34 BURS A BURSA-PASTORIS AND BURS A HEEGERI : 



As soon as it was demonstrated that there are two unit-differences 

 involved in these forms it was not difficult to discover in what these two 

 units consist. One of them is the elongation of the primary lobes, the 

 other the extension of the sinuses to the rachis and the presence of rounded 

 secondary lobes in the distal axils of the primary lobes. If we represent 

 the first by A and its absence by a, and the second by B and its absence 

 by b, then we may distinguish the four members of the ratio as 

 AB : Ab : aB : ab. In the first, AB, is denoted the presence both of the 

 elongated primary and the rounded secondary lobe, the distinguishing 

 characteristics of B. bp. heteris ; in the next, Ab, the primary lobes are 

 attenuate, but the rounded secondary is not present, this being the char- 

 acter of B. bp. tenuis; in the third, aB, the rounded secondary is present, 

 but the primary is not elongated, giving B. bp. rhomboidea; while in the 

 fourth, ab, both the secondary and the elongation of the primary are absent, 

 resulting in B. bp. simplex. 



The most remarkable corollary of this composition of these forms is the 

 possible resolution and recomposition of the several elementary species. 

 Thus, a cross between pure-bred B. bp. heteris and pure-bred B. bp. sim- 

 plex, or a cross between pure-bred B. bp. tenuis and pure-bred B. bp. 

 rhomboidea should again give in the second generation all four forms in the 

 ratio 9:3:3:1. The correctness of this prediction awaits the test of 

 further experimentation. 



Of the families representing the third generation of this hybrid 20 have 

 now been studied, and the results have agreed well with the interpretation 

 here given of the constitution of the original family. These 20 families 

 are briefly described below in the order of the pedigree -numbers by which 

 they were designated in the cultures and original notes. 



054.184: This plant was described as of the same general type as B. 

 bp. heteris, with long, slender primary lobes and a rounded secondary lobe 

 in the distal axil of the primary. The secondary lobe usually bore 1 or 2 

 sharp, erect lobes. Seeds of this plant were sown February 23, 1906, and 

 produced about 200 offspring. Owing to crowding of the propagating-house 

 at that season of the year, only 100 plants were potted and kept for obser- 

 vation, the remainder being discarded with the seed-pan. The specimens 

 potted up were taken from the pan in such manner as to prevent possible 

 selection of individuals. Of 82 specimens noted, 80 were regarded as B. 

 bp. heteris, and 2 as near B. bp. tenuis, but many of the specimens were 

 stunted and it appears certain now that some specimens of B. bp. tenuis 

 were looked upon as stunted B. bp. heteris, because the importance of the 

 secondary lobe was not fully appreciated at that time. That the parent 

 plant (054.184) was really a hybrid between B. bp. heteris and B. bp. tenuis 

 is further indicated by the behavior of one of its offspring, which was 



