162 Genetic Behaviour of the Hybrid Primula Kewensis 



Sterility. 



Before proceeding to a detailed description of our experiments, we 

 must refer to the very important part played by sterility. In crossing 

 the parent species with each other or with the hybrid Kewensis, viable 

 seed has only rarely been obtained. In these crosses the ovary may 

 fail altogether to swell, or the ovary may swell and very poor seed be 

 obtained, some of which may germinate. On the other hand, we have 

 often obtained apparently good seed, of which none, or only one or two 

 seeds, have germinated. We have sometimes also got apparently good 

 seed which has failed to germinate from Kewensis selfed. The method 

 of culture has been to sow the seeds directly they are ripe, or a few 

 weeks later. The results have been the same with both methods. 

 There has been occasional difficulty in germinating seeds of verticillata 

 and jioribunda, but we regard these species as fully fertile, and in 

 practice the fertility of Kewensis is not sensibly less. The condition of 

 the diploid form is not accurately known. 



Heterostyly. 



It is possible that some at least of the sterility observed is due 

 to the foct that the parent species are respectively monomorj)hic and 

 dimorphic as regards their flower structure. The two forms of flowers 

 found in heterostyle species are usually referred to as " pin " or long 

 styled and " thrum " or short styled respectively. The anthers in the 

 pin form are at the base of the corolla tube : the style is long and may 

 even protrude slightly beyond the corolla tube. In the thrum form the 

 anthers are situated near the mouth of the corolla tube, while the style 

 is very short and extends only a little way into the corolla tube. 



In the monomorphic types such as P. verticillata the anthers are 

 always near the mouth of the corolla tube, the style is long, but its 

 length is variable, the stigma being sometimes on a level with the 

 anthers, sometimes below the anthers, and .sometimes above them. 



P. fluribunda exhibits heterostyly differing from the usual condition. 

 The pin form is typical, but in the thrum form we have an arrangement 

 resembling that found in verticillata. The anthers are always near the 

 mouth of the corolla tube while the pistil is long styled. The stigma 

 may be on a level with the anthers, or it may lie above them or below 

 them, but we have not seen any plant with the style as short as in the 

 normal thrum condition described above. 



P. Kewensis resembles P. verticillata in being monomorphic, 



