324 GENETICS 



and therefore show diflerent characteristics. In varieties 

 having odd numbers of chromosomes to the "pair," as in 

 triploids and trisomies, the different offspring are bound to 

 receive different chromosome combinations, and therefore 

 to be of diverse types. Among the offspring are usually some 

 individuals in which the chromosome grouping is so irregu- 

 lar and unbalanced that they cannot live. It is only the cases 

 in which all the chromosome pairs are doubled — that is, the 

 tetraploids — that are inclined to reproduce themselves 

 normally, thus forming a lasting variety. Tetraploid varie- 

 ties are known in many plants. In tetraploids the cells, con- 

 taining twice the usual number of chromosomes, are larger 

 than in the common diploids. In consequence the individuals 

 themselves are stouter and larger, forming so-called giant 

 varieties. One of the well known "mutants" found by 

 De Vries, Oenothera gigas, was of this type. It was large 

 and thick-stemmed, and its cells carried 28 chromosomes in 

 place of the diploid number 14. 



The varied races produced by changes in chromosome 

 numbers may in many plants be perpetuated through vege- 

 tative reproduction; that is, by cuttings, bulbs, runners, off- 

 sets and the like. This method of reproduction does not 

 disturb the chromosome arrangements. In cultivated plants 

 that multiply vegetatively, alterations in the number and 

 arrangement of the chromosomes seemingly play a large 

 role in producing the great variety of types that are to be 

 observed. Presumably many of the extraordinary forms, 

 sizes and colors seen in an exhibition of Dahlias, Chrysan- 

 themums or the like are the result of such changes. Many 

 of these diversities, as we saw in Chapter XII, are due to 

 the recombinations of genes resulting from crossing of 

 different stocks. Others are the result of the chromosome 

 changes here under consideration. Both types may be per- 

 petuated in uniparental reproduction. 



