HETEROSTYLY 389 



pin-eyed pollen, a result confirmed by other workers. 

 Bateson and Gregory (1905) were able to prove that the 

 two types of flower are determined by a single Men- 

 delian factor. In Primula sine?isis, if the long-styled 

 flowers are pollinated by the same type only long-styled 

 flowers are found in the progeny. Short-styled flowers 

 selfed give either short-styled offspring only, or short- and 

 long-styled in the proportion 3:1. The "legitimate" 

 pollination gives either half and half short- and long- 

 styled or only short-styled. This proves that the short 

 style is due to a single dominant factor. Long-styled plants 

 are homozygous recessives. Short-styled plants are either 

 homozygous or heterozygous dominants. In the former case 

 they give only short-styled progeny, either when crossed 

 or when selfed ; in the latter case they give a mixture. 

 In nature, where only legitimate pollination occurs, the 

 short-styled plants are all heterozygotes, and the primroses 

 thus consist of two distinct races in each species, which can 

 exist only by continual crossing. It will be seen that 

 this is closely analogous to the relation between the male 

 and female plants of such diaxious species as Bryonia 

 dioica. 



Sensitive Stigmas and Stamens. — A type of floral 

 mechanism, the meaning of which is sometimes very 

 obscure, is that exhibited by flov/ers in which the stigmas 

 or stamens respond by rapid movements to the stimulus of 

 mechanical shock [seismonastic movement, like that of 

 Mimosa pudica). The two lobes of the stigma of a Mimulus 

 or an Incarvillea close together in a second or two if one is 

 lightly touched. This has been said to be of use in protect- 

 ing the stigma from deposition of " own " pollen when a 

 visiting insect is leaving the flower. As, however, the insect 

 in leaving brushes the hack of the stigma, such deposition 

 is in any case unlikely and the movement has probably not 

 much practical significance. Newcombe (1922) has shown 

 that in general sensitive stigmas which reopen quickly re- 

 main closed for longer if pollination has taken place, and 

 show a second and permanent closure a few hours later. 



