7 6 



The Living Plant 



Fio. 58. A diagram designed to illus- inside 

 trate the diffusion of a substance in 

 solution. The circles are water, and Outside 

 the crosses are the dissolving and dif- 

 fusing substance, e. g., sugar. The 

 molecules of water are supposed to 

 have a stronger attraction for the 

 molecules of sugar than these have for 

 one another. Magnified as in Fig. 6. 



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 

 of plants and animals as 

 of them. We are now 

 prepared to summarize diffusion 

 as another verity of nature, thus, 

 when substances are anywhere 

 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. What 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 4f 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 micella, which 

 are aggregates of molecules (or perhaps simply huge compound 

 molecules) that may be represented diagrammatically as cubical 

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

 are held closely together by virtue of a certain cohesive affinity 



