174 LIVING PLANTS 



40C, and steadily decreases from 40 to 50°C. 

 where activity wholly ceases. 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 

 adjustment to this condition the intensity of 

 the light impinging on the chlorophyll-bearing 

 masses of protoplasm is varied by altering the 

 position of the surfaces of the leaves by active 

 and passive movements. 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 alterations 

 in the position and distance of the chlorophyll 

 from the surface of the organ. 



In many plants growing in the bright glare 

 of the sun a thickened cuticle or a heavy coat 

 of hairs serves to protect the chlorophyll 

 against the more intense action of the rays. 

 Then, as will be shown later, other colors are 

 often developed in the external layers of the 

 leaf as a protection against intense illumina- 

 tion. 



Chlorophyll is generally formed in special 

 sponge-like masses of protoplasm ( chloro- 

 plasts) in the peripheral layers in the cell, and 

 is most abundant in leaves. 



As a rule the chloroplasts may form chloro- 



