OSMOSIS 269 



of plants there exists no power of absorbing free 

 nitrogen. 



The solution of the various nutrient salts absorbed by 

 root-hairs from the soil is very weak. Hence, in order 

 that plants may secure the necessary quantity of salts 

 they must take in much water, the excess of which 

 passes off as water vapour in the progress of transpira- 

 tion. True water-plants, which are wholly submerged, 

 get all the nutrient salts and gases they require from 

 the water, and in order that their demands may be 

 satisfied it is usual for them to have finely divided leaves 

 involving a sufl&cient absorptive surface. In some 

 instances such plants have true roots, which, of course, 

 add to the gain of nutrition effected by the leaves by 

 drawing also upon the soil. Land-plants which have 

 accommodated themselves to a partially aquatic environ- 

 ment, must not be confused with genuine aquatics. 



It appears that in some instances, at any rate, root- 

 hairs discharge an acid sap, which acts chemically upon 

 minerals in the soil and makes them soluble, so that they 

 may be absorbed. If a Bean be planted in an inch or so 

 of soU laid over a piece of polished marble, and allowed 

 to grow for six or seven weeks, on removing the marble 

 it will be found that the acid sap has eaten into the 

 polished surface, which will bear a tracing, or etching, 

 of the roots. 



In regard to the entrance of solutions into a plant by 

 way of root-hairs or other cells, the word " absorption " 

 is most apposite. Root-hairs are not open suction- 

 tubes operating like the pipe of a pump; they are 

 enclosed protoplasmic cells, and absorption takes place 

 through their permeable walls of cellulose by osmosis. 



