THE PHYSIOLOGY OF COLOR IN PLANTS. 



75 



converted into heat and other forms of energy. This energy is 

 directly available to the protoplasm containing the chlorophyll, 

 and by means of it the synthesis of complex substance may be ac- 

 complished. Moreover, the amount of synthesis accomplished by 

 plants exposed to separate portions of the spectrum will be directly 

 proportional to the amount of that portion which can be absorbed 

 and converted into useful forms of energy. This is graphically 

 illustrated in Fig. 2. The amount of synthesis is shown to be 

 greatest in the red light between B and C, where the greatest 

 absorption takes place. (See Fig. 1, 1.) 



Chlorophyll is a very complex and highly unstable substance, 

 and during the absorption of light it is slowly broken down, but 

 ordinarily it is rebuilt by the 

 protoplasm as fast as it is de- 

 composed. If, however, the 

 chlorophyll and the leaf con- 

 taining it are exposed to a light 

 of such intensity that the chlo- 

 rophyll is decomposed faster 

 than it can be rebuilt, then 

 damage must ensue, which if 

 sufficiently extensive will re- 

 sult in the death of the entire 

 leaf. The intensity of the light 

 which induces a maximum of 

 activity in any plant, and which 

 it may receive without damage, 

 is determined by its specific 

 constitution. The intensity of 

 light falling on a plant in an 

 open plain during twenty-four 

 hours ranges from almost total 

 darkness to the blaze of the 

 noonday sun, and varies almost 

 momentarily. As an adjust- 

 ment to this condition many 

 plants are able to regulate the 

 intensity of the light impinging on the chlorophyll-bearing masses 

 of protoplasm by altering the position of the surfaces of the leaves. 

 In others in which this movement is not possible such, for ex- 

 ample, as the leaflike duckweeds which float on the surface of the 

 water the intensity of the light received is regulated by alterna- 

 tions in the position and distance of the chlorophyll from the sur- 

 face of the organ. (See Fig. 3.) 



In many plants growing in the bright glare of the sun a thick- 

 ened cuticle or a heavy coat of hairs serves to protect the chloro- 



Fig. 3. Transverse Sections through the 

 Frond of Lemna Trisulca (Duckweed), 

 showing Different Positions of Chloro- 

 phyll Bodies. A, position in diffuse light ; 

 B, in strong light striking the surface per- 

 pendicularly ; C, in darkness. 



