NUTRITION 371 



if the stomala are only on the under surface, as they often are, the lower 

 cells are more favorably placed with respect to CO z ; and the more so as 

 the looser arrangement of these cells permits freer diffusion. The very 

 structure of the leaf is in large measure a response to these different 

 factors, and so perhaps the advantages and disadvantages balance one 

 another. A leaf which is directly shaded by another is obviously in a 

 decidedly disadvantageous situation; and we observe various arrange- 

 ments and positions that reduce shading. These result in leaf mosaics 

 of various kinds (see Part III, p. 543). A plant that grows in shade 

 is different from the same species grown in the sun ; indeed shade plants 

 have peculiarities which depend in large part on the difference in the 

 illumination (see Part III, p. 531). 



Energy obtained. — An ordinary thin leaf reflects and absorbs 40-70 

 per cent of the sunlight which falls upon it ; but of diffuse light it absorbs 

 about 95 per cent. The chlorophyll itself seems to absorb something like 

 20—30 per cent, but of this only a small part can be used for photo- 

 synthesis and so stored as potential energy in the carbohydrate made. 

 That amount is variously estimated from 0.5 to 3 per cent. The balance 

 is free to heat the leaf, whose internal temperature in the sun sometimes 

 rises 10-15 above that of the air. This surplus heat, of course, is partly 

 transferred to the air adjacent, but the greater part becomes latent in the 

 water, whose vaporization is accelerated thereby. This is the so-called 

 " chlorovaporization " (see p. 330). 



Deficiency in light. ■ — It will be evident from the foregoing that in 

 nature light is seldom lacking to drive the machinery rapidly enough to 

 dispose of all available C0 2 . Yet it may be reduced to an intensity at 

 which light, instead of the small supply of CO,, limits the output. For 

 example, some plants are so situated that they get only 2 per cent of the 

 total sunlight in the vicinity. From the point at which the effective 

 energy of the light absorbed is just equal to disposing of the available 

 COo, whether this is greater than natural or not, lessening the intensity 

 of the light results in a proportional diminution of the amount of the 

 product. 



Efficiency. — It will be further evident that the plant is a very in- 

 efficient machine, considering the relation of energy received to the 

 energy stored in the product. A steam engine which delivers as mechan- 

 ical power less than 10 per cent of the energy of the fuel consumed under 

 the boilers is fit for the scrap heap, and the best types are delivering above 

 15 per cent. Contrast this with the 0.5-3 P er cent °f tne plant economy. 



