33Q BOTAXICAL GAZETTE [October 



phenomena, etc., but certain others, which have not revealed them- 

 selves as hybrids in these ways, are clearly such, as a result of the 

 study of their spores. As an example let us take our mountain 

 ashes {Rosa or Riibas furnish just as good illustrations, or in fact 

 almost any of the larger genera of the Rosaceae in the Northern 

 Hemisphere). In Europe and Asia it is generally recognized that 

 the valid species of Sorbus have between them a number of natural 

 hybrids. In the eastern and northern region of North America, 

 we have two common species of mountain ash, Sorbus americana 

 and Sorbus sambucifolia. These have been shown in connection 

 with the present investigations to have pollen which is to a con- 

 siderable degree abortive. The same observation has been made 

 in regard to the European Sorbus aucuparia. In fact, the only 

 species of Sorbus yet examined in this connection with perfect 

 pollen is one growing in Japan. Or turning to another genus, 

 Prunus, most of the species examined had imperfect pollen. Of 

 the common species in New England investigated, the only one 

 with perfect pollen was Prunus serotina, which flowers somewhat 

 later than the others and is consequently phenologically isolated, 

 just as is the case geographically with the Japanese Sorbus men- 

 tioned above. Illustrations of a similar nature could be given to 

 an almost unlimited extent from studies on the Rosaceae, which 

 are as yet far from complete. We have consequently to distinguish 

 in the case of the Rosaceae, if the morphological features are taken 

 into account as well as the data of systematic botany, three kinds 

 of individuals, namely pure species, recognized species which from 

 the condition of their pollen are in reality concealed hybrids or 

 crypthybrids, and recognized hybrids. In some genera the multi- 

 plication of species through hybridization is very large; for example, 

 over 4000 species of Rubus have been described in Europe, and in 

 this country Sargent and others have described a very large 

 number of species of Crataegus. 



Nearly all our common garden herbaceous dicotyledons, which 

 from the nature of things are capable of hybridization, are hybrid- 

 ized. For example, we may take the carnations, the petunias, the 

 phloxes, the chrysanthemums, the calceolarias, certain primroses, 

 etc. Without concerning ourselves further with the dicotyledon- 



