THE TEST OF A PURE SPECIES OF CENOTHERA.^ 



By BRADLEY MOORE DAVIS. 



{Read April 23, T915.) 



There is probably no group of plants the genetic behavior of 

 which has received so much study as the species of (Enothera. No 

 group of plants is more prominently before the attention of experi- 

 mental plant morphologists, and yet to many botanists it may appear 

 that no group has yielded less of satisfaction. Among the workers 

 with these forms there is the widest divergence of opinion, and of 

 general conclusions there is little to show for the time that has 

 passed since the appearance of "Die Mutationstheorie " in 1901 

 and the many years of study that De Vries devoted to the group 

 previous to this date. 



Can we find the point around which the difficulties cluster most 

 thickly or from which the varied interpretations diverge most 

 sharply? And, finding such a point can we formulate lines of 

 experimentation that may clear the confusion of assumptions from 

 which the various workers have proceeded to follow the lines of 

 study that seemed to them to lead towards the light? To the 

 writer the center of the difficulties lies in the fact that we have no 

 accepted tests for the genetic purity of an CEnothera species. 



By the genetic purity of a species we mean such a constitution of 

 the germ plasm that a form is able to produce gametes of one type 

 only for each sex. That is to say all male gametes of the form 

 should have the same germinal constitution and thus be physio- 

 logically and morphologically equivalent, and all female gametes 

 likewise should be of the same type. The male and female gametes 

 may, however, differ in their respective effects upon the characters 

 of a succeeding generation as shown by the marked differences 

 exhibited by certain reciprocal crosses, for example, the reciprocals 

 between biennis and fnuricata, or between biennis and franciscana 



1 Genetical Studies on CEnothera — VI. 



226 



