SELF-STERILITY 393 



flower may fall. We have thus a graded series showing 

 different degrees of interference with " own " pollen, while, 

 in every case, pollination from another individual of the same 

 species is followed by fertilisation. 



It has frequently been assumed that pollen from any 

 other individual of the same species is effective, but, as 

 Bateson (19 13) writes, this " has always seemed to me a 

 self-evident absurdity, for it would imply that there can be 

 as many categories as individuals." In fact, we now know 

 for two cases that sterility exists not for each individual, 

 but within definite races of the species. Correns (1913) in- 

 vestigated Cardamine pratensiSy the cuckoo flower. " Own" 

 pollen just germinates on the stigma, but gets no further, 

 while pollen from another plant is effective. Correns 

 crossed two parents, B and G, and from the progeny selected 

 sixty plants. Each of these he crossed back with each 

 parent. Half were sterile with B, half sterile with G, and 

 half were fertile with G, half fertile with B. The relation 

 to one parent was completely independent of that to the 

 other, so that the daughter generation could be divided into 

 four equal classes, W fertile with both parents, X fertile 

 with B only, Y fertile with G only, and Z fertile with 

 neither. The species therefore consists of races, probably 

 many in number, inside each of which complete sterility 

 exists, while seed is set when pollination takes place from 

 another race. The attempt to explain the results by the 

 inheritance of two different inhibitors is not completely 

 satisfactory. The second case, that of Veronica syriaca, 

 described by Lehmann (19 18, 1922), is similar, though 

 apparently more complex. 



The actual cause of the failure of the pollen grains to 

 germinate, or of the tubes to grow, is not known. It may be 

 due to the presence of an inhibiting agent as Correns believes, 

 and in support of this the behaviour of the orchids is strong 

 evidence. Jost, on the other hand, thinks that the failure 

 is due to the lack of some essential growth factor. He 

 points out that in no case does a pollen tube attain its normal 

 length in a culture solution. Compton (1913), in a general 



