BREEDING SYSTEMS 



allelomorph (S) is dominant over the pin allelomorph (s). But 5, 

 as much as S, pollen grains of Ss plants behave in a thrum way, 

 while 5 pollen grains of 55 plants behave in a pin way. 



Thus, either o£ two different mechanisms of gene action with 

 which we are familiar elsewhere can determine incompatibility. 

 One acts directly through the haploid pollen grain; the other is 

 imposed upon the pollen through the cytoplasm of its diploid parent. 



LEGITIMATE UNION 

 Complete Fertility 



^ o^-- ^ 



ILLEGITIMATE 

 UNION 



Incomplete 

 Fertility 



kt 



I 



-•> 



LEGITIMATE 

 UNION 



Complete 

 Fertility 



ILLEGITIMATE 

 UNION 



Incomplete 

 Fertility 



PIN 

 Small Pollen 



THRUM 

 Large Pollen 



Fig. 62. — Distyly in Priinula, showing the incompatibihty associated with it. The 

 illegitimate unions, of pin X pin and thrum x thrum, are incompatible whether 

 the pollination is made within a flower, as shown in the diagram, or between 

 flowers, whether from the same or dilferent plants (after Darwin, 1877). 



This delayed action is made still clearer by a further elaboration in 

 the Purple Loosestrife, Lythrum salkaria, also studied by Darwin. Here 

 there arc three types of plants, each with its own form of flower. 

 There are three levels for the sexual organs within each flower, two 

 of which are occupied by anthers and one by the stigma, so that the 

 three types have long, mid, and short styles. Legitimate, that is 

 fertile, unions are always those between anthers and stigmata of 



2S0 



