WATER ABSORPTION 



71 



the surfaces of the rootlets themselves do a small part. 



Water-plants probably absorb 

 through the leaves and stems. 

 Most of the higher plants, how- 

 ever, growing in water, are pro- 

 vided with roots and root-hairs 

 and considerable absorption is 

 effected by these. Certain of the 

 water-plants have roots but 

 produce no root-hairs; others, 

 as the utricularia or bladder- 

 wort, have no roots whatever. 



149. Osmosis. — To under- 

 stand how water enters the 

 root-hair, it is necessary that 

 we study 



a great quantity of water 



To illustrate osmosis, 



122. Cross-section of root, enlarged, 

 showing root-hairs. 



the process of osmosis. A salt or sugar 

 solution, separated from water by a 

 semi-permeable membrane, will in- 

 C crease its volume, due to the passage 

 into the solution of some of the water. 

 This can be easily demonstrated. (Fig. 

 123.) Dissolve in one pint of water, one 

 ounce of either common household sail 

 (sodium chlorid) or saltpeter (sodium 

 nitrate). Saltpeter is a valuable plant 

 fertilizer. Tie securely over the large 

 mouth of the tube a piece of animal 

 membrane (hog's bladder is excellent 

 for the purpose). Now fill the enlarged 

 end of tube with either the common 

 salt or the saltpeter. Then sink the 

 tube, as in Fig. 123, in the bottle A. of 

 water, until the level of the water in the 

 tube stands at the same height as that 



