NUTRITION 371 
if the stomata are only on the under surface, as they often are, the lower 
cells are more favorably placed with respect to CO 2 ; 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 
2030 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 CO 2 . Yet it may be reduced to an intensity at 
which light, instead of the small supply of CO 2 , 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 
CO 2 , 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. 
