ABSORPTION OF MATERIALS IN GENERAL 12 7 



stances may enter roots from the soil by a mass streaming a|id nitration of the soil 

 solution through the peripheral cells, to the xylem vessels.) To enter plant cells, 

 these substances must be dissolved in water (or some other substance in the cell wall). 

 They diffuse through the peripheral, water-impregnated cell walls, into the proto- 

 plasm. Carbon dioxide and oxygen diffuse through the suberin or lignin of cell walls 

 that are impregnated with one of these substances, as well as through the imbibed 

 water. 



2. Diffusion of Gases. — The ultimate particles of every gas, and of every mixture 

 of gases, are considered as always in motion (somewhat as the individuals of a swarm 

 of gnats in the air) and as always tending to spread outward in all directions, until 

 some impermeable wall is encountered. They tend to distribute themselves uni- 

 formly throughout all the space that is available. This spreading movement of the 

 individual gas particles is called diffusion of the gas ; it is not to be confused with mass 

 flow and convection, by which the gas streams or flows as a whole, like wind. If two 

 masses of different gases are brought into contact (as in the two halves of a closed 

 chamber) and if no convection or stirring motion is present, the particles of both 

 kinds of gas diffuse outward, each kind into the space of the other kind, as though the 

 other kind were not present, and they eventually become uniformly mixed. Rates of 

 diffusion of different kinds of gases are proportional to the square roots of their respect- 

 ive densities; hydrogen diffuses four times as rapidly as oxygen (densities, 1:16), 

 temperature and pressure being the same for both. If septum or wall separates the 

 two original gas masses, diffusion takes place in both directions through the septum 

 if that is permeable to both gases; if the septum is permeable to but one of the gases, 

 diffusion occurs in one direction only. If the material of the septum is such that the 

 gas dissolves in it, then the gas diffuses through this material in the dissolved state 

 (as a solute) , Solutes (whether they are gases, liquids, or solids under ordinary con- 

 ditions) diffuse through the solvent in a manner analogous to that of gas diffusion, but 

 the rate of diffusion here is proportional to the concentration gradient in the liquid. 

 With a liquid-water septum separating two different gases (which are at the same 

 pressure and temperature), the rates of diffusion through the septum are proportional 

 to the solubilities of the two gases in water. There may also be mass streaming of the 

 septum material itself, which would apparently alter the rate of this diffusion, the solute 

 being carried by, rather than diffusing in, the solvent. 



3. Entrance of Gases into Plants. — Gases, as such, diffuse into (and out of) ordinary 

 plants through stomata and lenticels (openings in the peripheral layer of cells, con- 

 necting directly with gas-filled, irregular, intercellular channels in the tissues). Gas 

 diffusion continues in the intercellular spaces. There is also some mass streaming of 

 gases through intercellular spaces and their external openings. But beyond the cell 

 walls bounding these channels gas diffusion and gas streaming do not reach. Through 

 suberized, lignified, or cutinized cell walls, substances that are ordinarily gases diffuse 

 in solution in the substance of the walls, as well as in the small amounts of water held 

 by imbibition. Through ordinary, water-impregnated walls (and also through the 

 cell contents) they diffuse as solutes in the water. 



Most of the carbon dioxide and oxygen exchange of ordinary plants occurs through 

 the stomata or lenticels, the true absorption (or elimination) occurring, however, at 

 the peripheries of the intercellular channels, where the gases pass into (or out of) 

 solution in the imbibed water of the cell walls that bound these channels. Gas diffu- 

 sion through stomata occurs at a rate proportional to the linear dimensions of the 

 openings or pores, other conditions being constant; the rate is therefore relatively 



