122 BOTAMCAL GAZETTE [February 



different one. There must be careful distinctions made between the 

 3 classes of individuals; and the search for the true solution of the 

 problem of the origin of species becomes thereby a matter of great 

 complexity. As for the mutation hypothesis of DeVries, the 

 morphological and systematic evidence set forth with regard to 

 the conditions in Rosa, and the similar conditions brought out 

 with regard to Rubus, Crataegus, and the Rosaceae as a whole, 

 seem to lend it little support, since the mutability here is obviously 

 the result of hybridization in nature. 



Conclusions 



i. The species of Rosa are characterized by a large amount of 

 abortive pollen and also by great variability. 



2. Both pollen sterility and variability have long been recog- 

 nized as two main characteristics of hybrids. 



3. The species of Rosa, therefore, are largely of hybrid origin. 



4. On account of the great number of crypthybrids and phen- 

 hybrids in angiosperms, the term species has a very different 

 meaning from that which it has when applied to the lower vascular 

 plants and the gymnosperms. 



5. The mutability of the species of Rosa cannot properly be 

 used in support of the mutation hypothesis, since this phenomenon 

 is obviously the result of hybrid contamination in nature. 



In conclusion the writer wishes to express her most sincere 

 thanks to the Director of the Arnold Arboretum for permission to 

 collect material; and to Professor E. C. Jeffrey for advice and 

 assistance. 



) 



Harvard University 



EXPLANATION OF PLATES IV-VI 



PLATE IV 



Figs. 1-6. — Pollen. 



Fig. 1 . — Lycopodium complanatum; X 1 2 5 . 



Fig. 2. — Pinus divaricata; X125. 



Fig. 3. — Rosa alba; X250. 



Fig. 4. — Rosa alba; X375. 



Fig. 5. — Rosa rugosa; X250. 



Fig. 6. — Rosa rubiginosa; X375. 



