432 Morpho genetic Factors 



seed and the double recessive sksk tsts is thus female. If this is crossed 

 with a plant sksk Tsts, which is male, half the offspring are Tsts (male) 

 and half tsts (female). Interbreeding such plants, which are incapable 

 of self-fertilization, will continue to produce offspring of which half are 

 staminate and half pistillate, so that these plants, if prevented from 

 crossing with other types of maize, will constitute a dioecious race. In this 

 case chromosome 1, on which tassel seed is located, functions as a sex 

 chromosome although no morphological difference is visible. 



A detailed review of the genetic basis for sex expression in flowering 

 plants has been written by Allen (1940). 



Genes and Growth Substances. Since growth substances so powerfully 

 affect growth and development it is natural to expect that in many cases 

 gene action will involve the production and distribution of these sub- 

 stances, and in a considerable number of cases this has proved to be the 

 case. Thus in the profound changes in growth habit of "lazy" maize 

 (p. 390) the character is due to the fact that auxin, instead of accumu- 

 lating on the lower side of a horizontal stem, remains evenly distributed 

 so that the stem does not turn upward. Its failure to do so is not the re- 

 sult of mechanical weakness but of abnormal auxin relations. 



Mention has already been made (p. 264) of the single-gene difference 

 between the annual and the biennial varieties of Hyoscyamus niger and 

 the fact that this is apparently due to a growth substance which can 

 be transmitted by grafting, to make the biennial form flower in its first 

 season. It is probable that a growth substance may also be operative in 

 other similar cases, like those of beets and white sweet clover, where the 

 difference between annual and biennial forms has been shown to be due 

 to a single gene. In the single-gene mutants reported by Stein (p. 265) 

 which were grafted to normal stock, some of the effects of the gene seemed 

 able to cross the graft union but others did not. 



A particularly interesting case is reported by Scheibe ( 1956 ) for peas, 

 where a recessive fasciated mutant, differing from normal by a single 

 gene, has a higher concentration of natural auxin than the normal. 

 Furthermore, fasciation can be produced in the normal type by appli- 

 cation of indoleacetic or naphthaleneacetic acid. Here the difference be- 

 tween the two genes seems to be in their ability to produce a growth 

 substance. 



Genes are also concerned with photoperiodic response. The Mammoth 

 mutant of tobacco differs from most strains by a single gene which, 

 among other effects, has changed the normal day-neutral type to one 

 that flowers only under short day-lengths. Chandraratna ( 1955 ) has 

 shown that, in rice, races sensitive to photoperiod differ from day-neutral 

 types by a single gene. 



Goodwin ( 1944 ) crossed several races of a short-day species of Solidago 



