126 PHOTOCONTROL OF GROWTH 



tensities ~ 20,000 ergs/cnr sec) show results as indicated in Table I. 

 Also darkness is included as alternative treatment. 



Table I. Effect of Two Successive 2-hr Treatments as Indicated on Final Length 

 of Avcna Coleoptiles Grown in Darkness (De Lint, unpublished) 



Irradiation always decreases the final coleoptile length as compared 

 with darkness. In most cases, two subsequent spectral irradiations yield 

 shorter coleoptiles than one followed by darkness. Also irradiation 

 with near infrared (including longer wavelengths) yields shorter co- 

 leoptiles than complete darkness. It is curious, however, that near 

 infrared, following blue or yellow (the only regions tested so far) de- 

 inhibits the elongation process, yielding longer coleoptiles than those 

 obtained if the 2-hr color period were followed directly by darkness. 

 This de-inhibition, however, is not observed when the first irradiation 

 period also is near infrared (see Table I and de Lint, 1957). 



De Lint, furthermore, is running extensive light intensity-exposure 

 time series in blue and yellow light. The inhibition in length is fairly 

 proportional with the log It. Blue light shows a somewhat stronger 

 inhibition than yellow light, which seems qualitatively concordant with 

 Bensink's results on lettuce leaf shape, discussed earlier in this paper. 



Finally, we would like to mention that, in connection with horti- 

 cultural practice, Wassink and Wassink-van Lummel have started fol- 

 lowing growth of tomato plants in light of wide spectral regions. In the 

 series run so far, growth has been compared in equal incident intensi- 

 ties with white daylight fluorescent sources, red fluorescent tubes (un- 

 filtered), and incandescent light. In all cases, incandescent light yields 

 a more elongated growth type, accompanied by conspicuously higher 

 dry weight. It has so far not yet been determined whether this is due — 

 as one might be inclined to suppose — to the higher near infrared con- 

 tent of this light source, or to its greater predominance of red over 

 blue. Previous work of this laboratory (Wassink and Stolwijk, 1952) 

 has shown that in tomato, red light produces a more elongated and 



