316 The Physiology of Plants BOOK in 



Hydrangea, and by keeping them for some time in darkness 

 depleted them of their starch. He then threw upon them 

 a very pure spectrum, as bright as he could obtain, and 

 allowed it to remain upon the healthy leaves for five to 

 six hours. On subsequently treating the leaves with a 

 solution of iodine, starch was found to have been re-formed 

 along the limits of the spectrum, presenting an image of 

 the absorption spectrum in blue. The principal band 

 between B and C was very pronounced, and there was 

 a gradual falling off towards the blue end. This method 

 gave no evidence of absorption of carbon dioxide in the blue. 



Though Timiriazeff 's early experiments gave satisfactory 

 evidence of relation between the absorption bands of the 

 red end of the spectrum, they seemed to indicate that the 

 great number of blue and violet rays absorbed are not con- 

 cerned in photosynthesis. At the same time his researches 

 proved that the maximum effect is not in the yellow, but 

 is coincident with the band B-C. 



In 1881 Engelmann introduced, and in 1884 perfected, 

 his bacterium method of testing for traces of oxygen, to 

 which attention has been already drawn. The fact that 

 Bacterium termo is actively motile in the presence of free 

 oxygen and quiescent in its absence, was used to examine 

 the distribution of power in the spectrum. A filament of 

 an alga was mounted in oxygen -free water on a slide and 

 a number of the bacteria introduced. A cover-glass was 

 put over the preparation and its edges closed by a ring of 

 oil or wax, so that no oxygen could gain access from the 

 outside. A pure spectrum was then thrown upon the algal 

 filament. After a very short interval the bacteria became 

 actively motile and accumulated in the close vicinity of 

 those parts of the filament where oxygen was being exhaled. 

 The greater number were found to swarm in the band 

 B-C, confirming Timiriazeff 's observation made by another 

 method. Engelmann claimed to have found a secondary 



