Light 319 



but 16-16, 15-9, and 9-15. The 16-16 period was quite consistent and evi- 

 dently had been acquired by the plant. 



The physiology of photoperiodism presents many problems which are 

 too compiex to be discussed here. One hypothesis proposes that in the 

 light a substance is produced which persists in the subsequent dark 

 period. In the latter another is formed which is destroyed by a very 

 brief exposure to light. These substances interact to make a flower- 

 forming substance. In the short-day species Kalanchoe Blossfeldiana, 

 which has been studied intensively ( Harder, 1948 ) , a single leaf borne at 

 the tip of the plant ( those above it having been removed ) if exposed to 

 a long day will almost completely prevent flowering in the plant below. 

 Harder, Westphal, and Behrens (1949) conclude that in it is formed a 

 substance which inactivates the flower-forming hormone before this 

 reaches the floral primordium. Auxin presumably is involved in some of 

 these processes. This has been discussed by various workers, among them 

 Konishi (1956). 



In some cases the photoperiodic reaction can be changed. Working 

 with an early-blooming variety of peas which is day-neutral, Haupt 

 ( 1957 ) reduced flowering by removing the cotyledons and modifying 

 the soil nutrients. The plants now reacted as though they were long- 

 day and late-blooming types. He found that a true late-blooming 

 variety which is normally a long-day plant lost its photoperiodic reac- 

 tion and bloomed early if a scion from a blooming plant was grafted 

 into it. 



Earliness of blooming may be due to other factors than day-length. 

 Thus in an early-blooming and a late-blooming variety of Chrysanthe- 

 mum, both grown under short days, Doorenbos and Kofranek ( 1953 ) 

 found that the initiation of the florets took place at the same rate, but the 

 time from the end of this stage until the date of blooming was 28 days 

 in the early variety and 42 days in the late one. 



Photoperiodism has been studied chiefly in relation to the differentia- 

 tion of reproductive structures, but it has a pronounced effect on others 

 also. Among instances of this are the following: 



Pfeiffer ( 1926 ) grew buckwheat with daily illuminations of 5, 7, 12, 

 17, 19, and 24 hours and found that maximum stem length and diameter 

 were produced in the 17-hour day. 



The effect of the photoperiod is different on different parts of the plant 

 and under different conditions. Hall (1949) grew gherkins under green- 

 house conditions from seedling to maturity. At high nitrogen levels, 

 plants given a 16-hour day had larger stems than those under 8 hours 

 but at low nitrogen levels this was reversed and plants under the shorter 

 days grew larger. Under the 8-hour photoperiod more nodes and leaves 



