134 • A Miscellany 



flowering, sex expression, and leaf form, for example in hemp, 

 Cannabis sativa (J. and Y. Heslop-Harrison, 1958c), have provided 

 further evidence in its favor. Work of this sort also has implications 

 for the questions of juvenility and maturity mentioned in the 

 preceding chapter, but cannot be discussed in detail here. In addi- 

 tion, further information on the roles of other growth substances, 

 notably the gibberellins, will certainly be required before a truly 

 comprehensive hypothesis can be framed. 



GENETICS OF FLOWERING RESPONSES 



Flowering responses to photoperiod and temperature are of 

 course genetically controlled, and from the relative ease with which 

 "early" and "late" varieties of cultivated plants are bred, one might 

 guess that this control is often quite simple. Although practical 

 breeding work is not done with reference to narrowly defined 

 physiological responses, a number of specific investigations confirm 



this guess. 



The SDP characteristic of Maryland Mammoth tobacco has 

 been studied in crosses with Nicotiana tabacum var. Java. The ¥ 1 

 generation is not homogeneous, suggesting that the dominance of 

 Java's day-neutral (or, more accurately, weakly quantitative LDP) 

 characteristic is incomplete. In the F 2 , however, the SDP character 

 occurs in approximately one-fourth of the progeny, indicating 

 dependence on a single recessive gene. The "mammoth" (essentially 

 SDP) character apparently occurs frequently in various tobacco 

 varieties as a single-gene mutation, but its expression is affected by 

 other genetic properties of the variety. In the interspecific cross of 

 Maryland Mammoth with the LDP Nicotiana sylvestris, the LDP 

 character is completely dominant (Lang, 1948). In similar crosses 

 between the SDP Coleus frederici and the quantitative LDP Coleus 

 blumei the Fj plants are entirely SDP, indicating dominance of 

 this characteristic (Kribben, 1955). 



The difference between winter and spring varieties of Petkus 

 rye appears to be due to a single gene. In the ¥ 1 generation of a 

 (toss, the spring (noncold-requiring) habit is dominant; the F., 

 generation segregates in a spring:winter ratio of 8:1. However, 

 the dispersion in flowering time within the spring and winter classes 

 of the F 2 indicates that the situation may not be quite as simple 



