Short- day Plants — 137 — Sperm 



Somatic. — Pertaining to the body. Having two sets of 

 chromosomes, one set normally coming from the male parent 

 and the other from the female. 



Somatic Doubling. — A doubling of the chromosome comple- 

 ment brought about as a result of an aberrant mitosis in which 

 chromosome division takes place without nuclear division. 



Somatic Mutation. — A mutation in a somatic cell, resulting 

 in a chimaeral individual with respect to the mutant character 

 (J. Heredity). 



Somatic Pairing. — See under Pairing of Chromosomes. 



Somatic Segregation. — The production of two genetically 

 dissimilar cells at a somatic cell division, caused by some sudden 

 qualitative or quantitative change in the nuclear constituents, 

 or by the segregation of two dissimilar cytoplasmic consti- 

 tuents at the somatic cell divisions (Chittenden). 



Somatogenesis. — The study of the emergence under favour- 

 able surroundings of bodily form out of hereditary sources 

 (Walter). 



Somatogenic Variations. — Environmentally produced varia- 

 tions, not heritable. 



Somatoplastic Sterility. — The collapse of fertilized ovules 

 during the early developmental stages (Hayes & Immer). 



Spay. — To destroy or excise the ovaries. 



Spaying. — The castration of a female by removal or destruc- 

 tion of the ovaries. 



Speciation. — The processes whereby new species are formed. 



Speciation, Sympatric. — See Sympatric Speciation; Sym- 

 patric Speciation, Instantaneous. 



Species. — Groups of actually or potentially interbreeding 

 natural populations, which are reproductively isolated from 

 other such groups (Mayr, 1942). 



Specificity. — The degree of variation in the actual qualita- 

 tive nature of the effect of a gene. Specificity is similar to 

 expressivity but whereas the latter refers to the amount of 

 effect caused by a gene, the former refers to the quality of that 

 effect; clearly no hard and fast line can be drawn between 

 specificity and expressivity. 



Specific Modifiers. — Genes which modify the effect produced 

 by other genes. They may be without effect themselves (Ford, 

 1945). 



Speltoids. — A group of three types of wheat mutants which 

 resemble Triticum Spelta. 



Sperm. — The male gamete in animals. See under Spermato- 

 gonitim- 



