54 On Hybridisation of some Species of Salix 



those of Ruhus, is highly probable, though not yet actually proven. 

 This problem will be one of the objects of my future study. 



That in the case of Salix the formation of false hybrids is due 

 neither to parthenogenetic development of the oosphere nor to the 

 vegetative production of embryos from nucellar cells is quite evident in 

 view of the fact that female inflorescences covered with paper bags 

 were never able to bear even single fruits. Many authors think ^ that 

 in the formation of false hybrids pollen has nothing to do with fertilisa- 

 tion, but acts merely by irritating egg-cells in some way and enables 

 them to develop into embryos without being fertilised ; such process is 

 called pseudogamy, a word first proposed by Focke^ Giard-^ thinks 

 that false hybrids of purely paternal type are derived from maternal 

 cytoplasm with the male nucleus alone, the female nucleus degenerating 

 (Merogony !), and that those of purely maternal type are derived by 

 pseudogamy, some stimulus to development being given by the pollen- 

 tube without entrance of the sperm-nucleus into the egg. All these 

 are however mere hypotheses which are simply more or less probable, 

 and which ought to be proven cytologically. The only cytological 

 investigation on false hybrids of plants is that of Strasburger on 

 Fragaria*, which did not confirm the hypothesis of Giard above stated. 

 Thus in the hybrid F. virginiana % x F. elatior ^ the former author 

 could observe no degeneration of the egg-nucleus, while, on the contrary, 

 not only was he able to see clearly the fusion of the sperm- and the 

 egg-nuclei, but he was able to count in the mitosis of the" fusion-nucleus 

 the diploid number of chromosomes. 



False hybrids have also been observed in animals, and there is a 

 series of papers concerning hybrids of purely maternal type, though 

 they never reached the adult stage. In these hybridisations, or hetero- 

 geneous fertilisation, as it is often called, eggs of the Echinoids 

 (as Sphaer echinus J Strongylocentrotus, Echinus, Arbacia, etc.) were 

 fertilised by sperms of the Echinoids, the Crinoids (as Antedon), the 

 Mollusks (as Mytilus), the Vermes (as Chaetopterus). Since in these 

 hybridisations the systematic affinity of the two parents is always very 

 remote from each other, only the larvae, in more or less advanced stages 

 of their development, were obtained, and these have always proven to be 



1 For instance, Hurst, I.e. p. 106 ; Winkler, I.e. p. 333. 



2 Die Pflanzenmisehlinge, Berlin 1881, p. 525. 



3 Gomptes-rendus de la Soe. de Biologic, einquantenaire de la Soc. 1899, Vol. jubilaire, 

 p. 665 ; Gomptes-rendus hebdomad, des Seances de la Soc. de Biologie de Paris, Vol. lv. 

 1903, p. 779 (original not seen ; cited according to Solms-Laubach, I.e. p. 53). 



4 Histol. Beitrdge, Heft vii. 1909, pp. 43—46. 



