68 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



Finally triploids arise by hybridisation between diploids and 

 tetraploids of the same species (CEnothera, Primula, Zea, Datura) or 

 of another (Rubus, Primus, Fragaria, Avena, Triticum) {v. Table 28d). 



(iii) Polysomy. Polysomic forms arise in a diploid through two 

 daughter chromosomes passing to the same pole at mitosis, or at 

 meiosis. The first abnormality is doubtless stimulated by adverse 

 external conditions, but disproportionately small chromosomes are 

 especially liable to it [v, infra). The same abnormality affects 

 the ordinary members of the chromosome complement less fre- 

 quently. Navashin (1930) has observed the two halves of one 

 chromosome passing to one pole in the plant of Crepis tectorum with 

 the ring-chromosome. In Drosophila, irregular division in this way 

 is the cause of gynandromorphism when a female cell with two 

 X-chromosomes gives a daughter cell with one X which yields 

 a male sector in an otherwise female animal, and of " diminished 

 mosaics " when a fourth chromosome is lost (Mohr, 1932 ; cf. 

 Crew on Melopsittacus, Fig. 121). 



The second abnormality has been shown to follow in meiosis : 

 (a) failure of pairing ; {b)" non-disjunction " of paired chromosomes 

 which then pass to the same pole ; {c) " non-disjunction " or 

 irregular distribution of the components of a multivalent 

 configuration in a polyploid or of a ring in a structural hybrid. 

 The first of these cannot strictly be called non-disjunction, although 

 this term is used by geneticists to cover the results of all three types 

 of abnormality, when they cannot be directly identified. 



Trisomic plants regularly arise by crossing diploid with triploid 

 in Solanum, Crepis and elsewhere [v. Ch. VI). 



They also appear through two daughter chromosomes passing 

 to the same pole in mitosis, either spontaneously, as has happened 

 in Datura and S cilia (D., 1926) or following abnormal treatments 

 which will be described later. How often trisomic nuclei arise in 

 mitosis we do not know, because they are probably often eliminated 

 in competition with the normal diploid nuclei. There is evidently 

 a struggle for existence amongst meristematic cells, at least in the 

 higher plants. This struggle favours the normal diploid at the 

 expense of the abnormal haploid or trisomic or tetraploid, so that 

 the abnormal types appear to be more changeable than the normal. 



