Knight — 44 — Dictionary 



Disc Floret. — A regular tubular floret typically found in 



the inner portion of a capitulum. 

 Discontinuous Variation. — A mutation, or sudden heritable 



variation. 

 Discrete Variations. — See under Continuous Variations. 

 Disease Garden. — A nursery in which plants can be grown 



under optimum conditions for any or several particular diseases 



as a means of studying resistance. 

 Disjunction. — The moving apart of chromosomes at ana- 

 phase. 

 Dislocated Segments. — Homologous pairs of segments in a 



different linear sequence from other segments in a structural 



hybrid (Darlington). 

 Dislocation. — Structural change, q.v. 

 Disomaty. — The production of somatic nuclei with twice the 



normal somatic chromosome number ; tetraploidy. 

 Disome. — A diploid chromosome complement; a chromosome 



set having paired members, as in normal somatic tissue. 

 Disomic. — Of two homologous chromosomes, or genes. 

 Disparate Chiasmata. — Chiasmata which produce an asym- 

 metrical relationship between chromatids in that the second 



chiasma does not restore the relationship altered by the first. 



0pp. Comparate Chiasmata. 

 Dispermic Fertilization, Dispermy. — Fertilization of an 



ovum by two spermatozoa. 

 Dispireme. — The stage of karyokinesis in which the spireme 



thread has formed in both daughter nuclei. 

 Dissogeny. — Dissogony, q.v. 

 Dissogony. — Having two periods of sexual maturity during 



the life cycle, one during the larval stage and the other during 



adulthood. 

 Distal. — Of a chromosome, that part which is furthest from 



the centromere. 

 Distal Chiasma. — (i) A chiasma formed in that portion of 



a chromosome which is distal to an inversion loop i.e. so that 



the loop lies between the chiasma and the centromere, (ii) 



A chiasma formed further away from the centromere than a 



particular other chiasma. 

 Ditokous. — Producing two young (or eggs) at a time. 

 d/i Values. — The ratio of differential distance to interference 



distance. See Differential and Interference Distances. 

 Dizygotic. — (Generally of twins) originating from two 



fertilized eggs. 



