Genetics of Flowering Responses • 135 



as the gross segregation suggests (Purvis, 1939). Sarkar (1958) has 

 confirmed and extended earlier work on the cold requirement in 

 Hyoscyamus niger. Here again, crosses between the annual and 

 biennial strains indicate a single-gene difference in this regard, but 

 there is no dominance. The F 3 is intermediate between homozygous 

 annuals and homozygous biennials. The heterozygote will eventually 

 respond to long days without a previous cold treatment, but does 

 so more rapidly with it; a given cold treatment has a greater effect 

 on the heterozygote than on the pure biennial; and the former 

 reaches a vernalizable stage earlier in development than the 

 latter. 



Not all vernalization requirements appear to depend on single 

 genes. Napp-Zinn (1960) reports in one paper of a continuing study 

 on Arabidopsis thaliana that the difference between summer and 

 winter annual strains depends on at least two genes. In addition, 

 the relation between developmental stage and susceptibility to 

 vernalization is under further genetic control, which has not been 

 completely analyzed. 



This brief survey will be sufficient to suggest the nature of 

 such investigations. Two general observations are worth making in 

 this connection. In the first place, it seems evident even from the 

 little that is known that specific requirements for flowering are 

 not necessarily genetically deep-seated, but may be easily acquired 

 or lost. Hence conclusions about the distribution— geographical 

 or geological— of species and families on the basis of the present-day 

 response characters of certain members (for example, Allard, 1948), 

 although stimulating, should be entertained with the greatest 

 caution. Second, and perhaps more important, there is clearly room 

 for much more work on the genetic control of flowering require- 

 ments. Cold requirements, at least, are currently receiving con- 

 siderable attention (see Napp-Zinn, 1960) but genetic studies are 

 notably inconspicuous or absent in most of the recent literature on 

 photoperiodism. The difficulties should not be underestimated— 

 particularly those involved in finding SDP and LDP sufficiently 

 closely related to allow crossing, a difficulty that in itself may be of 

 great importance. However, with the increasingly precise knowledge 

 that research in flowering may be expected to gain from investiga- 

 tions as diverse as those on the red, far-red system and with chemical 

 controlling agents, a biochemical genetics of flowering as envisaged 



