THE BUiiAKDOWN OF CONTROL 



a small seed-set obtained in this way is easier than a large seed-set 

 obtained from legitimate pollinations by hand. This process of un- 

 conscious selection for self-compatibility has had its results recorded 

 at intervals over nearly half the 120 years it has been at work. They 

 show a steady increase in the success of self-pollination of the pin 

 plants, a steady decay in the effectiveness of incompatibility 

 (Table 23). 



TABLE 23 



THE BREAKDOWN OF INCOMPATIBILITY IN PRIMULA 

 SINENSIS (MATHER AND DE WINTON, 1941) 



* Measure of fertility of illegitimate pollinations relative to legitimate ones, used in order to correct 

 for the decline in fertility introduced into the later results by the accumulation of recessives genes in 

 material used for genetical experiments. 



In spite of this partial breakdown of the incompatibility mechan- 

 ism, its action is still controlled by the pin-thrum gene, S-s. This 

 gene can therefore control incompatibilities of different strengths, 

 whose differences are governed by other genes. In other words the 

 S-s gene acts as a switch directing development into one of two 

 alternative paths, of which the precise course depends on other genes, 

 or rather polygenic systems, whose variation is not normally 

 detectable. 



Ordinary incompatibility is controlled in the same way: it has 

 the same genetic structure as heterostyly. For example. Petunia 

 violacea has the usual multiple allelomorph system of pollen-style 



255 



