xii INTERMEDIATES 133 



genus of plants exhibits an extraordinary profusion of 

 forms differing from one another sometimes in a single 

 feature, sometimes in several. The question as to how far 

 these numerous forms were to be classified as distinct 

 species, how far as varieties, and how far as products of 

 chance hybridisation, was even at that time a source of 

 keen controversy among botanists. There is little doubt 

 that Mendel undertook his experiments on the Hawk- 

 weeds in the hope that the conception of unit-characters 

 so brilliantly demonstrated for the pea would serve to ex- 

 plain the great profusion of forms among the Hieraciums. 

 Owing to the minute size of their florets, these plants offer 

 very considerable technical difficulties in the way of cross 

 fertilisation. By dint of great perseverance and labour, 

 however, Mendel succeeded in obtaining a few crosses 

 between different forms. These hybrids were reared and a 

 further generation produced from them, and, no doubt 

 somewhat to Mendel's chagrin, every one of them proved 

 to breed true. There was a complete absence of that seg- 

 regation of characters which he had shown to exist in 

 peas and beans, and had probably looked forward with 

 some confidence to finding in Hieracium. More than 

 thirty years passed before the matter was cleared up. 

 To-day we know that the peculiar behaviour of the hybrid 

 Hieraciums is due to the fact that they normally produce 

 seed by a peculiar process of parthenogenesis. It is pos- 

 sible to take an unopened flower and to shear off with a 



