Mumeek — 59 — Research in Photoper!od!sm 



long days in cucumber and by short days in some tobaccos, both of which 

 apparently are day neutral plants (Danielson, 1944; Dennison, 1945). 

 Photoperiodic inhibition has an important bearing on nutrition and metab- 

 olism of plants and thus on flower development and performance. 



Conclusion : — The discovery of photoperiodism and the subsequent 

 disclosure of the effects of temperature on plants have emphasized force- 

 fully that the genetically determined rhythm of development can be modi- 

 fied strikingly by the environment. It has advanced our knowledge on 

 certain aspects of latitudinal and seasonal adaptation of a considerable 

 number of species and varieties. 



The present commonly used classification of plants into short-day, long- 

 day and neutral types, however, is not acceptable any more, in considera- 

 tion of recent investigations. In nature probably no plant throughout its 

 life is adapted strictly to one kind of photoperiod, excepting those growing 

 in equatorial regions, but many plants will tolerate to a considerable extent 

 a uniform length of day, as when grown under such conditions for experi- 

 mental purposes. Most likely a specific length of day or night is most 

 conducive to one phase of sexual reproduction of particular plants and an- 

 other or others to succeeding phases. There is urgent need for more de- 

 tailed studies of the effects of the light period not only on initiation of the 

 sexual state but especially on flower development and fruit and seed pro- 

 duction. 



We may well expect to gain a clearer understanding of the often very 

 close adaptability of plants to their ecological environments from further 

 studies on the reciprocal influences between photoperiodism and tempera- 

 ture and possibly other environmental factors. The information thus 

 gained should be of increasingly greater importance in cultural practices of 

 many economic species and varieties. Application of photoperiodism and 

 control of night temperature has already been of considerable practical 

 value in connection with greenhouse culture of certain flowers and vegeta- 

 bles and in the breeding and selection of a large variety of field and other 

 crop plants. 



Despite the extensive investigational work that has been conducted on 

 the problem, exact information on the mechanism of photoperiodism is 

 still lacking. It is fairly certain, however, that, as a result of the photo- 

 periodic stimulus, a catalytic substance of hormone-like nature is formed 

 in the leaves, whence it moves to the terminal meristems resulting in initia- 

 tion of floral primordia. If the inductive action, either during the light 

 or dark periods of the diurnal cycle, is of a photochemical nature and in a 

 way analogous to photosynthesis then detailed physico-chemical studies are 

 in order and many difficulties may be encountered in analysis of the prob- 

 lem. Considerable advance in our understanding of photoperiodism would 

 be gained if the flower inducing hormone ("florigen"), unstable as it may 

 be, could be isolated in pure form and its chemical constitution determined. 

 This would seem to be the first crucial problem for solution. 



The function of the photoperiodically formed hormone in altering the 

 activity of terminal meristematic cells, from production of vegetative to 

 reproductive tissues, has received practically no attention so far, beyond 

 some general histological observations. Cytological and chemical studies of 



