Spontaneous Generation — 141 — Structural Heterozygot* 



Stem Body. — The equatorial portion of the spindle-cytoplasm 

 between the two groups of daughter chromosomes at late 

 anaphase, which, by elongating, helps to separate the two 

 groups of chromosomes more widely. 



Stenoplastic. — Having little or no modificational plasticity, 

 q.v. N. Stenoplasty. 



Stenotribal. — Of, or pertaining to, flowers whose anthers 

 are so arranged as to dust their pollen on the under part of 

 the thorax of their insect visitors (Carpenter). 



Step Allelomorphism. — The theory that the locus of one 

 gene may contain a group of sub-genes arranged in linear 

 order and each contributing its definite and characteristic 

 quota to the phenotype. Each such sub-gene would be only 

 partially allelomorphic to the other sub-genes within the one 

 gene locus, cf. Fractionation. 



Stereoplasm. — The more viscous portion of the protoplasm. 



Stigma. — That portion of the style which is adapted for 

 receiving pollen. 



Stigmasterol. — A steroid from soya beans used in the synthe- 

 sis of progesterone and testosterone for clinical use. 



Stilboestrol, — dipropionate. — See Oestrogens. 



Stock. — The rooted plant onto which a graft is made. 



Stolon. — A runner, or creeping stem capable of forming ad- 

 ventitious roots and acting as a means of vegetative propaga- 

 tion. Adj. Stoloniferous. 



Strain. — A group within a variety which constantly differs 

 in one or more genetic factors from the variety proper. 



Strepsitene. — Late diplotene when the successive loops be- 

 tween chiasmata coming to lie at right angles give the appear- 

 ance of merely twisting round one another; a misnomer 

 (Dixon; Darlington). 



Structure. — The linear arrangement of the genes in the 

 chromosomes. 



Structural Change. — Change in the genetic structure of the 

 chromosome. May be intra-radial or extra-radial with respect 

 to arms, internal, fraternal or external with regard to chromo- 

 somes, symmetrical or asymmetrical with respect to the posses- 

 sion of a centromere, eucentric or dyscentric with respect to 

 the direction of a segment in relation to the centromere. Sec- 

 ondary , is change in structure resulting from cross- 

 ing-over between two homologous segments in chromosomes 

 which are structurally different on both sides of these segments 

 (Darlington). 



Structural Heterozygote. — Structural Hybrid, q.v. 



