CONTROL OF LEAF GROWTH 163 



chemicals and light and their interaction in 48 hr, whereas tests such 

 as those on flowering require several days or weeks. In addition, this 

 system appears to have advantages over the pea internode and the 

 Avena cyHnder test, because of the almost essential requirement for 

 light in the case of leaf discs. 



Our system is a modification of that of Bonner et al. (1939) as 

 modified by Miller (1952) and again modified by us (Scott and 

 Liverman, 1956) better to fit our own conditions. The growth medium 

 used in the experiments described below consisted of the following 

 essentials: 3% D-glucose, 0.08M potassium nitrate, 0.09M potassium 

 dihydrogen phosphate, and a 0.01 8M potassium-sodium tartrate 

 buffer, pH 5.6. Five milliliters of this basal solution was added to a 

 petri dish so as to just moisten two sheets of Whatman No. 1 filter 

 paper. Growth promotion due to an additive would be growth over 

 and above that occurring in the basal medium, and the reverse would 

 occur for inhibition. As a source of red light we used fluorescent 

 lamps filtered through two layers of Dupont 300 MSC red cellophane. 

 The far red was obtained by passing light from frosted incandescent 

 bulbs through two layers of the above red cellophane and two layers 

 of Dupont 300 MSC dark blue cellophane. The red filter transmits 

 only those wavelengths of light longer than 6000 A except for a slight 

 transmission (about 3%) in the region of 5500 A. Since the trans- 

 mission limit for the fluorescent lights is about 7000 A, this filter 

 system gives essentially only photoperiodically active red light. The 

 blue plus red cellophane transmits about 3 to 5% in the region of 

 5500 A, transmission is almost nil between 6000 A and 7000 A, and 

 rapidly reaches 60% beyond 7000 A. These filter systems are in 

 general use now in a number of laboratories with apparently satis- 

 factory results. 



Several experiments have shown that the initial size and physio- 

 logical age of the leaf are particularly important, so that leaf material 

 must always be uniform. The plants were grown in washed sand for 

 6 to 7 days in a darkroom maintained at a temperature of 25° ± 1°C 

 until the leaves reached an area of about 2 to 3 cm-. Discs were 

 removed under a dim green or dim orange safelight and placed on the 

 filter paper with the upper epidermis against the paper. The discs were 

 then exposed to light with and without supplements of chemicals to 



