232 A TEXTBOOK OF BOTANY [Cn. V, 5 



roots, however, air is absorbed from the supply contained in 

 the porous soil. Roots have no stomata, or other openings 

 in their equivalent for an epidermis ; but the air in the soil 

 becomes dissolved in the water, and goes in solution through 

 the saturated walls into the cells of the root, from which it 

 passes to the air spaces, where it re-collects in the gaseous 

 form and thus reaches other parts of the root. The carbon 

 dioxide produced in respiration diffuses out to the soil by 

 exactly the reverse process. It is because of self -adjustment 

 to a more abundant air supply (aerotropism) that most of the 

 roots of great plants do not commonly penetrate far into the 

 ground, but keep close to the surface. This is also the reason 

 why trees commonly die when their roots are deeply buried, 

 as sometimes happens in grading around new buildings. 

 Protection of roots against desiccation, the ever present 

 danger to leaves and stems, is effected incidentally by their 

 position within the damp ground. Thus it is possible for 

 the young tips to dispense with a cutinized epidermis, which 

 would be inconsistent with their absorptive function. The 

 older roots develop a bark, but it is thin as compared with 

 that of the stems. 



5. OSMOTIC PROCESSES IN PLANTS 



The absorption of water by roots is only one of several 

 important plant processes in which osmosis has part. It is 

 important to recall that osmosis is a physical process, though 

 living protoplasm may regulate the conditions of its opera- 

 tion : that it occurs wherever in Nature two solutions of 

 different strengths are separated by a membrane they can 

 wet : that in such case there is always a movement from the 

 weaker to the stronger solution : that the movement in- 

 volves both solvent and dissolved substance in case of per- 

 meable membranes, but the solvent only in the semi-permeable 

 kind : that the stronger solution will swell and rise if free, but 

 when confined will develop pressure which can become very 

 great. Also its rate is directly proportional to temperature. 



