42 



INTRODUCTION TO BOTANY 



can penetrate has received the special name of osmosis. Os- 

 mosis is not quite so simple as diffusion, since the movement 

 of particles of the liquids is a good deal affected by the nature 

 of the partition. In Chapter II it was stated that soil water may 

 be taken up by root hairs. It may now be seen how osmotic 

 action affects this process of taking up water by root hairs. 

 By osmosis soil water may pass through the root-hair walls 



into the interior of 

 the root hair and 

 thence to other root 

 cells. Obviously, 

 when the liquids of 

 the soil are more 

 dense than the liq- 

 uids within the root 

 hairs, the root hairs 

 will lose some of 

 their water to the 

 soil outside, and if 

 enough water is 

 lost in this way, 

 the contents of the 

 root hair may be- 

 come plasmolyzed. 

 41. How food is carried through plants. Applying the prin- 

 ciples of osmotic action to the case of a starch-loaded leaf, it 

 is evident that, as fast as the starch grains temporarily de- 

 posited in the chloroplasts of the leaf are changed into sugar, 

 some of the sugar in the denser cell sap thus produced will 

 pass on to the more watery sap of adjacent cells. From these 

 cells, in turn, portions of sugar will pass on to still more dis- 

 tant cells. In a similar way, when a potato tuber is planted 

 and begins to sprout, the sugar formed from the reserve starch 

 in the potato passes into the more watery sap contained in 

 the sprouts. This sap is constantly losing sugar that is used 

 as building material for the young growing stems and leaves, 



FIG 29. Root hairs 



A, in normal condition ; B, the same root hairs after 

 being treated with a solution of common salt 



