46o BREAKDOWN OF GENETIC SYSTEMS 



In the organisms in which meiosis immediately follows 

 fertilisation, on the other hand, there is no evidence of any such 

 mechanism. Since every cell which is diploid in these undergoes 

 meiosis it seems possible to consider that meiosis, which in the 

 higher organisms must be regarded as induced in specialised cells 

 by their relationship with the rest of the organism, is in these simple 

 forms determined directly by the condition of diploidy (cf. D., 

 1932, p. 458). In this connection the parthenogenetic Chara crinita, 

 Ernst, 1918) is of interest. It is diploid, unlike its sexual relations ; 

 but since, as Ernst supposes, it is probably a hybrid, the normal 

 diploid condition, the presence of pairs of identical chromosomes, is 

 lacking. Parthenogenesis wiU therefore follow the failure of meiosis 

 in such an individual as a matter of course, if it happens to be 

 female like the regenerated diploid gametophytes of Sphcerocarpiis 

 (Lorbeer, 1927). Hybridity is then seen as a simple and immediate 

 condition of parthenogenesis. 



(iv) Meiosis in Rosa canina. The type of sexual reproduction 

 found in Rosa canina and its relatives approaches closely to apomixis 

 and can best be considered in relation to it. All species of Rosa 

 have multiples of 7 chromosomes. Aneuploid forms reported by 

 Tackholm (1922) and Erlanson (1929) are exceptional and probably 

 sterile. Most diploid and even-multiple polyploid species have 

 fairly regular pairing, although they are none the less to be regarded 

 as hybrids (Blackburn and Harrison, 1921 ; Tackhohn, 1922 ; 

 Erlanson, 1929), and some have been shown to be interchange 

 heterozygotes (Erlanson, 1931 c). They are sexually reproducing, 

 with occasional apomixis in some species. All species of the 

 section Canince (European and Asiatic) are facultatively apomictic. 

 For example, in 38 seedlings of R. coriifolia solanifolia x glauca 

 concolor, 37 were maternal in character, one was a hybrid (Tack- 

 holm, 1922). Gustafsson (1931) has shown that this apomixis is 

 not spontaneous, but requires the stimulus of pollination, especially 

 of cross-pollination, for self-fertilisation probably leads to successful 

 sexual reproduction {v. infra). 



All the CanincB roses have, according to Tackholm and Hurst 

 (1931), seven pairs of chromosomes and 21 univalents (or, in a few 

 species, 14 or 28). The pairs are distinguished by the regularity of 



