BURSA BURSA-PASTORIS AND BURSA HEEGERI 5 



In this table the pedigree numbers 06196 and 06197 represent 

 families in which the bursa-pastoris is the mother, while No. 06212 is 

 the reciprocal cross. It will be seen that there is no essential differ- 

 ence between these two crosses. In the rosette characters the num- 

 bers of offspring belonging to the several types 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 that of 

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

 any but a chance way with the form of the capsule. This indepen- 

 dence of characters is thus seen to be 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 four elementary species of 

 B. heegeri. 



While there is thus seen to be perfect agreement with Mendelian 

 ratios in rosette characters, the capsules gave a surprising departure. 

 The capsule-form is perfectly alternative; there never appeared in 

 my cultures any intermediate condition. As the bursa-pastoris form 

 is dominant, simple Mendelian expectation would require the ap- 

 pearance of one B. heegeri in every four F 2 individuals, providing 

 this capsule difference be assumed to be due to the presence or ab- 

 sence of a single unit. Out of 2540 plants of the F 2 generation 

 observed, only 111 were B. heegeri, or approximately one in twenty- 

 three. That this result should be consistently given in three differ- 

 ent pedigrees representing reciprocal crosses adds greatly to the 

 weight that is to be attached to it. The greatest frequency in any 

 pedigree was one in nearly 18, and the least frequency was one in 

 25. I take the fact that the reciprocal crosses gave similar ratios 

 to indicate that the B. heegeri capsule is dependent upon something 

 carried by the germ-cells and cannot be a pathological condition trans- 

 missible from mother to offspring somatically, a possibility that might 

 account for apparent heritability of characters in a self -fertilized 

 line, but could not well account for equal results in reciprocal crosses. 

 Normal Mendelian phenomena are believed to rest pretty securely 

 on the method of formation of the chromosomes during the reduction 

 division, but no scheme of behavior occurs to me which would result 

 in the distribution of the heegeri determiner to but one cell in 20. 



It is clear that in this case the capacity of B. heegeri to maintain 

 itself in competition with its parent would be very slight were it not 

 for the fact that cross-fertilization in Bursa is relatively infrequent. 



Briefly summarized my conclusions are : 



(1) Bursa heegeri has the same Mendelian units in its leaves as 

 occur in B. bursa-pastoris, which serves further to confirm its deri- 

 vation from that form. 



(2) The crossing of B. heegeri with B. bursa-pastoris gives rise to 



