426 



M 01 pho genetic Factors 



by various workers to differ in some types by a single gene from normal 

 flower. Fasciation is a single-gene recessive to normal in Nicotiana 

 (O. E. White, 1916). In both peloria and fasciation individuals are found 

 which are usually normal but occasionally produce these abnormalities, 

 presumably because of some genetic predisposition in this direction. These 

 should be of particular interest in studies of the basis of gene action. 



There are two traits in the nature of abnormalities ("tufted" and 

 "polycladous"), presumably gene-determined, in the liverwort Spliaero- 

 carpos (Allen, 1924, 1925) which are of particular interest in that they 

 occur in the haploid gametophyte generation so that the effects of a single 

 gene are directly visible, unmodified by the influence of its allele. 



Fig. 19-5. Cross section of the stem of Aquilegia. Left, normal plant. Right, mutant 

 with thicker cell walls. ( From Anderson and Abbe. ) 



Anatomical characters have also been shown to be directly affected 

 by genes. In Aquilegia canadense a dwarf race with bushy, compact 

 growth and stiff, brittle branches has been shown by Anderson and 

 Abbe (1933; Fig. 19-5) to differ from normal by a single gene. The 

 direct effect of this gene is to cause precocious secondary thickening of 

 the cell walls, from which all the other differences follow. Piatt, Darroch, 

 and Kemp ( 1941 ) report that in wheat, solid stem differs from normal 

 hollow stem by three or four pairs of recessive genes. 



In all these cases where the form or structure of a particular plant part 

 has been found to be controlled by a gene or group of genes, much 

 valuable information as to the method of control may be gained by a 

 study of the development of this structure. In a few cases this has been 

 done, as in cotton leaves and cucurbit fruits described above; in dwarf 

 mutants in maize by Stephens (1948); in structure and growth rates of 



