THE PHYSIOLOGY OF COLOR IN PLANTS. 77 



braces many well-known species, among which are the " foliage " 

 plants of the gardener. 



A very large category of plants have become adapted to living 

 in the deep recesses of swamps and jungles, and in underbrush, 

 where the direct rays of the sun never penetrate. These plants 

 must carry on the synthesis of food material by the aid of the 

 diffuse light which reaches them, and stand a little danger from 

 its over-intensity. Still another group is found upon the higher 

 slopes of mountains in regions of low air temperatures. In both 

 instances these plants need all the energy they may be able to 

 derive from the light which falls upon them. They are not able 

 by means of their chlorophyll to absorb all this light, and some 

 of it would ordinarily be transmitted through the leaf without 

 advantage to the plant. As an adaptation to this condition, their 

 leaves are provided with layers of coloring matter, which are 

 placed near the lower surfaces in such manner that any light 

 passing the chlorophyll will be absorbed and converted into heat. 

 It is noticeable that plants growing in the swamps and on the 

 mountain tops are not provided with layers of coloring matter, if 

 the leaves are arranged in such manner that light passing the 

 upper leaves will fall on those underneath. The heat-saving 

 color screen is not needed in this instance, but is present most 

 frequently in leaves which form a low, simple rosette, or which 

 lie close to the ground. That the presence of anthocyan in flowers 

 is also often for the purpose of converting light into heat seems 

 well authenticated, from the number of plants which bear color- 

 less petals or glumes at ordinary temperatures, yet develop color 

 in these organs at lower temperatures at the beginning or end of 

 the season, or at higher altitudes. That these bright colors in 

 living plants do convert light rays into heat follows as a conclu- 

 sion of the following experiment devised by Kny : Three similar 

 glass vessels with parallel walls were filled with distilled water. 

 In one vessel a number of green leaves of canna were placed, and 

 in another such number as to offer the same amount of surface as 

 those in the first, but which contain a large amount of anthocyan. 

 The third vessel is left unchanged, and all are placed in sunlight 

 of equal intensity. A certain rise in temperature naturally ensues 

 in the water in the third vessel ; a greater rise occurs in the 

 first, showing that chlorophyll converts a portion of the light into 

 heat, while the greatest increase takes place in the second, where, 

 in addition to the action of the chlorophyll, the converting power 

 of the anthocyan is exerted. The difference between the tem- 

 perature of the vessels containing the green and red leaves often 

 amounts to 4 C, which is due entirely to the action of the antho- 

 cyan. 



It is often necessary for the plant to transport complex food 



