554 WHITE— STUDIES OF INHERITANCE IN PISUM. 



White flower X white flower always gives white-flowered prog- 

 eny. 



Tschermak has carried out and pubHshed (84) the results of a 

 very complete series of back-crosses of F^'s, Fo's, Fg's, F^'s and Fj's 

 with pure varieties and of the segregates of each type from several 

 of these generations with each other. This work of Tschermak's, 

 together with that of Mendel and Lock has put the genetics of flower 

 color in Pisiwi on a very strong basis of fact. 



All these and other studies on Pisum flower color have shown 

 colored flowers to be always associated with colored seed coats, 

 colored leaf axils, indent seed, etc., while white-flowered races are 

 always characterized by their absence. Further, of the two colored 

 flower types, purple flowers are always associated with reddish 

 purple axil color and purple dots on the seeds, while pink-flowered 

 varieties are associated with reddish leaf axils and reddish dots on 

 the seed coat. Both purple- and pink-flowered forms are known or 

 have been obtained through crossing which lack axil color or dotted 

 seed coats, though all have the gray-brown seed coat for which the 

 factor Gc stands. 



Exceptional Cases. — In several cases both Tschermak and Fru- 

 wirtli have secured purple flowers in F^ from crossing two pink- 

 flowered plants, where only pink was expected. Tschermak tenta- 

 tively regards these pinks which give rise to purples as individuals 

 which were really purples genetically, but for some reason the union 

 of the factors A and B failed to produce purples when they were 

 expected. Later B became active again. These exceptions are 

 still under investigation. 



Interpretation. 

 According to Tschermak, flower color in peas is due to the pres- 

 ence and absence of two factors — a chromogen factor A and a color 

 modifier or blueing factor B. When A only is present the plants 

 have salmon-pink flowers, when both A and B are present the pink 

 color is modified to a purplish red. When both A and B are absent 

 the flowers are white. When A is absent and only B is present the 

 flowers are also white, so that B cannot express itself in the absence 

 of A. All white-flowered varieties so far tested have shown the 



