XIII] BUBBLE-COUNTING 133 



with approximate accuracy the energy of assimilation 

 under various conditions, and has been much used to 

 determine which rays of light are most effective in as- 

 similation. 



If the apparatus is covered with a red glass bell, which 

 only allows those rays of the solar spectrum on the red 

 side of the blue end of the green to pass i.e. the less 

 refrangible rays the number of bubbles per minute is 

 not much fewer than in ordinary daylight, care being 

 taken to ensure that other conditions are equal. If, how- 

 ever, a blue bell-jar is used which cuts off all the fore- 

 going rays, and only transmits the more refrangible blue 

 and violet rays, the bubbles diminish and fall to a 

 minimum. 



By the use of more refined apparatus, and placing the 

 Elodea successively in the red, orange, yellow, green, blue, 

 indigo and violet rays of the solar spectrum, a curve of 

 intensity of assimilatory activity has thus been obtained, 

 and although more accurate methods show that the bubble- 

 counting method is not sufficiently reliable to warrant 

 our regarding this curve as perfect, we may nevertheless 

 trust the results for general purposes. They show that 

 not only are the most active rays those in the red half of 

 the spectrum, but that it is especially the red-orange rays 

 which are most effective. 



It is beside my purpose here to discuss the details of 

 differences found by using different methods and different 

 plants, but the student may accept as generally true that 

 the process of carbon-assimilation in green leaves is due 

 especially to the action of the red -orange rays in decom- 

 posing the carbon-dioxide, in the chlorophyll-apparatus 

 of the living cells of the leaf. 



The foregoing facts help us to understand why gar- 

 deners must allow plenty of air to circulate through their 



