MIXED REPRODUCTION 465 



with the central nucleus to give the endosperm ; the other degenerates, 

 but the first fusion stimulates the development of the unreduced 

 egg-cell. Pollination is also necessary to stimulate apogamy and 

 apospory in Allium odorum (Haberlandt, 1923) and Zygopetalum 

 (Suessenguth, 1923). 



In Potentilla (Miintzing, 1928) several species which fail to set 

 seed without pollination produce entirely maternal offspring when 

 cross-pollinated, as well as when self -pollinated. Some of these 

 species have good pollen, others bad. In Riihus (Lidforss, 1914 ; 

 Gustafsson, 1930) the position is more complicated. In many 

 European species of blackberries (section Eu-hatus) crosses with 

 closely related forms yield a high proportion of hybrids, crosses 

 with more distant species yield entirely maternal offspring — " false 

 hybrids." These are identical with the female parent, yet when 

 such plants are selfed they yield the heterogeneous progeny 

 characteristic of a sexually reproducing hybrid. Evidently the 

 proportion of sexual embryos diminishes with increasing remoteness 

 of the male parent. Such a situation indicates competition between 

 the sexual embryos and their substitutes, the asexual, probably 

 aposporous, ones. The same conditions probably apply to the 

 Caninae roses, although the evidence here is still scanty (Hurst, 

 1931), and to Ohmachi's orthopteran hybrids (1929 h). 



Thus pseudogamy may stimulate either apospory (in Allium, 

 probably in Rosa and Potentilla) or diploid parthenogenesis 

 (Rhabditis, Zephyranthes). 



In all these cases we may perhaps not inappropriately look upon 

 pseudogamy as a physiological response that has been genetically 

 conditioned. Originally development of the egg-cell was no doubt 

 stimulated solely and directly by fertilisation. But fertilisation 

 has been accompanied throughout a long period of evolution, 

 that is of adaptive change, by certain invariable antecedents, such 

 as the penetration of the Qg^ by the sperm in animals and pollination 

 in plants. Many organisms have therefore acquired by adaptation 

 an equally satisfactory response by which the antecedents 

 determine development as effectually as the act of fertilisation 

 itself. 



(ii) Injury. By electrical, chemical and mechanical stimuli the 



