Thimann — vii — Foreword 



and Purvis that vernalization of cereals may be reversed points in the 

 general direction of control by special substances rather than by the succes- 

 sive completion of determinative "phases." However, it is important to 

 note that Sen and Chakravarti^ have been unable to reverse the vernaliza- 

 tion of mustard either by high temperature or by dry storage for a year. 

 Mustard differs, however, from, rye in that the excised embryos can be fully 

 vernalized in pure water, while rye embryos require sugar for complete 

 and rapid vernalization. Whether there is any connection between this need 

 for carbohydrate and the reversibility of vernalization is, of course, not 

 known yet. However, the metabolism which accompanies vernalization 

 may well be worth analysis. Indeed, the way may have been opened to 

 such an analysis by the recent experiments of Purvis', which indicate that, 

 during a period of starvation of the rye embryo, some materials necessary 

 not only for vernalization but also for growth are metabolized away. Per- 

 haps at this point our developing knowledge of the special nutritional re- 

 quirements of young embryos in culture may be brought to bear. A very 

 recent paper by Lang and Melchers,* unfortunately received too late for 

 inclusion in the text, brings the two ideas together in another way. Biennial 

 Hyoscyamus niger, which flowers after vernalization only if kept in long 

 days, can be devernalized if given ten short days at 38°. This treatment 

 must, however, be applied immediately (within four days) after the ver- 

 nalization by cold. Thus the flowering condition or substance is destroyed 

 before it has had time to act. Another recent piece of evidence strongly 

 suggestive of the former, or hormonal view, is supplied by Holdsworth 

 and Nutman's° study of the flowering of Orobanche. This parasite evi- 

 dently initiates flowers only when its host, red clover, does so; in other 

 words, the receptors for the flowering "hormone," whose production depends 

 on day-length, are not only the buds of the host but also those of the parasite. 

 The formation and destruction of special substances, or alternatively the 

 balance between their production and its inhibition, is, of course, the general 

 line of interpretation adopted by the workers in photoperiodism. The 

 former of the two alternatives is essentially that of Hamner and of Borth- 

 wiCK, Parker and their co-workers at Beltsville, the latter that of 

 Melchers and his collaborators. It is needless to add, however, that the 

 nature of these hypothetical substances and the metabolic conditions under 

 which they are produced remain completely unknown. Nevertheless, this 

 vast hiatus does not at present interfere seriously with the development of 

 the field, since these ideas are little more than interpretations and are not 

 specifically formulated theories which can stand or fall by experiments 

 designed to test them. 



Another group of questions which we are perhaps not yet ready to 

 formulate concerns the mode of action of the stimulus (or the substances). 

 In the case of vernalization of the grasses the impetus to flower formation 

 seems to appear as a change in the primary meristem ; in the dicotyledons 

 the contribution of Roberts and Struckmeyer suggests that it may be the 

 secondary meristem which shows the initial and determining responses. 



2 Sen, B. and Chakravarti, S. C. in Nature 157 : 266, 1946; 159: 783-4, 1947. 

 s Purvis, O. N. in Ann. Bot. N.S. 11 : 269-283, 1947. 



*Lang, a. and Melchers, G. in Zeit. Naturforsch. 2b: 444-449, 1947 (received 

 May 1948). 



'Holdsworth, H. and Nutman, P. S. in Nature 160: 223, 1947. 



