438 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 19 3 



factors which the extra material contains. It will be noted from our 

 previous illustration (pis, 5 and 6) that there must be a different 

 set of factors in the two halves of the 1 • 2 chromosome since the 

 secondary Sugarloaf , which has the 2 • 2 chromosome extra, does not 

 resemble the secondary Polycarpic, which has the 1 • 1 chromosome 

 extra. In no chromosome does there appear to be a greater similar- 

 ity between the groups of factors in the two halves than between the 

 groups of factors in two different normal chromosomes. 



The unbalance which extra chromosomes exert over the normal 

 balanced condition may best be shown by capsules of the " Globe " 

 mutants. The Globe chromosome is next to the smallest of the 12, 

 but the changes which it brings about when present in excess are 

 very distinct. As shown in Plate 9, we have two diploid (2n) 

 Globes, or, more properly speaking, two diploids modified by extra 

 chromosomes in the Globe set. The (2n+l) Globe has one extra 

 chromosome in the Globe set giving an unbalance of one over the 

 normal complement of 24. The (2n + 2) Globe, with two extra 

 chromosomes in the same set, has a greater unbalance (2 over 24) 

 and in consequence all Globe characters, such as depression of the 

 capsule, are heightened in expression. In similar manner starting 

 with the capsule of a tetraploid at the right, it will be seen that the 

 tetraploid Globes have their capsules relatively more depressed and 

 their spines relatively stouter as we pass from (4n+l ) to (4n+2) 

 and (4n + 3) Globes. A (4n + 4) Globe has not yet appeared in our 

 cultures, but we might expect it, since the unbalance which its extra 

 chromosomes exert would be 4 over 48 which is the same un- 

 balance found in the (2n + 2) Globe (2 over 24). Similarly we 

 might expect a haploid (ln + 1) Globe with an unbalance of 1 over 

 12, but this unbalance, found as yet only in the rare (2n+2) Globes, 

 may be the limit of unbalance which a plant can endure, and may 

 be possible only in the diploid series. In the center of the picture 

 we have a (3n + l) Globe obtained by crossing a (4n + l) Globe with 

 a 2n pollen parent. 



We have discussed primary or unmodified chromosomes and sec- 

 ondary chromosomes in which the two ends are alike. A (2n+l) 

 type appeared not infrequently in our cultures which was neither a 

 primary nor a secondary type. It always came from hybrids between 

 our standard line 1 and certain " B " races. From the attachment 

 of the extra chromosome to both the white (1-2) and the stippled 

 (17-18) chromosomes, Belling concluded that the extra chromosome 

 consisted of part of the white and part of the stippled chromosome. 

 From these findings he proposed the hypothesis of interchange be- 

 tween segments of these nonhomologous chromosomes to account 

 for the origin of " B " races from the standard type. The chromo- 



