THE AXGIOSPERMAE 



1475 



embryo sac, we have classed with a pomictic pheno- 

 mena. A clear division is not always possible {e.g., 

 the development of unfertilized synergidae is really 

 apomictic) and it is undesirable to try to classify 

 cases of polyembryony into "true" and "false". 

 Nevertheless embryos arising outside the embryo 

 sac, that is from sporophyte cells, are clearly in a 

 different category from those arising, in whatever 

 w^ay, from gametophytic cells, even when the 

 sporophvtic embryos invade the embryo sac and 

 develop inside it. 



The term apomixis in its widest sense covers 

 all non-sexual methods of reproduction and there- 

 fore logically includes vegetative propagation and 

 reproduction by non-sexual spores, etc, in Crypto- 

 gams. There is a good scientific sanction for this 

 usage, since extensive vegetative reproduction is 

 linked in many species with failure or abnormality 

 of the sexual process. The older and narrower use 

 of the term confines it to cases where a sexual 

 mechanism exists but functions without sexual 

 fertilization taking place, as the word itself, signi- 

 fying "away from mingling", implies. It is in this 

 sense that we use it here. If the wider concept 

 of apomixis be adopted, the more restricted pheno- 

 mena are called agamospermy, a term that 

 may be criticized on the ground that it does not 

 cover aposporic reproduction in Ferns, which is 

 biologically similar. 



Several other terms are in use in this connection, which it is well to get 

 clear. 



Diplospory. A diploid embryo sac is formed directly from a mega- 

 spore mother cell, and an embryo is formed from the diploid oosphere 

 without fertilization. Sometimes called somatic parthenogenesis 

 or diplo-parthenogenesis. 



Apospory. A diploid embryo sac is formed from a somatic cell, e.g. of 

 the nucellus. Embryo development as above. 



Parthenogenesis. A haploid oosphere is formed and develops an embryo 

 without fertilization. (Diplo-parthenogenesis, if the oosphere is 

 already diploid.) 



Apogamy or Apogamety . One of the cells of a haploid or diploid game- 

 tophvte, apart from the oosphere, develops an embryo without 

 fertilization. 



Adventitious Emhryony. Embryos are produced directly from somatic 

 cells of the nucellus or integuments and develop usually within the 



Fig. 1353. — Xeliimbo lutea. 

 Two embryo sacs in 

 axillary order and with 

 reversed orientation, 

 the oosphere in the 

 smaller sac being at 

 the chalazal end. 

 {After M. T. Cook.) 



