No. 638] 



TYPES OF MUTATIOXS 



261 



to the influence of each specific chromosome upon the 

 morphology and physiology of the datura plant. Whil^; 

 there seems to be but a single chromosomal set rcsixvn- 

 sible for the presence or absence of purple piginciitnt ion. 

 probably each chromosome has an influence iiixm the 

 strength of expression of the pigment since the sc\cr;il 

 mutants appear to differ \vid('l>- in coloi- win n homo- 

 zygous for the main purple f;ict(»r. 'riiu> (iIo--y is 

 darker purple than normals, while Cockh'hur i- .list iiietly 

 lighter. In normal i)lants tiiere is a balanced ndjustmen't 

 between the modifying factors in the different chromo- 

 somes. When this balance is disturbed by tlie addition 

 of only a single extra chromosome to one of the 12 sets, 

 profound changes are brought about in tlie ontogeny of 

 the resultant plant. When all of tlie sets li.iw an extra 

 chromosome, however, as is the case in triploids. ik. ureal 

 disturbance of the balance is brought about and the plant 

 is not greatly different from normals. Mven in tetra- 

 ploid plants where all the sets are e(piall\- affected, al- 

 though the total number of chromosomes is doubled, the 

 difference from iioniial is not so -reat as in mutants of 

 the (i|..b.. and I'oin.ettia --ei-i* ^. Th.' lea \ . ^ of t ( t I'a ploid 

 ].l.int^. uhen <Mrr>inu the la--tor loi' nian> node.. ma> bt" 

 diMincll> hamer than tln-e of nonnaN. l-\-w-nodi'd tetra- 

 ploids, hox\,Arr. are ea^lx distinguished. The best 

 diagnostic ehai-actei- has been the u'lobose shape of the 

 capsule, and \ et i»lants know n to be tetraploid from cyto- 

 logical exideiiee ha\e been foun.l thi- pa-t seaSDU with 

 capsules perfectlv normal In appearance. 



What i^ the bearim; of the phenomena of chromosomal 

 dui)lication in datui-a ui)on the mutation theoi-y.' In the 

 first place, the mutants of the (iloiu' type apparently cor- 



which an (uld s,,nia- ic Vh ronn.some ha^ hr.u determined. 



