250 Inheritance in the Groundsel 



garden for two or three years, crossed with a local radiate form in 1908, 

 and 48 plants were brought to the flowering stage in 1911. As the 

 results of analysis are peculiar and difficult of interpretation, it is 

 necessary to state that of the 112 specimens which were originally 

 planted, 64 perished of a disease induced probably by the rich soil 

 and close planting. The disease had no selective action ; it simply 

 destroyed the plants in the middle of the bed. The 48 plants which 

 matured consisted of two types : 24 were RR and 24 NR plants. The 

 seed-bearing plant was certainly NN in appearance ; the pollen was 

 from an ordinary RR plant specially introduced into the garden for 

 the purpose of making this cross. The expectation was, as self-pollina- 

 tion was not excluded, that a large number of non-radiate {NN) plants 

 would be secured and a few F^ hybrids, NR in type. It was rashly 

 assumed that the non-radiate type would produce non-radiate plants 

 only, if selfed, as is invariably the case in non-radiate forms of Senecio 

 vulgaris. This ambiguous result may be tentatively explained by 

 assuming (1) that the non-radiate (NN) plant produced equal numbers 

 of R and N ovules, was indeed in reality a heterozygote (NR), and 

 (2) that the. pollen of the radiate type (RR) is prepotent and, in the 

 presence of both types, is the only one that is effective. Accepting 

 these assumptions, all the N ovules would be fertilized by R pollen 

 grains and produce NR plants, and all the jK ovules by R pollen grains 

 and produce RR plants. The explanation is purely provisional, and not 

 the only possible one. The experiment is recorded at this stage to 

 show how necessary it is to avoid the inference that what is proved 

 to be true for one species is also true for others, however closely allied 

 the species may be. 



The ray character has hitherto been examined experimentally in 

 Senecio vulgaris and S. Jacobaea only, Gentaurea nigra, as is well 

 known to many botanists, possesses radiate and non-radiate forms, and 

 the mode of occurrence of these in Glamorgan suggests that the ray 

 character segregates as in the groundsel. Other similar variations are 

 by no means infrequent in the Compositae, and are well worth in- 

 vestigation by the experimental methods of genetics. The problem 

 of the inheritance of the ray character in groundsels can, however, be 

 regarded as solved, and it seems desirable, with a view to securing 

 uniformity and brevity of notation, to adopt the theory of dominance 

 to the extent of using the signs RR, Rr and rr instead of RR, RN and 

 NN for the three types of ray character. R signifies radiate, r non- 

 radiate. 



