30 



THE AMERICAN NATURALIST [Vol.LVI 



tion from a particular part of a single mutant, the Grlobe. 

 A more detailed study of changes in external and internal 

 morphology brought about by the presence of specific 

 extra chromosomes in the several mutants is being under- 

 taken in cooperation with Dr. Sinnott. 



The unbalancing effect of an extra chromosome is 

 showTi in the lessened vigor of mutant plants. Thus from 

 Globe parents as an example of (2^ + 1) mutants, ordi- 

 narily only one quarter of the offspring to reach record- 

 able size are Globes, instead of the 50 per cent, expected. 

 Moreover, when the plants are crowded the proportion of 

 Globes surviving is considerably lessened. 



We have been discussing the unbalance as affecting the 

 sporophytic generation. In the gametophyte, the un- 

 balance is doubled. Thus from (2w + l) Globe plants 

 with an unbalance of 1 over 2n the pollen grains w^ith the 

 extra chromosome have an unbalance of 1 over n. This 

 extreme unbalance hinders their functioning and brings 

 it about that the Globe character is transmitted to only a 

 slight extent through the pollen (under 2 per cent, in a 

 considerable series of crosses). It is of interest in this 

 connection to note the results of selfing and crossing 

 Globes of the tetraploid series. The unbalance in a 

 Globe is 1 over 4n, while the unbalance in its pol- 

 len grains which carry the extra chromosome is 1 over 2n. 

 Due to this lessened unbalance in comparison with pollen 

 of {2)1 + 1) Globes, the pollen of the (4w -f 1) Globe trans- 

 mits the Globe character to a higher percentage of its 

 progeny (14 per cent, in the single pedigree tested), and 

 partially for the same reason we have obtained higher 

 proportions of Globes in the offspring from selfing such 

 (4*1-1-1) Globes (a total of about 60 per cent, in a single 

 experiment). A more specific study of the effect of ex- 

 tra chromosomes upon the gametophyte is being under- 

 taken in cooperation with Dr. Buchholz. 



It will not be advisable at the present stage of our in- 

 vestigations to discuss the possible external and internal 

 factors which may induce the chromosomal aberrations 

 which form the basis of our common mutations in Datura. 



