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THE AMERICAN NATURALIST [Vol. XLIX 



where it occurs it is accompanied by the phenomenon of 

 spore abortion. 



If we turn to the Angiosperms with their nearly one 

 hundred and fifty thousand recognized species, we find 

 that hybridism is very commonly recognized. It would 

 take us much too far to discuss the situation here at any 

 length. The consideration of a single important family 

 must suffice. The one chosen, as being of particular sig- 

 nificance in the present connection, is the Bosaceaa. We 

 have had a recognition for many years past on the part of 

 systematic botanists in this country and in Europe that 

 hybridism is extremely common as a natural condition in 

 certain genera of the Bosaceae. The inference in such 

 cases is generally based on the blended character of the 

 hybrids themselves, which show to a large extent a com- 

 bination of the characters of their parent species. Pro- 

 fessor Brainerd has recently made some very interesting 

 investigations in this direction in the case of American rep- 

 resentatives of the Bosaceae. The recognized hybrid forms 

 in the Bosaceae are usually characterized by a considerable 

 degree of pollen sterility, unless the parents happen to be 

 species not very remote in relationship. In addition to 

 the recognized hybrids of the rosaceous species, the work 

 carried on in the Harvard laboratories has revealed a 

 large number of hidden hybrids or crypthybrids, which 

 are quite constant in their characters and are recognized 

 by systematists as good species, but differ from normal 

 species in the fact that their reproductive cells are to a 

 greater or less degree abortive. Species of this kind are 

 extremely common among those rosaceous genera, which 

 have become of economic importance, such as Rubus, 

 Rosa, Pyrus, Mains, Sorbus, Crataegus, etc. Taking Rosa 

 as an illustration, in addition to numerous recognized 

 hybrids, there are many types recognized as good species, 

 e. g., Rosa blanda, in which the pollen is normally largely 

 abortive, in still other species, frequently those which are 

 isolated geographically, the pollen is quite sound, e. g., 

 Rosa rugosa of J apan. The latter type of species must be 



