NUTRITION 365 
thus accumulate to 25 or even ipo times as much as in the air. This 
puts water plants in a very advantageous position so far as a supply of 
CO 2 is concerned. 
Admission of CO 2 . Of course in all plants that present an uncutin- 
ized (and consequently a wet) surface to the air, the CO 2 enters directly 
at the surface; in fact it can enter, in proportion, wherever water 
can evaporate. As the cuticular evaporation in most of the higher 
plants is small, the quantity of CO 2 entering through the epidermis 
is trifling. Into some epiphytic seed plants which have no stomata 
(e.g. TillandsM), the leaves of mosses, the thallus of liverworts, etc., CO 2 
enters directly. 
The supply for the great majority of the larger land plants, however, 
passes through the stomata. These openings are ample to admit not 
only what is necessary, but five or six times more than actually passes 
through them in nature. 
It has been shown that CO 2 will diffuse through a multiperforate partition, 
placed over some ready solvent like sodium hydroxid, as freely as it would enter 
the solvent were the partition absent, provided the perforations are not farther apart 
than ten times their diameter. The epidermis is like such a multiperforate parti- 
tion in which the area of the openings is scarcely more than i per cent of the total 
surface. But the CO 2 dissolves so readily in the wet cell walls bounding the inter- 
cellular spaces that its pressure in the internal passages is usually o; so it may 
traverse the stomata as rapidly as is permitted by the gradient of pressure, 0.228 mm. 
outside to o inside. The speed of the molecules is found to be greatly accelerated 
as they swirl through the narrow passage of a stoma; in fact, they traverse it at a 
speed about 50 times as great as when diffusing freely into sodium hydroxid. 
Even when the orifice of the stoma is partly closed, though this reduces 
proportionally the amount of gas passing, the supply of CO 2 is not likely 
to fall below the maximum that can be used. As in good light the sto- 
mata are usually more than half opened, even though the evaporation 
is excessive, an adequate supply of CO 2 is thus assured, so far as admis- 
sion to the aerating system is concerned. 
Deficiency in CO 2 . As a matter of fact, however, the supply of CO 2 
is often less than could be utilized by the chloroplasts. This is shown 
by the fact that photosynthesis is increased when, in good light, the 
amount of CO 2 in the air around the plant is artificially increased. The 
increase may go to a hundredfold or more with positive benefit, at least 
so far as brief experiments show. Any increase in the air means in- 
creased pressure of CO 2 in the aerating passages; and this means the 
solution of more CO 2 in the wet walls, and consequently faster diffusion 
