NUTRITION. 129 



171. Solvent action. — The root hairs also exert a slightly 

 solvent action upon the soil particles themselves by reason of 

 the carbonic acid and the acid salts which they excrete. By 

 this means various minerals, which could not be dissolved by 

 the water alone, may be brought into solution. 



EXERCISE XXIII. 



To show excretion of acid salts by roots. 



Fill a wide-mouthed bottle holding 250 cc. with tap water; add 2-3 

 drops of ammonia and several drops of phenolphthalein. * If the water 

 does not now remain pink add a drop or two more of ammonia. Select 

 a vigorous seedling bean grown in sawdust; rinse roots well to remove 

 impurities. 



Cut in two a cork which fits the bottle; in the halves cut two cor- 

 responding notches of such size that with a little cotton for packing the 

 plant will be firmly held. Place the plant with enough cotton to secure 

 it in the cut cork and set in bottle with roots immersed. 



As the plant grows from day to day watch for the disappearance of 

 color in the solution, which will indicate when the alkaline fluid has be- 

 come acid. Arrange a control experiment in exactly the same way, but 

 without plant. Surround each bottle with opaque shade of heavy paper, 

 to a void effect of light on the roots and fluid. 



To show the corrosion of carbonate of lime by the carbonic acid 

 excreted by the roots. 



Cover a polished marble slab to a depth of 5 cm. with clean sand, 'in 

 which plant corn or beans. After the plants are 10-15 '-'"• high, remove 

 sand carefully and rinse off the marble. Examine the surface by reflected 

 light. A little graphite rubbed into lines etched by roots will make them 

 more readily visible. 



172. Movement of water within the plant. — Once the 



water has gained entrance to the plant, it must move to those 

 parts where it is to be used — i.e., to all the organs of the 

 plant, but especially to the leaves, since from these there is 



* An indicator for acids, colorless when a fluid in which it is dissolved 

 is acid, rose pink or darker when alkaline. For use the crystallized 

 phenolphthalein is dissolved in alcohol. 



