70 THE NATURE AND WORK OF PLANTS 



are converted into heat. This may be demonstrated 

 if two naked-bulb thermometers are procured. Next 

 find a species which has some of its leaves green and 

 others red, such as canna or coleus. Wrap a green 

 leaf around the bulb of one and secure it with a 

 small string. Place a red leaf around the bulb of 

 the second instrument. Expose them side by side 

 to the sunlight for twenty minutes, then read the 

 height of the mercury in both instruments. 



84. Red color as a heat saver. — Examine a num- 

 ber of plants growing in a deep forest in the shade. 

 Many of those species which have large leaves 

 lying against the ground, or floating on the water, 

 will exhibit a layer of color on the lower side 

 of the leaf but none on the upper. Light which 

 strikes such leaves will be used partly by the 

 chlorophyl, and the remainder on reaching the lower 

 side of the leaf will be mostly converted into heat 

 by the red pigment. Again, it is to be said that it 

 is not definitely known that the color is formed here 

 for that purpose. The color may be produced as a 

 result of the fact that it is subjected to low tempera- 

 tures at night, but it certainly does bear the above 

 relation to light. 



