Temperature 341 



Different plants respond very differently to vernalization, and in some 

 it is without effect (Kondratenko, 1940). Vernalized plants may be de- 

 vernalized, usually by high temperature following the cold treatment, 

 and they may sometimes even be revernalized (Lang and Melchers, 

 1947). 



The induction of flowering by low temperature is by no means limited 

 to the cereal grains or to seed treatment. Young plants beyond the seedling 

 stage may be vernalized and thus forced into flower, and some biennial 

 varieties will flower in their first season if subjected to cold. The grow- 

 ing stem tip is the region sensitive to the vernalizing influence. Low- 

 temperature effects on growing plants, particularly as to flowering, 

 have been widely investigated. This work is reviewed by Thompson 

 (1953). 



The exact way in which low temperature produces its effects in ver- 

 nalization is not clearly known but in some cases it has been thought to 

 influence the production and distribution of auxin and perhaps also of 

 substances that stimulate flowering (p. 397). Hatcher (1945), however, 

 finds that the auxin content of grains of winter and spring races of 

 cereals is the same and that there is no detectable amount of auxin in 

 the embryos either at normal or low temperatures. He concludes that it 

 is not concerned in the process of vernalization. 



Although the most conspicuous effect of vernalization is the acceleration 

 of flower development, vegetative characters may also be affected, such as 

 leaf size. Hansel ( 1953 ) found that early leaves were longer if germina- 

 tion temperatures were slightly below 0°C than if they were slightly above 

 this (Fig. 15-3). Internal differentiation is also affected (Roberts and 

 Struckmeyer, 1948). 



The literature of vernalization and of its relation to photoperiodism 

 and phasic development is extensive. The history of research in this gen- 

 eral field has been reviewed by McKinney (1940) and Whyte (1948). 

 The latter is one of a series of papers on this general subject brought to- 

 gether by Murneek and Whyte (1948). Among other related publica- 

 tions are those of Gregory and Purvis ( 1938), and Whyte (1939). 



Other Temperature Effects. There are many other instances where 

 morphogenetic effects of temperature have been observed. Some typical 

 examples of recent investigation in this field are the following: 



Burstrom ( 1956 ) finds that under higher temperatures the final length 

 of cells in roots is reduced because of the shorter period of cell elonga- 

 tion. Cell-wall plasticity and calcium requirement are also reduced. 



Schwabe (1954), working with Chrysanthemum, limited low-tempera- 

 ture treatment to the growing tip and confirmed earlier conclusions that 

 this is the region where the stimulus of vernalization is perceived. The 

 stimulus did not p^tss across a graft union but it was translocated to 



