54 THE SCIENCE OF LIFE. 



green chlorophyll which it forms beneath the epidermis, 

 especially in the leaves, under the influence of light alone 

 breaks up carbonic acid for the supply of carbonaceous 

 food. The influence of the more luminous rays, as the 

 yellow and orange, is greater in this respect than the 

 others. Gardeners blanch certain plants by raising them 

 in the dark, yet in the first part of the germination of 

 seeds Light is injurious rather than beneficial. The in- 

 fluence of Light upon the direction of the growing parts 

 of plants, the opening and closing of flowers, etc., may 

 be chiefly owing to its influence upon the chlorophyll 

 referred to above, or it may be in some degree a direct 

 mechanical stimulus. The same amount of Light, how- 

 ever, is not required for all plants. Some require a very 

 different amount than others. Among animals Light has 

 considerable influence upon colors, and still more upon 

 the process of development. Persons who live in cellars 

 or in dark streets are apt to produce deformed children, 

 while recoveries from disease are promoted by the access 

 delight. 



To every species of plant and animal there is a conge- 

 nial and favorable temperature, although great varieties 

 exist in this respect, as well as in the power of adapta- 

 tion to extreme conditions. Many plants, for example, 

 perish with the slightest frost, yet the little fungus 

 (Torula) which is the principal agent in yeast, does not 

 lose its vitality at 76 below zero, although requiring a 

 somewhat elevated temperature for its active growth. 



Electricity possesses the power of exciting the con- 

 tractility of tjie muscular fibers and the nervous force 

 in animals in a remarkable degree. It has, however. 



