CHAPTER II. — ABSORPTION OF FOOD. 229 



plant through the leaves or other chlorophyll-bearing parts. 

 Other food-materials are also to some extent absorbed by leaves. 

 Young branches, glandular and other hairs, aerial roots, and to 

 a limited extent also mature stems, particularly those of herba- 

 ceous plants, may act as absorbing organs. Air plants must 

 obtain their entire supply of food in this way. Nutritive material 

 may, in fact, be absorbed by the entire surface of the plant, as is 

 the case with many aquatics. But the great majority of the 

 higher plants obtain their saline food-materials in aqueous solu- 

 tion, chiefly from the soil, by means of the innumerable delicate 

 roots and root-hairs with which they are provided. They are 

 conveyed, by a process presently to be described, through the 

 membranous cell-walls of these organs, and thus find their way 

 into the interior of the plant. 



To understand this process, it is important that the student 

 first clearly comprehend the laws of liquid diffusion and osmose. 

 It is well known that if two miscible liquids, for example, alcohol 

 and water, be placed in contact with each other carefully, so as 

 to cause as little commingling as possible, they will, nevertheless, 

 after the lapse of a little time, be found to be completely mixed, 

 so that the mass has exactly the same composition throughout. 

 This is due partly to molecular motion, and partly to the attrac- 

 tion which the molecules of the different liquids have for each 

 other. The phenomenon is called diffusion. If a thin animal or 

 vegetable membrane be stretched between two different liquids, 

 both of which are capable of wetting the membrane, the same 

 thing will take place, only somewhat more rapidly. Currents will 

 move to and fro through the membrane, until the composition 

 of the liquids on either side becomes the same. In case both 

 liquids are capable of wetting the membrane, but one is denser 

 than the other, usually the stronger current will be toward the 

 denser liquid, and on that side of the membrane the level of the 

 liquid will rise ; if, for example, pure water be on one side and a 

 solution of sugar in water be on the other, the stronger current will 

 be toward the sugar solution. But the general rule has some 

 exceptions : for instance, if water be on one side and alcohol on 

 the other, the stronger current will be from the water toward 

 the alcohol, although the former is the denser liquid. It is an 

 important fact also that, if on one side be placed an aqueous 



