THE FOOD OF THE PLANT 



11 



seems that there is some solvent in a root which acts 

 on substances that are not soluble in water. 



If a little hydrochloric acid be poured over a piece 

 of marble, effervescence will be noticed and the marble 

 will slowly dissolve away, thus showing that some 

 substances that are insoluble in water dissolve in the 

 presence of an acid. 



It is possible, quite simply, to find out whether there 

 is present in a root an acid by means of which an 

 insoluble substance, such as marble, may be dissolved. 



Experiment 5 



Aim. — To determine whether any acid is present in 

 a root. 



Method. — (a) Using litmus solution. 



Litmus solution is blue in colour, but turns pink on 

 the addition of a drop of acid. 



Fill two test-tubes with a weak solution of litmus. 

 Into the mouth of one put a broad-bean seedling with 

 a radicle of about an inch in length. 



(b) Using litmus paper. 



Litmus paper also changes colour. Alkaline litmus 

 paper is blue, and becomes red when 

 brought into contact with an acid ; simi- 

 larly, acid litmus paper will be changed 

 from red to blue in presence of an alkali. 



A cylindrical gas-jar is lined with a 

 piece of blotting-paper, and the paper 

 is then made thoroughly damp. 



A soaked broad-bean and some strips 

 of blue litmus paper are inserted between 

 the glass and the blotting-paper. One 

 strip of the litmus paper is arranged so 

 that the growing radicle will come into 

 contact with it. 



Observations. — (a) After a day or two 

 the colour of the solution in which the 

 bean root is growing has become pink ; 

 the other solution remains unchanged in colour (Fig. 3) 



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Fig. 3 



Blue colour iR denoted 

 by darker tint, pink 

 by lighter tint. 



