LIGHT AND LIFE. 109 



and that those of the greatest illuminating power are also 

 those which act with most energy on plants, Prillieux un- 

 dertook to examine what influence will be exercised on 

 plants by rays different in color, but known to be equal in 

 intensity, and whether this influence differs in the case of 

 different colors, or is the same, provided they do not vary 

 in illuminating power. The long and conscientious re- 

 searches of this experimenter led him to the conclusion 

 that rays of different colors act with equal force on the 

 green parts of plants, and produce an equal release of gas, 

 when they have the like luminous intensity. He holds 

 that all luminous rays effect the reduction of carbonic acid 

 by vegetables in proportion to their illuminating power, 

 whatever their refrangibility may be. If the yellow and 

 orange rays are more active in this respect, it is because 

 their luminous glare is much greater than that of the ex- 

 treme rays. 



The luminous rays also promote the production of green 

 tissue, the green matter of all vegetables. Gardeners 

 blanch certain plants by raising them in the dark. They 

 thus obtain plants of a pale yellow, spindling, without 

 strength or crispness. They are attacked by a true chloro- 

 sis, and waste away, as if sprung from barren sand. The 

 sun also aids the transpiration of plants, and the constant 

 renewal of healthy moisture in their tissues. On failure 

 of the evaporation of moisture, the plant tends to grow 

 dropsical, and its leaves fall, from weakness of the stem. 



This love of plants for light, which is one of the most 

 imperious needs of their existence, displays itself also in 

 other interesting phenomena, which show that solar rays 

 are, in very truth, the fertilizer that produces color. The 

 corolla of vegetable species growing at great heights on 

 mountains has livelier colors than that of species that spring 

 in low spots. The sun's rays, in fact, pass more easily 

 through the clear atmosphere that bathes high summits. 



