162 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
roots imbibe, sufficient to complete those complex compounds so 
indispensable to life. 
It has been estimated that a square mile of dense hardwood forest 
may use over 500 tons of carbonic acid gas and over 1,000,000 tons 
of water in a season for such chemical activities. In dry countries 
such prodigality with water would be, of course, impossible. This 
figure represents a depth of nearly 114 feet of quid water over 
the whole area, which is from one-third to one-sixth of the total 
yearly rainfall of very moist climates, and exceeds by fivefold the 
yearly rainfall of some of the great deserts. 
Only a small part of the imbibed water is retained by vegetation. 
The leaves have a multitude of little mouths, called stomata, which, 
when under the influence of light, suck in carbonic acid gas and 
exude oxygen and water vapor. In darkness, plants exude carbonic 
acid gas slowly. This seems to be an attribute of all living cells, plant 
as well as animal. It no doubt goes on in the light with plants 
also, but is obscured by the opposite reaction just mentioned. The 
combined area of all the stomata hardly amounts to 1 per cent of 
the area of the leaves, so that it is hard to see how so much material 
can pass through such tiny orifices. It has, indeed, been shown that 
if one-half the leaf area were kept wet with fresh, strong caustic 
potash solution, it could not absorb carbonic acid gas faster than 
the stomata. 
Brown and Escombe resolved the puzzle. They showed by lab- 
oratory experiments that when carbonic acid gas is admitted through 
a small orifice into a medium which absorbs it as fast as admitted, 
the amount transmitted is proportional, not to the area, but to the 
diameter of the orifice. For example, the same area of opening, if 
split up into four parts, will admit twice as much carbonic acid gas 
as when forming only one orifice, since the diameter of the large 
orifice is but twice the diameters of the four small ones. This 
paradox, of course, depends on a more rapid rate of flow of the gases 
through the smaller apertures. 
Nature avails herself of this strange secret by crowding stomata 
something like 1,000,000 to the square inch. She thus adapts her 
leaves to suck in their sustenance and give out the waste products 
almost as rapidly as if the whole leaf were one aperture, while really 
about 99 per cent of its surface is closed to protect the delicate cells 
within. 
Even this is not the whole story. The stomata, like mouths of 
animals, may be either wide open, shut, or partly open, and they go 
through all of these variations. It is not known exactly how they 
are regulated. We, at least, do not suppose that the plants use 
volition as men do in opening their mouths. Yet it is conceivable 
