Testicle, Testis — 149 — Tractile Fibre 



Thermocleistogamy. — Self pollination taking place within 

 flowers the opening of whose perianth has been inhibited by 

 low temperature. 



Thiamin, Thiamine. — Aneurine; vitamin Bj (Ci2HigN^ 

 SOClg), the anti-neuritic vitamin. 



Thixotropy. — The property of liquefying when agitated and 

 of returning to the 'gel' condition on standing. This property 

 is shown by some colloids. Adj. Thixotropic. 



Three-breed Crossing. — A system of breeding in which breed 

 A is crossed with B and the F^ with C, this hybrid is then 

 crossed with B and the product is crossed with A to produce 

 a hybrid which is crossed with C; the next cross is with B 

 and then A and so on. 



Three-way Cross. — A cross between an Fj and another 

 variety so that three parental types enter into the resulting 

 hybrid. 



Thremmatology. — Plant and animal breeding for domestic 

 uses. 



Throwing-back. — The reappearance of a character after a 

 lapse of one or more generations due to the character being 

 recessive or being dependent on complementary factors, or as a 

 result of back-mutation; atavism. Such an aberrant individual 

 is called a throw-back. 



Thrum-eyed. — Having the anthers at the throat of the corolla 

 and the stigma below, enclosed within the corolla-tube. 



Tocopherol. — A group of alcohols which show vitamin E 

 activity. a-Tocopherol, C^^H^qO^, and /^-tocopherol, CggH^g 

 Og, have been isolated from wheat-germ-oil and y-tocopherol 

 occurs in cotton seed oil. 



Top-cross. — Inbred-variety cross, q.v. 



Topoclines. — GeocHnes (q.v.), particularly where the dis- 

 tances involved are very great. 



Torsion-pairing. — Non-homologous association at pachytene 

 which releases a torsion without satisfying an attraction (Dar- 

 lington). 



Trabant. — A short segment of chromosome separated from the 

 main body of the chromosome by one or more constrictions. 

 An intercalary trabant has a constriction at each end, whereas 

 a terminal trabant being situated at the end of a chromosome 

 has but one constriction. A lateral trabant has only one 

 constriction but is itself a branch of the chromosome proper. 



Tractile Fibre. — A 'fibre' connecting the centromere with 

 the pole at cell division. 



