246 Parthenogenesis in Hieracimn [ch. 



non-Mendelian interpretations which in almost every case it 

 has been found possible to replace by simple Mendelian 

 formulae. Where this reduction to a common plan has 

 not been yet effected, the difficulty, we feel fairly confident, 

 is ge^ierally created rather by the disturbance of environ- 

 mental causes or by the influence of undetermined factors 

 than by any more profound aberration in physiology. 



It is the object of the present chapter to discuss some 

 of the more prominent of the phenomena which are really 

 or apparently unconformable. As will be seen, they are of 

 a miscellaneous nature. 



I. Ci^osses breeding true without Segregation, 

 A. Parthenogenetic Cases, 



Of crosses breeding true there are two types quite 

 distinct in nature and significance. In the first we now 

 know that the absence of segregation is due to the fact that 

 the reproduction is parthenogenetic, or apogamic, as it is 

 more often called by botanists. The famous example of 

 this kind of reproduction is provided by Hieracium (Hawk- 

 weeds). Mendel himself investigated the inheritance of 

 Hieracium experimentally, and his paper is given in trans- 

 lation at the end of this book. This o^enus beino- one of 

 the most strikingly polymorphic, he chose it after his 

 discovery regarding the inheritance of peas, as the subject 

 of further research. We may surmise that he expected to 

 find in it illustrations of the new principles. The technical 

 difficulties were extreme. The minuteness and the delicacy 

 of the flowers made the operation of castration almost 

 impossible to carry out on a large scale. It is recorded 

 that in the course of this work he injured his eyesight ; 

 and after all precautions were taken he was mortified by 

 what seemed to be frequent failures. Seeds that were 

 supposed to be hybrid gave plants of pure maternal type, 

 which he could, in those days, only attribute to accidental 

 pollination from anthers of the mother-plant, caused by 

 imperfect emasculation. In a few cases however he did 

 succeed in raising genuine and obvious hybrids. These 

 hybrids were partially sterile, but the seeds which they did 

 give reproduced the hybrid form again. 



