1004 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



in ordinary air, and 30-40 mg. in air enriched with carbon dioxide, and not 

 the much higher yields found by Blagoveshchenskij, or Monch.) We 

 therefore suggest that in Table 28. VI, too, the results of Purevich and Bose 

 should be given preference over those of Brown and Escombe. However, 

 the whole problem is in need of renewed and more exact experimental 

 analysis, which alone could link the rate of photosj^nthesis under natural 

 conditions (strong illumination and limited carbon dioxide supply) to 

 the much better known rates in weak light and in the presence of an ample 

 amount of carbon dioxide. 



We now turn to experiments of longer duration, in which the total yield 

 of photosynthesis of an assemblage of plants was compared, over an ex- 

 tended period of vegetation, with the integral of insolation over the same 

 period. 



Noddack and Komor (1937) studied two plots of grass, one of 9 m.^ 

 and another of 74 m.^ In two consecutive periods of 20 days each, they 

 measured the total solar radiation falling on these two plots, S I dt; after 

 this, the grass was mowed, dried and combusted, and the heat of combus- 

 tion, AHf, was measured. Here are the results: total irradiation in 20 

 days, 2.6 X 10^ cal./cm.^ (average irradiation 0.0015 cal./cm.^ sec, or ap- 

 proximately 6000 lux) ; -pro-portion of incident energij stored in the hay, first 

 plot, Mic/f I dt = 0.67% (first period) and 0.80% (second period), sec- 

 ond plot, 0.41% and 0.64%, respectively. 



In these measurements, the growth of the root system was not taken into 

 consideration. This correction is difficult to estimate; but it should bring 

 the average value of AHc/Sl dt up to almost 1%, and e (= l^EJ f A dt) 

 close to 2.5%. 



In comparing these results with those of the short-time experiments 

 listed in Table 28.VH, one has to consider that some obvious factors tend 

 to decrease the long-time average value of energy conversion by a large 

 assemblage of plants growing under natural conditions, compared with 

 that of a few isolated plants or leaves, averaged over a few hours of full 

 sunlight ; but that other less obvious factors may act in the opposite direc- 

 tion. Such favorable factors are the lower average light intensity (which 

 decreases S ^ <^f without reducing strongly the rate of photosynthesis), 

 and, possibly, partial retention of the respiratory gases in the dense foliage, 

 permitting reutilization of exhaled carbon dioxide. On the unfavorable 

 side we can anticipate that in a large assemblage of organisms a certain 

 proportion will not be in a healthy state, and others will be "resting"; 

 sometimes the temperature will be too low for maximum photosynthetic 

 efficiency; sometime it will bo so high as to cause inhibition (cf. chapter 

 31). Some leaves will be in the shade of others, at least part of the day. 

 These unfavorable influences seem to predominate, judged by the fact 



