THE CHLOKOPHYLL APPAKATUS 157 



views of Usher and Priestley should prove to be correct, part 

 of it is applied directly to the original decomposition of the 

 absorbed carbon dioxide without the intervention of the 

 living substance of the plastid. A very ingenious method of 

 demonstrating that the energy is derived from the rays of 

 light absorbed by the pigment was devised by Engelmann. 

 He observed that certain bacteria were excited to active 

 movement only in the presence of free oxygen. He placed 

 a filament of a green alga upon a glass slide in a fluid 

 containing a number of the bacteria, covered it with a glass 

 cover-slip, and sealed it with wax. He kept it in darkness 

 till the microbes had come to rest, and then by the aid 

 of a microspectroscope he threw a spectrum upon the 

 filament and observed in what parts of it the bacteria 

 accumulated as soon as they began to move. These places 

 corresponded with the positions of the absorption bands 

 which we have seen to be characteristic of the chlorophyll 

 spectrum, the maximum effect being produced by the deep 

 band in the red region. These were evidently the places 

 at which the chlorophyll apparatus of the filament was at 

 work, the movements of the bacteria showing that oxygen 

 was liberated there. Tmiriazeff proved the same thing by 

 throwing the spectrum of solar light upon a darkened leaf, 

 when he found that starch was produced only in the 

 positions of those same absorption bands, indicating that 

 those were the only places of photosynthetic activity. 



The process of photosynthesis has been found to pro- 

 ceed under certain circumstances in light which is too 

 feeble in intensity to cause the development of chlorophyll. 

 It is effected in these cases by the etiolin, which we have 

 seen to be the antecedent of chlorophyll. The photo- 

 synthetic power of etiolin is, however, exceedingly small. 



The percentage of carbon dioxide admitted to the 

 chloroplasts has some influence upon the activity of the 

 process. Normal air contains a mere trace of the gas, less 

 than 4 parts in 10,000. A more copious supply is, how- 

 ever, distinctly advantageous, and the activity increases as 



