90 CARNEGIE INSTITUTION OF WASHINGTON. 



strongly that Cocklebur has its extra chromosome in the set carrying the 

 genes for armed or inermis capsules on account of the ratios which it throws 

 when heterozygous for these factors. A selfed heterozygous plant of the 

 variety Wedge threw a 3 : 1 ratio, which is typical of disomic inheritance and 

 seems to indicate that there are in this (2n+l) variety only two chromosomes 

 carrying genes for armed and inermis rather than 3 as expected. More data 

 are desirable, but offspring from selfed and back-crossed parents now in the 

 garden seem to confirm the tentative conclusion from the winter's cultures. 

 Our preliminary hypothesis is that the variety may have a part of one of 

 the chromosomes in the trisome deficient (in this group for the armed-tner- 

 mis genes), either by inactivation of this part or by an actual loss of a por- 

 tion of the chromosome. A tentative scheme of the possible chromosomal be- 

 havior has been worked out which squares measurably well with the differences 

 observed between the Cocklebur mutant and its variety Wedge in respect 

 to the inheritance of Mendelian factors and in respect to the fact that the 

 variety regularly throws the main mutant, while the latter rarely throws the 

 variety. The scheme is merely a tentative working hypothesis, however, 

 since it is susceptible of various tests, especially from segregation of Mendelian 

 factors and from the cytological relations. Dr. Belling is undertaking a 

 detailed study of the size relations between the main mutants and their 

 varieties and, if our preliminary hypothesis be correct, should soon determine 

 whether or not there is actually a shortening of the extra chromosome in the 

 varieties instead of a deficiency unrepresented by any morphological differ- 

 ence, as seems to be the condition in reported cases of deficiency in Drosophila. 

 The relatively few varieties to each main mutant (generally less than one) 

 speak for a definite region of fragmentation. 



"It has been previously mentioned that the Spinach group showed possible 

 relationships to another group. It is also true that the proportion of unrelated 

 mutants thrown by this group is unusually large. The most extreme example 

 is a culture this summer derived from a cross between the two members of 

 the Spinach group. Out of 161 offspring, 95 (nearly 60 per cent) were mutants 

 of various types. The cause of such anomalous behavior is being studied 

 further, both from the cytological and the breeding standpoints. The evi- 

 dence from the Spinach group, however, indicates that it will be unsafe to 

 apply, without modification, our hypothesis for varieties to all our (2n+l) 

 mutants. 



"In an early stage of our study, when we had only 12 (2n+l) mutants, 

 which number was to have been expected on the basis of an addition of a 

 single unaltered chromosome to the constellation of 12 diploid pairs, it seemed 

 that a method was opened up for an analysis of the chromosomal constitution 

 by the unbalancing effects produced when a given extra chromosome was 

 present. The finding of more than 12 (2n+l) mutants made any attempt at 

 an extensive analysis of this kind unwarranted. With the discovery, however, 

 that the main mutants (at least 11 of the 12) can be distinguished from their 

 varieties, the possibility of such an analysis is restored, and, if deficiency, 

 fragmentation, or other alteration of chromosomes proves to be a means of 

 modifying mutants into varieties, we have an additional method of analyzing 

 the chromosomes by studying the unbalancing effect of the assemblage of 

 factors in the fragments. Already, from only a superficial study of the main 

 mutants and their varieties, we can conclude that each of the chromosomes 

 contains factors which affect the somatic characters of habit of growth and of 

 intensity and distribution of purple pigmentation, the various mutants 

 varying in plus or minus direction from the balanced condition in normal 

 diploids. Dr. Sinnott is analyzing the effects of extra chromosomes upon the 

 internal anatomy and Dr. Buchholz the effects upon pollen-tube growth. 



