102 



BOTANY OF THE LIVING PLANT 



Iodine solution, the exposed areas will take a dark purple stain, indicat- 

 ing the presence of starch, while the shaded parts will be yellow (Fig. 74). 

 Microscopic examination of the mesophyll of the exposed parts will 

 show that small starch-grains are included in the chlorophyll-cor- 

 puscles, while those of the shaded parts contain none. Thus starch 



FIG. 74- 



Results of experiment to show that the effect of light in Photo-Synthesis is 

 strictly local. See Text. 



appears only where the light falls, and it appears actually in the 

 chloroplasts, which are thus recognised as the active agents in the process. 



An experiment indicating that the decomposition of carbon dioxide does 

 take place at the expense of the rays absorbed by the chlorophyll has been 

 arranged by Timiriazeff (Fig. 75). Into each of a series of glass tubes contain- 

 ing air and a certain percentage of carbon dioxide a green leaf (uniform in 

 size, and from the same plant as the rest) was inserted, and the tubes exposed 

 to the spectrum of the sun in a dark room, so that rays of different refrangi- 

 bility played upon each (n). After a few hours, analysis of the gases in the 

 several tubes was made, and the results compared. It was found that the 

 rays which are not absorbed by the chlorophyll do not decompose carbon 

 dioxide, whereas the rays which are absorbed do decompose it, and that the 

 activity is in proportion to the completeness of their absorption. The results 

 are shown for the rays of the red end of the spectrum by the curve in Fig. 75, in. 



