No. 577] 



MUTATION THEORY OF BE VRIES 



13 



able. Some of the grains are full size and present dark 

 contents. Others are considerably smaller and are devoid 

 of protoplasm. The latter are abortive or sterile grains. 

 We have in fact before us a hybrid derivative of E. hir- 

 sutism, commonly found near ballast in New England and 

 not unfrequently cultivated in gardens. Other species of 

 EpUobntm in the stricter sense of the generic appellation, 

 show similarly abortive pollen development and the con- 

 clusion reached by old world systematists on the external 



characters, that hybridization is common among the spe- 

 cies of Epilobium proper, is entirely confirmed by the 

 study of the pollen. It need hardly be emphasized in this 

 connection, that imperfect pollen development has been 

 recognized for nearly a century by scientific plant breed- 

 ers, as a criterion of hybrids. 



The genus (Enothera may now be profitably considered. 

 Fig. 6 presents a magnified view of a transverse section of 

 a mature flower bud of one of the commonest of eastern 

 species of (Enothera, namely (Enothera biennis. The 

 floral envelopes are more voluminous than in the two 

 genera illustrated above. Within are the stamens and in 

 the center of the figure the style appears as a large 

 rounded structure. Even with the low magnification em- 

 ployed, it is easy to discern that the contents of the anther 

 sacks present a very different appearance from those of 



