84 



PHOTOCONTROL OF GROWTH 



inhibitor contents of the leaves and shoot apices of seedlings grown 

 under long-day or short-day conditions. Samples of the leaves and 

 shoot apices were taken from each series after 2, 5, 10, and 33 days 

 following the commencement of the treatments. It was found that 

 there was consistently more inhibitor present in the leaves and shoot 

 apices of short-day seedlings than of long-day seedlings (Fig. 2). A 

 difference between the two series was detectable even after two short- 

 day cycles, but this became greater after five cycles (Fig. 3). We have 



WATER CONTROL 



2 5 lO 33 



NO. OF CYCLES OF PHOTOPERIODIC 



TREATMENT BEFORE EXTRACTION 



> 



Fig. 3. Coleoptile section assay of the inhibitory eluates (Rf 0.55-0.88) 

 from chromatograms of extracts of mature leaves. The extracts were 

 prepared from plants growing under controlled photoperiodic conditions; 

 the leaf samples were made at the start of the experiment, and after 2, 

 5, 10, and 33 cycles of long day or short day. Extract from 1.0 g dry 

 weight of tissue was chromatographed in each case. Upper curve, long day; 

 lower curve, short day. 



confirmed these results several times. When the chromatograms were 

 assayed by planting seed of lettuce var. New Market (a non-light- 

 requiring variety) on the various zones, it was found that germina- 

 tion was markedly inhibited by the same region as were wheat coleop- 

 tiles, and that there were very great differences between the short-day 

 and long-day extracts in this respect. Recent experiments with extracts 

 of seedlings of birch grown under long and short days have given 

 similar results. 



These results do not, of course, necessarily imply a causal relation- 

 ship between the production of inhibitor and the induction of dor- 



