SHULL: DUPLICATION OF A LEAF-LOBE FACTOR 441 



of the structure of the triangular capsule of Bursa bursa- pastoris as 

 compared with the Heegeri type of capsule, is strong evidence against 

 the duplication of the factor for this complex character through inde- 

 pendently repeated mutations affecting different chromosome pairs 

 (Shull, 1914, p. 141). The character under consideration in the 

 present paper, namely the B lobing of the rosette leaves, appears to 

 justify the same observation. The production of leaves divided at 

 frequent intervals by sinuses reaching to the midrib, and bearing 

 characteristic secondary lobes and sinuses, involves the control of the 

 number and direction of cell divisions through very long and com- 

 plexly branched cell lineages, and it is scarcely conceivable that such 

 specific control of these long cell lineages should be exactly duplicated 

 by independent mutations affecting different chromosomes. 



It appears to me much more logical to assume that such a rearrange- 

 ment of the genotype has taken place that the two B determiners 

 which are allelomorphic to each other in the homozygous monomeric 

 strains, assumed new positions, whereby they became associated with 

 chromosomes belonging to different pairs, and thus capable of inclu- 

 sion in the same germ cell. 



As these two factors, B and B', are apparently entirely independent 

 of each other, it may be taken for granted that they are associated 

 with different chromosome pairs. They could become thus asso- 

 ciated by either of the two methods, (b) or (c), but in the absence of 

 known linkage relations, there is nothing to indicate which of these 

 two methods has been the more probably responsible for the dupli- 

 cation of the B factor, — ^whether a rearrangement of -whole chromo- 

 somes or the rearrangement of parts of chromosomes through a so- 

 called "longitudinal crossing-over." 



All these suggestions as to the origin of duplicate determiners 

 assume the duplication to be a derivative condition; but it may also 

 be in some cases the primitive condition from which monomeric and 

 recessive strains may have arisen as a result of repeated "loss" muta- 

 tions, as stated in my previous paper (1914, p. 137). Studies of the 

 geographical distribution of the duplicated factors may throw some 

 light upon the relative age of the monomeric and polymeric types, 

 for if wild biotypes almost universally possess the duplicated factors, 

 it may be assumed that this condition is either primitive or at least 

 relatively old, while a much restricted and more or less definitely 

 circumscribed range may be accepted as a criterion of relatively recent 

 origin from the monomeric condition. 



• In regard to the B leaf-lobe factor it will be noted by reference to 

 the tables, that plants showing the duplication (Table 4) have been 

 found at Tucson, Arizona, at Cardiff, Wales, at Groningen, Holland, 



