ABSORPTION OF MATERIALS IN GENERAL 



lOI 



While the area of the smallest opening (diameter 2. 11 mm.) was less than a 

 hundredth of that of the largest (diameter 22.7 mm.), the amount of gas passing 

 the former was one-tenth, rather than one-hundredth, of the amount passing the 

 latter. From this it follows that if a vessel of sodium hydroxide solution is 

 covered with a thin plate perforated with very small openings, the quantity of 

 carbon dioxide absorbed may be as great as though no cover were present at 

 all. The total area of all the openings may be only a small fraction of the total 

 surface of the plate, however. It was found that diffusion was most rapid when 

 the distances between the openings were each ten times as great as the diameter. 

 This proportion holds approximately for the distribution of stoma ta in most 

 leaves. Therefore the velocity of gas absorption is as great when the stomata 

 are open as it would be if no cuticle were present and if the whole leaf were cov- 

 ered with a wet membrane of pure cellulose. 



Investigations of movements of gases in water plants' have shown that the 

 air of the intercellular spaces has about the same composition as that of the ex- 

 ternal atmosphere. 



§4. Diffusion of Dissolved Substances.^ — Many substances that are not 

 gases at ordinary temperatures are soluble in water, but not all substances are 

 appreciably so; oils, for example, are generally practically insoluble in water. ^ 

 Whether the dissolved substance is a gas, liquid or solid under ordinary 

 conditions, it forms an aqueous solution when it is dissolved in water. The 

 dissolved substance is usually called the solute and the water in which it 

 dissolves is the solvent. A solution may contain many different kinds of solutes, 

 all dissolved in the common solvent. All dissolved substances diffuse in all 

 directions withipi the limits of the solution or solvent, and tend to become equally 



' Devaux, Henri, Du m^canisme des 6clianges gazeux chez les plantes aquatiques submerg6es. Ann. 

 sci. nat. Bot. VII, 9: 35-179. 1889. 



2 Dastre, M. A., Traits de physique biologique 1 : 466. Paris, 190 1. 



° The following discussion of osmotic pressure and related phenomena is largely due to the 

 editor, but the spirit and apparent intent of the author is followed as closely as possible, at the 

 same time avoiding the author's curious conceptions that dissolved substances are liquids and 

 that "osmosis" and diffusion are essentially different. For another attempt at presenting 

 these phenomena to the student of physiology, see; LiTingston, B. E., The r61e of diffusion 

 and osmotic pressure in plants. Chicago, 1903. Also see: Findlay, Alexander, Osmotic pres- 

 sure. London, 1913. Washburn, Edward W., An introduction to the principles <5f physical 

 chemistry. XXV-I-4S P- New York, 1915. The last-named discussion is the most 

 thorough from the physical and mathematical point of view.' — Ed. 



