404 GROWTH FACTORS AND FLOWERING 



sensitive. In 1930, Arthur et al. tested many plants under constant 

 illumination as contrasted to various other day lengths. Tomatoes, 

 Coleiis, and geranium gave less growth under continuous artificial light 

 than with some dark period every 24 hr — the growth inhibition was 

 greatest in tomato, "not quite as severe" in Coleus and geranium. 

 For all three plants, the inhibition was decreased by using sunlight 

 for 12 of each 24 hr of continuous light. Under sunlight plus a 12-hr 

 supplement of artificial light, tomato was still strongly inhibited, but 

 Co/^w5' showed little or no inhibition. [The decreased growth of Coleus 

 under continuous artificial light was also found by Garner and Allard 

 (1931).] 



Thus, only a few of the plants tested showed inhibition of growth 

 by fight during the "night," and tomato was far more sensitive than 

 Coleus. 



Hillman (1956) has further investigated these photoperiodic effects 

 in tomato, and among his interesting findings is the report that if 

 older leaves are removed from intact plants, the plants show increased 

 sensitivity to continuous light. He suggests that some substance from 

 older tissues perhaps is destroyed by light. In view of our findings 

 with Coleus, an alternative possibility is that excising some leaves 

 caused compensatory growth in those which were left, with a con- 

 comitant increase in sensitivity. 



In the light of these earlier papers, three aspects of our methods 

 with Coleus would tend to obscure a growth-inhibiting effect on 

 normal growth: ( 1 ) the use of sunlight for most of the daily illumina- 

 tion; (2) the use of a 2-hr interruption of the dark period (instead 

 of continuous illumination); and (3) in measuring growth, the use 

 of only leaves on the main shoot — thus [in contrast to Garner and 

 Allard (1931), and Highkin and Hanson (1954)] efiminating 81% 

 of the plant's leaf weight (Fig. 5). 



Why should light during the middle of the night inhibit the vegeta- 

 tive growth of some plants but not of others? An intriguing answer 

 is suggested by parallel studies on the developmental anatomy of 

 Coleus. From round-the-clock collections we have found evidence that 

 leaf initiation in our clone occurs between 1 1 p.m. and 1 a.m. (Jacobs 

 and Morrow, 1957, Table 1, where leaf lengths of 1300± microns 

 mark the plants which are initiating new leaf primordia at the shoot 



