64 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



have been found in the callus of a haploid tomato by Lindstrom 

 and Koos (1931), and in this way tetraploid, octoploid and other 

 abnormal nuclei may arise in the diploid. In the haploid Triticum 

 multiple fusion occurs and in a root-tip of Crepis tectorum Navashin 

 (1926) has found a dividing cell with 500 chromosomes instead of 

 the normal 8 [i.e., 125^;). Tetraploid (4^:) plants first arose in this 

 way in grafting experiments in Solanum (Winkler, 1916) and Crane 

 (cf. Jorgensen and Crane, 1927 ; j0rgensen, 1928 ; Sansome, 1931) 

 developed the method to produce tetraploids systematically. 

 Usually about 6 per cent, of adventitious callus shoots are tetra- 

 ploid in 5. Ly coper sicum. Poisoning and ill-treatment of various 

 kinds have been used to induce doubling of the chromosome number 

 in root-tips (cf. De Mol, 1921 ; Nemec, 1929). 



Doubled nuclei having arisen with normal frequency probably 

 develop with exceptional vigour in haploids ; such plants have 

 arisen by failure of fertilisation. They are weak in growth and are 

 often distorted and modified by the occurrence of diploid tissues, 

 with larger cells either as islands or as whole shoots [cf. Hollingshead 

 1930 ; V. Ch. VI). Thus amongst no root-tips of haploid Crepis 

 capillaris there were 42 wholly diploid and 28 partly diploid, and 

 one partly tetraploid. Amongst 82 root-tips of haploid Nicotiana 

 Tahacum, 22 were wholly diploid and 8 partly diploid (Ruttle, 1928 ; 

 cf. Webber, 1933). Datura and Oryza haploids, like those of Crepis 

 (cf. Hollingshead, 1930 a) have yielded diploid shoots (cf. Meurman, 

 1933, on Acer ; Hruby, 1935 h, on Salvia). 



In tissues in which polyploid nuclei have arisen, these nuclei have 

 been observed to divide by multipolar mitoses. A spindle is then 

 formed with three or four poles, as in a doubly fertilised echinoderm 

 Qgg, and the chromosomes are distributed at random to these poles. 

 This has been found both where the polyploid cells were developed 

 in galls and following chloralisation. The result is the production 

 of nuclei with chromosome numbers once more reduced (Nemec, 

 1926, 1929). This is perhaps the basis of rare variations such as the 

 halving of the number in the tetraploid, observed by Winkler (1921) 

 in Solanum nigrum. Randolph (1932) by treatment of the parents 

 with high temperatures, has induced the formation of tetraploid 

 cells (as well as other abnormalities) in young embryos from which 



