50 



BURSA BURSA-PASTORIS AND BURSA HEEGERI : 



that much better-developed heteris rosettes occur in the F 2 than are seen in 

 pure-bred B. heegeri, though these heteris characteristics must have come 

 directly from the heegeri side of the cross. 



The numerical results of these crosses may be tabulated thus : 



Bursa bp. simplex (abC) X Bur so. h. heteris (A Be} 



(056) (059) 



Bursa bp. heteris-rkomboidea (ABCabc) 

 (0688 and 0689) 



In this table the pedigree-numbers 06196 and 06197 represent families 

 in which B. heegeri was the mother, while 06212 is the reciprocal cross. 

 It will be seen that there is no essential difference between these two 

 crosses. In the rosette-characters these numbers are very close to the 

 Mendelian ratios, among both those having bursa-pastoris capsules and 

 those having heegeri capsules, showing by these facts that the heegeri 

 rosette has the same allelomorphic composition as the rosette of B. bursa- 

 pastoris heteris, and that these rosette-characters are not coupled in any 

 but a chance way with the form of the capsule. This independence of 

 characters is thus a matter of great importance in the production of new 

 elementary forms ; for before this cross was made there existed, so far as 

 is known, but one elementary species of B. heegeri, while out of the cross 

 came 4 distinct elementary forms of this species. 



While there is a perfect agreement with Mendelian ratios in rosette- 

 characters, the capsules give a surprising departure. The capsule-form is 

 perfectly alternative; there has never appeared any intermediate condition 

 in the plants in my cultures, and the "reversions" observed by Solms- 

 Laubach and Laubert were pathological. As the bursa-pastoris type is 

 dominant, simple Mendelian expectation would require the appearance of 

 1 B. heegeri in every 4 F 2 individuals. Out of 2,540 plants of the F 2 gen- 

 eration observed, only 111 were B. heegeri, or approximately 1 in 23. That 

 this result should be consistently given in 3 different pedigrees represent- 

 ing reciprocal crosses adds greatly to the weight that is to be attached to 



