NUTRITION 365 



thus accumulate to 25 or even too times as much as in the air. This 

 puts water plants in a very advantageous position so far as a supply of 

 C0 2 is concerned. 



Admission of C0 2 . — Of course in all plants that present an uncutin- 

 ized (and consequently a wet) surface to the air, the C0 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, entering through the epidermis 

 is trifling. Into some epiphytic seed plants which have no stomata 

 (e.g. Tillandsia), the leaves of mosses, the thallus of liverworts, etc., C0 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 CO2 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 C0 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 C0 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 C0 2 is thus assured, so far as admis- 

 sion to the aerating system is concerned. 



Deficiency in C0 2 . — As a matter of fact, however, the supply of C0 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 C0 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 C0 2 in the aerating passages; and this means the 

 solution of more C0 2 in the wet walls, and consequently faster diffusion 



