Sammelreferat. I43 



zygote plants had no influence on the degree of awning of their respective 

 F3 descendants. Of the Fg awnless plants, only eleven were grown on; 

 and of these, 5 bred true and fi scattered like the partially awned F, plants. 

 There are, therefore, plants which are awnless and yet behave genetically 

 as if they were partially awned, that is, plants in which avnilessness is per- 

 fectly dominant (5). 



Surface and Zinn find that the strongly awned condition is dominant 

 to the weakly awned condition , and they seem to get a clear 3 : 1 segre- 

 gation (10). 



All are agreed that the degree of awning is affected by changes in 

 the environment, but there is no concensus of opinion as to the specific 

 effect of any particular external condition. An exact study of the physio- 

 logical factors which cause the degree of awning to vary is clearly called for. 



An interesting feature is that the factor for yellow colour acts as an 

 inhibitor for awns. In crosses involving the two characters, no plant has 

 been found which is at once yellow and awned (7) and (11). Furthermore, 

 in cases where the yellow colour is masked by the epistatic Black factor, 

 the inhibiting character of the yellow factor is nevertheless able to function, 

 and prevent the awns appearing in the black plants which carry the hypo- 

 static yellow factor (7). In the case of Fraser's second yellow factor, it is 

 claimed that this differs from the usual yellow factor in that it has not the 

 power to inhibit the production of awns (2). 



Pubescence. 



In the wild oat, the backs of both the grains are pubescent, whereas 

 in the cultivated types they are smooth. Surface distinguishes between the 

 pubescence on the upper grain and that on the lower and his experiments 

 seem to show that each is controlled by a specific factor, of which one is 

 dominant and the other recessive: so that in the F, between fatua (pubes- 

 cent) and sativa (smooth), the lower grains are pubescent and the upper ones 

 quite smooth. These factors, although not linked (in Morgan's sense) are 

 connected with each other to the extent that in the absence of the factor 

 for pubescense on the back of the lower grain, the factor for pubescence 

 on the upper grain is unable to produce hairs (9). 



These factors, though segregating independently of one another, yet 

 are linked to other factors. There is evidence that there is slight linkage 

 between the factor for pubescence on the upper grain and the factor which 

 controls the fatua type of base: also the factor for pubescence on the lower 

 grain seems to be linked to the factor for black colour. In both cases, 

 however, the numbers concerned are not sufficiently large to give at all a 

 j)recise measure of the degree of linkage (9). 



