xiv] Parthenogenesis in Hieracium 247 



In the earlier discussions which followed the rediscovery 

 of these papers we all were inclined to follow Mendel in 

 supposing that Hieracium illustrated a distinct kind of 

 sexual inheritance in which segregation was absent, and it 

 seemed natural to suspect that the association of this pheno- 

 menon with partial sterility was not accidental. As we 

 now know, there is a distinction between the inheritance 

 exemplified by these true breeding hybrids, but it is not a 

 speciality of Hieracium, or in any sense to be regarded as 

 an exception to the general rule that in sexual heredity 

 segregation of characters occurs. The meaning of the facts 

 was first discovered by Ostenfeld (219) following up an 

 experiment of Raunkiaer (228). These two observers 

 found that both Taraxacum and many Hieracia have the 

 power of setting seeds parthenogenetically without any 

 fertilisation. Such plants may set seed profusely when all 

 the anthers and stigmas are cut off in an unopened bud. 

 The cytology has been investigated by Rosenberg (233), 

 and he has shown that in the ovules which are capable of 

 parthenogenesis no reduction-process occurs. In the strict 

 sense therefore this method of reproduction is not sexual, 

 but asexual, being comparable with the reproduction from 

 buds or cuttings in which, as is well known, the parent type 

 almost always reappears without modification. 



This fact explains not only why Mendel's hybrid 

 Hieracia bred true, but also why he so often failed to 

 produce a hybrid when all precautions were taken to avoid 

 self-fertilisation. The plants which were supposed to be 

 due to accidental self-fertilisation were in reality partheno- 

 genetic. From Ostenfeld's experiments it was shown that 

 some only of the ovules are capable of fertilisation ; and it 

 is practically certain that the ovules with unreduced nuclei, 

 though they may give rise to plants by parthenogenesis or 

 apogamy, are incapable of being fertilised. The evidence 

 that the unreduced egg-cells merely reproduce the maternal 

 type is of course a strong support to the view that it is 

 in the reduction-division that the segregation of factors is 

 effected. 



It is by no means alone in the Compositae that partheno- 

 genesis has been observed. Fresh cases are continually 

 being discovered among flowering plants, and in any 



