EFFECTS OF HIGH-INTENSITY IRRADIATION 119 



feel inclined at present to ascribe the elongation in blue to the blue 

 radiation itself and thus give preference to the first explanation. If, 

 indeed, the effect still were due to near infrared contamination, the 

 infrared would have to be extremely effective — so much so that one 

 might ask whether near infrared generation by chlorophyll fluorescence 

 might not play a part. 



We now have experiments under way in which further improvement 

 of the light qualities of our cabinets is envisaged, especially with 

 respect to the removal of traces of red and near infrared in the cabinets 

 in which spectral purity is desired. 



HIGH LIGHT INTENSITY FORMATIVE EFFECTS IN LETTUCE 



Heading of lettuce is possible only under fairly high light intensities. 

 This phenomenon has been studied in detail in our laboratory by 

 J. Bensink during recent years (Wassink, 1957). The following data 

 appear of interest for the present discussion (cf. also Bensink, 

 1958a,b). 



The shape of successive leaves in young lettuce plants changes 

 gradually. The plant starts with the formation of relatively narrow 

 leaves, with a length-breadth relation >2. Successive leaves gradually 

 become broader, so that under normal conditions of illumination the 

 length-breadth relation approaches 1.0 from about the eighth leaf 

 onward. The latter leaves make heading possible. 



The relation outlined is subject to changes due to several factors, 

 both external and internal. Those investigated up to the present are 

 light intensity, night temperature, nitrogen supply, variety of lettuce, 

 and spectral region. Lower Hght intensities "lift up" the curves, i.e., a 

 certain leaf has a higher length-breadth ratio than it has under higher 

 light intensities. The same holds true for higher night temperatures, 

 for higher nitrogen concentrations, for certain varieties with respect 

 to others, also for yellow light as compared with blue light. 



The two varieties investigated were Meikoningin and Wonder van 

 Voorburg; the latter has higher light requirements than the former. At 

 the lowest light intensity used (25,000 ergs/cm- sec daylight fluo- 

 rescent tubes). Wonder van Voorburg shows a remarkable phenome- 

 non. Instead of the normal behavior in which successive leaves grad- 



