Coleiii biianei Benth 401 



day; in B5, the long-day plants were given 4 hr during the middle of 

 the night. Final leaf lengths for B3 are shown in Fig. 6 of Jacobs and 

 Bullwinkel (1953). Although the plants in these two experiments 

 were exposed to the photoperiods for 54 and 40 days, respectively, 

 there was no detectable effect of photoperiod on the number of plants 

 that formed flower buds during the experimental period. (The num- 

 ber of days to flowering averaged 5 days less in SD plants than in LD 

 plants in both the axTllaries-off treatments, but the difference was 

 not statistically significant.) 



An interesting "differential appeared when growth was compared 

 under "extended day" (B3) and "interrupted night" conditions (B5), 

 the total hours of illumination being the same. Interrupted nights gave 

 results in the control plants that showed no apparent differences from 

 extended days. For instance, the ratio of LD/SD summated leaf 

 lengths was 1.08 for extended days, 1.06 for interrupted nights [in 

 both cases, reflecting the slightly larger leaf lengths in plants under 

 LD when compared to plants under SD (see Fig. 6, Jacobs and 

 Bullwinkel, 1953)]. The increased stem length in control plants like- 

 wise showed no differential as a result of the two different long-day 

 treatments. However, interrupted nights had markedly different effects 

 on the axillaries-off plants. Although compensatory growth of the 

 leaves did occur under interrupted nights, it was only 24% greater 

 than the controls, instead of 45 to 49% greater as under extended-day 

 or short-day conditions. This difference is apparent from the ratio of 

 LD/SD summated leaf lengths in the axillaries-off plants; the ratio 

 was only 0.91 for interrupted nights, whereas it was 1.11 in the 

 replication using extended days. Stem lengths, as would be expected 

 from the relations between leaf and internode growth described by 

 Jacobs and Bullwinkel (1953), also showed much less compensa- 

 tory growth under interrupted nights (19% increase over controls, 

 versus 52 to 56% for extended or short days). But, as one would also 

 expect from the frequent use of interrupted nights as a substitute for 

 extended days in research on flowering, we found no detectable differ- 

 ential effect of extended days versus interrupted night on the flower- 

 ing rate. 



Since there were the usual large number of uncontrolled variables 

 in the greenhouse that we used for the above experiments, we were 



