Il8 BOTANICAL GAZETTE [February 



Three species of this last group remain to be mentioned. R. seti- 

 gera shows a large percentage of microsporic degeneracy. R. virgini- 

 ana plena with about 90 per cent of its pollen bad, and R. virginiana 

 alba with but 25 per cent of its pollen imperfect, are varieties 

 of the R. virginiana Mill, of Gray's Manual. The latter, known 

 sometimes as R. lucida Ehrh., is a dwarf wild rose found on the 

 margins of swamps and rocky shores from Newfoundland and 

 eastern Quebec to New York and eastern Pennsylvania. 



The preceding statistics show clearly that the species of Rosa are 

 in a very marked degree characterized by abnormal pollen. It is 

 true likewise that abnormal pollen is largely sterile. Pollen sterility 

 for nearly a hundred years has been recognized by plant breeders 



prominent 



Lybrids; and another well known 

 extreme variabilitv. Since both 



extreme pollen sterility and variability are prominent 



of hybrids, the conclusion seems inevitable that most of the so-called 

 species of Rosa are in reality hybrids. 



This conclusion is most interesting when viewed from an evo- 

 lutionary standpoint. Are new species the result of gradual 

 changes or sudden leaps? The answer to this depends largely 

 upon the definition of the term species. In the lower vascular 

 plants the conditions of spore abortion and hybridization appear 

 to be very rare. The term species in these cases, therefore, is 

 used to distinguish groups of plants wholly distinct from one 

 another and probably genetically pure. Jeffrey 6 has shown by 

 microscopical investigation that morphologically sterile pollen 

 does not occur in plants that are monotypic, isolated geographically 

 or through the time of flower maturity. He has likewise made a 

 comparison of the "conditions of sporogeny found in the lower 



gymnosperms 



enormous m 



;enic features of the angiosperms in which multiplication o 

 5 has run riot." In this comparison he found that in tin 

 forms of Embryophyta, from the Bryophyta to the gymno 



sperms, " 



ism 



6 Jeffrey, E. C, Some fundamental morphological objections to the mutation 

 theory of DeVries. Amer. Nat. 49:5-21. figs. 7. 1915. 



