ABSORPTION OF MATERIALS IN GENERAL 105 



velocity of diffusion of undissolved gases depends upon the density of the dif- 

 fusing gas (temperature and pressure being the same) and is inversely propor- 

 tional to the square root of this density. For instance, the density of hydrogen 

 is approximately 1, while that of oxygen is 16, and the velocities of diffusion 

 of these two gases are to each other as 1 is to 4; i.e., hydrogen passes through 

 a dry porous clay septum four times as rapidly as does oxygen when the two 

 gases have the same temperature and pressure. 



In the diffusion of dissolved gases the density of the gas plays no direct 

 part. Here the velocity of the movement is directly proportional to the co- 

 efficient of solubility of the gas in the solvent contained in the septum. In 

 the absorption of gases by plant cells, it is diffusion of dissolved gases that is 

 encountered, since the cell walls are impregnated with water. According to 

 the law of gas diffusion, carbon dioxide should enter plant cells more slowly 

 than do any of the other gases encountered; on the basis of the principle of 

 diffusion of dissolved gases, it should enter more quickly than the others, since 

 it possesses the greatest solubility in water (and in water-impregnated mem- 

 branes). Thus it happens that carbon dioxide, in spite of the small amount of 

 it in the air, is still absorbed by plant cells in adequate amounts. 



§3. Absorption of Gases. — Plants possess various structures that favor gas 

 absorption and gas movement, among which are stomata, lenticels, and numer- 

 ous intercellular passages traversing the plant body in all directions. The 



clearness and is frequently confusing. We have two kinds of diffusion with which to deal here, 

 one being the intermingling of gases as such and the other that of substances (such as carbon 

 dioxide, alcohol, potassium nitrate, etc.) while dispersed (dissolved) in a solvent; the solvent is 

 usually liquid (water), but substances may dissolve in solid material — as carbon dioxide in the 

 wax-like, cuticular material of many exterior cell walls. Diffusion of undissolved gases is met 

 with in the inward and outward movement of water vapor, carbon dioxide and oxygen through 

 stomatal openings and from place to place in the plant body through gas-filled intercellular 

 spaces, but gases do not pass through the cell walls or protoplasm of active cells, and therefore 

 cannot get inside the cells, unless they are first dissolved, usually in water. (See below, in 

 text.) Of course, when water vapor is dissolved in liquid water it simply becomes a part of 

 the liquid, being condensed from the gaseous to the liquid state. This and the following para- 

 graphs have been subjected to some modification, in accordance with these principles. It 

 may be added at this point that, besides the diffusion of gases and that of dissolved sub- 

 stances, there is another kind of movement met with in plants, namely that of molar stream- 

 ing. This occurs with gases and liquids and also (but not commonly in the plant) with 

 suitably sub-divided solids (as sand). When a gas or liquid is forced through openings, by 

 pressure, it is this molar movement that has to be considered. Diffusion may go on at the 

 same time, in the liquid or gas stream, its direction being independent of the direction of 

 the streaming. If diffusion and streaming are in the same direction, the rate of movement 

 is the sum of the rates of diffusion and streaming; if they are in opposite directions the difference 

 is the rate of movement. — Ed. 



c Carbon dioxide is about three times as soluble in water as is oxygen. It is as a gas, 

 however (undissolved in either liquid or solid), that carbon dioxide first enters the ordinary 

 green plant through stomatal openings. See: Blackman, 1895. [See note 5, p. 36.] Brown, 

 1899. [See note 2, p. 34.] Brown and Escombe, 1900. [See note 2, p. 34.] Having entered 

 by gas diffusion, carbon dioxide soon passes into solution in the water with which the cell walls 

 abutting on the sub -stomatal intercellular spaces are impregnated, and it diffuses as a dissolved 

 substance through these walls and into the cells. — Ed. 



