36 



ELEMENTS OP BOTANY. 



The rise of liquid in the tube is evidently due to water making its way 

 through the thin membrane which lines the egg-shell, although this mem- 

 brane contains no pores visible even under the microscope. 



63. Experiment 16. Osmose in a Begonia 

 Leaf. — Place a little powdered sugar on the upper 

 surface of a thick begonia leaf under a small bell 

 glass. Watch for several days to see whether mois- 

 ture from the inside of the leaf affects the sugar. 

 The upper surface of this leaf contains no pores, even 

 of microscopic size. 



64. Inequality of Os^notic Exchange. — The 

 nature of the two liquids separated by any 

 given membrane determines in which direc- 

 tion the greater flow shall take place. 



If one of the liquids is pure water and the 

 other is water containing solid substances 

 dissolved in it, the greater flow of liquid will 

 be away from the pure water into the solu- 

 tion, and the stronger or denser the latter, 

 the more unequal will be the flow. This 

 principle is well illustrated by the egg-osmose 

 experiment. Another important principle 

 is that substances which readily crystallize, like salt or sugar, 

 pass rapidly through membranes, while jelly-like substances, 

 like white of egg, can hardly pass through them at all. 



55. Osmose in Root-Hairs. — It is very easy to understand, 

 from the principles just stated, that the soil-water (which is 

 like ordinary spring or well water), separated by the delicate 

 walls of the root-hairs and a thin lining of jelly-like liv- 

 ing matter from the more or less sugary or mucilaginous sap 

 inside them, will pass rapidly into the plant, while very little 

 of the sap will come out. Probably most of the selective 

 action, which causes the flow of liquid through the root-hairs 

 to be almost wholly inward, is due mainly to the living layer of 

 proteid material known as protoplasm (Chapter XIII), which 



Fig. 21. — Egg on 

 Beaker of Water, 

 to show Osmose. 



