The Semantics of Vernalization • 63 



very general situation. If so, then perhaps the erosion of the word 

 vernalization is fortunate. 



The plasticity of some vernalization requirements is illustrated 

 by celery, Apium graveolens var. dulce. If the plants are kept at 

 usual vernalizing temperatures (about 7° C) for a month, they will 

 flower rapidly when transferred to cool (10-16°) or moderate 

 (16-21°) but not warm (about 24°) conditions. The initial vernaliza- 

 tion is not absolutely necessary for flowering, which will also take 

 place eventually under constant cool conditions, or under the 

 moderate conditions after two weeks under cool conditions. No 

 temperature pretreatment of any kind will permit flower initiation 

 under the warm conditions (Thompson, 1953). In short, vernaliza- 

 tion is only weakly inductive and can take place at temperatures 

 high enough to allow growth. The latter of course is true to a lesser 

 extent even of Hyoscyamus, and one can still see in celery the 

 occurrence of vernalization and devernalization in the Hyoscyamus 

 sense, but the effective temperatures are considerably closer 

 together. 



The flowering response of stocks, Matthiola incana, as sum- 

 marized by Kohl (1958), represents a situation in which it is uncer- 

 tain whether the term vernalization can be applied or not. Neither 

 germinating seeds nor seedlings can be induced by low tempera- 

 tures, but maturing plants require at least three weeks at 10 to 

 16° C for flower initiation. If the temperature rises above 19° for 

 as little as 6 hours per day, initiation is completely inhibited; the 

 plants must remain at the favorably low temperatures until full 

 differentiation of floral primordia has occurred. After this, however, 

 they remain induced and produce new flower primordia even at the 

 higher temperatures. This behavior can of course be regarded as 

 vernalization with a very low degree of induction and a small 

 difference between vernalizing and devernalizing temperatures, but 

 speaking simply of optimum and maximum temperatures for 

 flower initiation seems to be as accurate. Many plants probably 

 respond in a similar fashion, with optima and maxima varying 

 widely depending on the species. 



Also relevant here is another temperature effect on plants, 

 thermoperiodism. This term indicates the responses of plants to 

 differing day and night temperatures— growth and development in 

 mo6t of those tested are favored by night temperatures markedly 



