102 



BOTANY OF THE LIVING PLANT 



Iodine solution, tlte 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 



I'lG. 7.1. 



Kfsult^ !■[ ex]«Tiim'iit lo shiiw that the cftirl nf light in I'hcito-Syuthciib ib 

 strict!)' local. Sec Text. 



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

 chloroplasts, 7ehich 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 ])erccntage ol carbon dioxide a green leaf (uniform in 

 size, and from the same ])lant 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- 

 Vnlity played upon each (11). 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 arc not absorbed by fhc chlorophyll do not decompose carbon 

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

 activity is m proportion to the ccunplctcncss ol their absorption. The results 

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



