124 BOTANY FOR BEGINNERS CHAP. 



EXPT. 113. Place a green plant under a blue bottle, and expose to 

 light for a few days. Examine a leaf by the same method given in 

 Expt. 112. Note 



(i) The leaf does not turn so dark in colour with the iodine solution. 



(ii) This is due to the blue glass cutting off all the rays except the 

 blue and violet, and these are not the active agents in assimilation. 



Experiments show that the active rays are those from 

 the red end of the spectrum, but as white light contains 

 all these rays in addition to blue and violet, there will be even 

 more starch produced in white light than under the influence of 

 the red, orange and yellow rays. 



EXPT. 114. Obtain a strip of tinfoil and fix it over a portion of a 

 leaf, so as to protect it from the light. Leave the leaf on the plant for 

 a few days. Treat the leaf in the way advised in Expt. 112. Note 



(i) The part of the leaf which has been covered with tinfoil is of a 

 pale yellow colour. 



(ii) The remainder of the leaf is of a dark blue colour. 



(iii) This shows 'that light is necessary for the formation of starch. 



EXPT. 115. Take a plant with variegated leaves, and place it in 

 darkness for a few days. Now place the plant in bright light for two 

 hours, and treat a leaf as described in Expt. 112. Note 



(i) That it is only the green parts of the leaf which colour blue. 



(ii) This shows that it is only the green parts of plants which can 

 assimilate. 



Other Elements in the Food of Plants. Hydrogen. 

 All plants can obtain hydrogen from water and ammonium salts. 

 Hydrogen is necessary for the life of the plant, for it enters into 

 the composition, as we have seen, of cellulose, starch, proteids, 

 and protoplasm. 



Oxygen. Plants can take up oxygen in a free state, in 

 combination in water, and in mineral salts. 



Respiration. Free oxygen is necessary for the life of 

 nearly all living beings. The taking in of free oxygen and the 

 giving out of carbon dioxide is spoken of as respiration. Every 

 living cell in a plant requires oxygen for its activity. 



// must be distinctly understood that respiration and assimila- 

 tion are two distinct processes. Green plants are alone able to 

 assimilate under the influence of light; they take in carbon 

 dioxide and give out oxygen. 



Respiration is carried on by all parts of plants no mattct 

 what their colour may be, and at all times, from the commence- 

 ment of germination until the plants die. Oxygen is taken in 



