ABSORPTION OF GASES. 



39 



light, or in light the intensity of which is reduced by means of one 

 or more plates of ground glass (Fig. 35), the number of bubbles 



Fig. 35. 



Box blackened on the inside. (After Oels.) a a, ground-glass plates; b, 



shoot of Elodea. 



given off decreases, so that the dependence of photosynthesis upon 

 light can be seen directly. 



30. Physical Properties of Chlorophyll. — Chlorophyll is a sub- 

 stance of extremely complex and unstable constitution. It is 

 generally found in certain definite masses of protoplasm in the 

 cell, although in some plants it appears uniformfy diffused 

 throughout. Its presence is sometimes masked by other color- 

 ing matter, as in the Red Sea-weeds and colored leaves of the 

 foliage plants of the garden. Some of the autumnal tints of 

 leaves are due to coloring substances resulting from the oxida- 

 tion of chlorophyll. The spectrum of sunlight which has 

 passed through a solution of chlorophyll in alcohol shows 

 several dark bands. The portions of light thus absorbed are 

 converted into heat and other forms of energy needed for 

 photosynthesis and other processes. If different portions of the 

 spectrum are allowed to act on a plant, the relative amount of 



