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THE AMERICAN NATURALIST [Vol.L 



time at my disposal, shows clearly that propinquity, geo- 

 graphical or phenological, is to a large extent correlated 

 with pollen imperfection in the group. 



Limitations of time make it necessary to proceed sum- 

 marily with other illustrations. Next may be cited the 

 Betulaceae and Fagacese. Each of these orders has one 

 strikingly polytypic species, Betula in the one case, and 

 Quercus in the other. Interestingly enough, it is in these 

 two genera that variability and gametic sterility coincide. 



One might continue at length through the Dicotyledons, 

 but one other example must suffice for this division of the 

 Angiosperms. The Solanaceae have one huge genus, Sola- 

 tium itself, in which there are nine hundred species. In 

 this genus not only is there extreme variability, but also 

 a large degree of pollen sterility. In the monocotyledo- 

 nous division we may start with the grasses. Monotypic 

 grasses have perfect pollen, as is illustrated, for example, 

 by the wild rice, Zizania aquatica. In the genus Alope- 

 curus, with numerous and propinquitous species, on the 

 contrary the pollen conditions frequently indicate gen- 



