iqi4] JEFFREY— SPORE CONDITIONS 335 



Summary 



1. Spontaneous hybridization is comparatively rare among 

 lower plants, but very common in the angiosperms. 



2. A long recognized criterion of hybridism is sterility, partial 

 or complete, of the reproductive cells. In plants this is recognized 

 with particular ease in the case of the pollen. 



3. In forms which are ordinarily recognized taxonomically as 

 species, pollen infertility frequently indicates past genetical con- 

 tamination. 



4. In families such as the Rosaceae and the Onagraceae, we 

 find grading into each other recognized species and recognized 

 hybrids, having in common the character of partial or complete 

 reproductive sterility, most easily recognized in the organization 



* 



of the pollen. 



5. This situation points inevitably to the hybrid origin of these 

 abnormal species. 



6. Species of this type may conveniently be called crypthybrids. 



7. The species of Oenothera and many of those of Epilobium 

 and probably those of Fuchsia as well are crypthybrids. 



8. This condition must be clearly recognized in connection with 

 any investigation in regard to the origin of species based on material 

 of this sort, that is, on crypthybrids such as Oenothera Lamarckiana, 

 O. biennis, etc. 



9. Although there appears to be good evidence that hybridism 

 has been an important cause of the multiplication of species, there 

 seems to be no logical support for the view that it has to do with 

 their actual origin. 



Harvard University 



Fig. 

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 Fig. 5 

 Fig. 6 



EXPLANATION OF PLATES XXII-XXV 



PLATE XXII 



Spores and elaters of Marchantia polymorpha; X250. 



Spores of Polytrichum commune; X250. 



Spores of Equisetum hiemale; X 200. 



Spores of £. variegatum, var. Jesupi, a hybrid; X200 



Spores of Angiopteris evecta; X200. . 



Pollen of Zamia floridana; X250. 



