176 The Living Plant 



cules of a gas (figure 58). And if the reader objects at this 

 point that diffusion in a solution takes place at a temperature 

 too low to permit this explanation, I remind him that days far 

 too cold for our comfort are yet hot from the physical point of 



view, for there is heat in the air 

 at all temperatures above the ab- 

 solute zero, which lies no less than 

 four hundred and fifty-nine de- 

 grees below zero of our ordinary 

 thermometer. And the phenomena 

 of diffusion are precisely the same 



Fig. 5S.— a diagram designed to illus- insidc of plauts and auimals aS 

 trate the diffusion of a substance in r i ttt 



solution. The circles are water, and OUtSlde of them. We are UOW 

 the crosses are the dissolving and dif- j , • ]*£C„„:^^ 



fusing substance,-e. g., sugar. The prepared to Summarize diffusion 

 molecules of water are supposed to ^g another Verity of uature, thus, 



have a stronger attraction for the "^ 



molecules of sugar than these have for — when suhstauces are anywJiere 



one another. Magnified as in Fig. 6. . 777 



brought into a state, whether by 

 conversion to a gas or by solution in a liquid, such that their mole- 

 cules are separated from one another, then those molecules, set into 

 energetic action, and thereby given a mutually-repulsive motion, by 

 heat derived from the surroundings, spread, or diffuse, forcibly out- 

 ward from places of greater to those of lesser concentration. 



Thus much for diffusion; we turn next to the other condition 

 involved in osmosis, — the nature of the membrane. ^\Tiat can 

 be the constitution of a body which, possessing no discoverable 

 openings, will permit water and other substances to pass through 

 with a freedom well-nigh as uncanny as if a fourth dimension 

 were concerned? The membrane, of course, is composed of mole- 

 cules, but there is also good reason to believe that, in walls at least, 

 the membrane is composed of larger units, called micellae, which 

 are aggregates of molecules (or perhaps simply huge compound 

 molecules) that may be represented diagranmiatically as cubical 

 (figure 59). Now these micellae, although structurally separate, 

 are held closely together by virtue of a certain cohesive affinity 



