APOMIXIS AND RELATED PHENOMENA 409 



degeneration. In (Enothera, Haberlandt (1921, 1922) finds that wound- 

 ing brings about the formation of adventitious embryos and he attributes 

 this to the action of a wound hormone originating in the injured tissue. 



The importance of apomixis to the geneticist and field taxonomist 

 is well illustrated by the condition in the Canince section of the genus 

 Rosa (Tackholm, 1920, 1922). Among these roses there are types which 

 had long been regarded as distinct species because of their remarkable 

 constancy in external characters and the lack of intermediate types. 

 Tackholm found that the section includes tetraploid, pentaploid, and 

 hexaploid forms, and that they all show most strikingly the Drosera 

 type of chromosome behavior in meiosis. As a result the microspores 

 and male gametes in the pentaploid forms, for example, contain from 7 

 to 22 chromosomes, while the eggs usually have 28. These facts clearly 

 indicate the hybrid nature of the Canince roses. They are stable geneti- 

 cally because they are apomictic : the embryo arises not from sexual cells 

 but by budding from the nucellus. They are therefore not pure species 

 but asexually reproducing clones which maintain their hybrid chromo- 

 some complements and the same external characters from generation to 

 generation because the reassorting of chromosomes and genes at meiosis 

 plays no part in determining the make-up of the next generation. Apo- 

 mixis obviously retards the process of forming new combinations, but it 

 preserves new types once they are formed, just as in other vegetatively 

 propagated plants. The importance of such facts for students of the 

 origin of species and varieties can scarcely be overestimated. 



ANIMALS 



The best know^n examples of natural parthenogenesis in animals are 

 found among the rotifers, crustaceans, and insects, this being the regular 

 mode of reproduction in some species. Other modes may also occur in 

 such organisms under certain conditions or after a certain number of 

 generations. Some species produce parthenogenetic and sexual eggs 

 which show conspicuous structural differences. Moreover, parthe- 

 nogenesis may be artificially induced in the eggs of other animal groups, 

 notably echinoderms, mollusks, and amphibians. Around this fact 

 centers much significant work in modern experimental biology. ^^ 



Parthenogenesis. — In true parthenogenesis the egg develops without 

 syngamy or other nuclear fusion. 



1. Reduced Parthenogenesis. — The developing gamete has the reduced 

 (gametic) chromosome number, two maturation divisions having effected 

 haplosis as usual. 



" Robertson (19316) introduces the useful term parthenote for an individual derived 

 from an egg with only one gametic nucleus. 



