THE ROLE OF LIGHT 



349 



ing factor, the rate of photosynthesis is approximately proportional to the 

 light intensity. For the maximum concentration of carbon dioxide indicated 

 in Fig. 88 (o.ii per cent) the maximmii light intensity used was not great 

 enough for the carbon dioxide factor to have become limiting. It should 

 also be noted that the maximum light intensity employed — lOOO foot candles — 

 is much inferior in intensity to usual summer sunlight, which at noon on a 

 clear day usually is equivalent to 8,000-10,000 foot candles. These results 

 indicate that light is not a limiting factor in photosynthesis when leaves are 

 exposed to full sunlight if the carbon dioxide content is within the range 

 normally found in the atmosphere. Under such conditions maximum photosyn- 

 thesis per unit of leaf area is attained in most and probably all species at light 

 intensities considerably less than that of full sunlight. 



Fig. 87. Per cent transmission of various wave lengths of light by leaves of box elder 

 {Acer tiegundo). Data of Seybold (1933). 



Results such as those depicted in Fig. 88 will be obtained only when a 

 single leaf or a small plant, all parts of which are well illuminated, is used 

 as the experimental material. When the effect of light on photosynthesis is 

 considered in terms of an entire tree, however, a different relation holds. 

 Thus Heinicke and Childers (1937) showed that the rate of photosynthesis 

 for an entire apple tree increased progressively with increase in light intensity 

 up to that of full sunlight. This is undoubtedly due to the fact that many 

 of the interior leaves on a large tree are heavily shaded. Although, in gen- 

 eral, maximum rates of photosynthesis are attained in the leaves of most 

 species at light intensities considerably below that of full sunlight (in the 

 apple tree at one-fourth to one-third of this value) these investigators showed 

 that many of the interior leaves of an apple tree receive i per cent or even 

 less of the sunlight received by peripheral leaves. Even in full sunlight, 

 therefore, many of the leaves on a tree do not photosynthesize at their maxi- 

 mum capacity. The lower the light intensity the greater the proportion of 

 the leaves of which this will be true. Hence the greater the intensity of the 



