50 BOTANY: PRINCIPLES AND PROBLEMS 



feet freedom through the iiieinl3rane, or that the meinbi'ane is 

 permeable to them. All osmotic membranes are readily perme- 

 able to water, but we find that they differ markedly in the ease 

 with which dissolved substances of various sorts can diffuse 

 through them. One membrane may be perfectly permeable to a 

 given substance; another may allow it to pass slowly and with 

 difficulty, and another may exclude it altogether. Nor does even 

 the same membrane display an equal degree of permeability to 

 all substances, for some may pass through it easily, others with 

 difficulty, and others not at all. To what these differences in 

 permeability are due we do not know, but they are presumably 

 caused by the relations between the structure of the membrane 

 and the size and character of the molecules of the dissolved 

 substance. 



A membrane which allows water to diffuse through it but does 

 not allow a given dissolved substance to do so is called semi- 

 permeable, and it is a highly important biological fact that all 

 membranes in living cells seem to belong to this class. The 

 membrane of a root-hair cell, for example, allows water to pass 

 readily but is impermeable to such substances as sugar, which are 

 dissolved in the sap solution. The cell is thus able not only to 

 retain these valuable materials within itself, unwasted by outward 

 diffusion, but to use them as a permanent means of drawing in 

 osmotically a supply of water from the soil, since their presence 

 within the root-hairs normally maintains the sap of these cells at 

 a higher concentration than the adjacent soil solution. This 

 same membrane, however, is permeable to most of the mineral 

 salts present in the soil, which are thus able to diffuse readily 

 into the root-hair. 



Other Principles of Osmotic Action. — Before we attempt to 

 apply the principles of osmosis to the living plant, however, we 

 should fix clearly in mind certain facts with regard to osmotic 

 phenomena in general about which confusion frequently arises. 

 First, substances which are not soluble or which for any reason 

 are not in solution cannot diffuse and have no osmotic effect 

 whatever. Sugar, for instance, is soluble and is osmotically 

 active, but the moment it is converted into starch, which is an 

 insoluble substance, it loses its osmotic effect entirely. Second, 

 the osmotic strength of a solution, and consequently its power to 

 attract water, is determined not by the chemical nature of the 

 dissolved substances but by the total concentration of material, 



