae, Birches, Oaks, etc., there is 

 often a large degree of pollen sterility. Where isolation, 

 geographical, phenological or specific, is present the con- 

 tents of the anther sacs are strikingly perfect in their 

 development. In other words, where interspecific cross- 

 ing is possible, there is often clear evidence of its presence 



