USES OF LEAVER. 147 



derived their nourishment, was scarcely less than that 

 of boiling water. 



But to promote these effects, Light is an agent of in- 

 dispensible importance. We already know that when 

 it is excluded, leaves become sickly and pale, their 

 stems lose their natural elasticity and strength, for the 

 former derive no nourishment from the air, and the lat- 

 ter no sustenance from the leaf. The fluids are retain- 

 ed, no oxygen escapes, there is a deficiency of alkali, 

 on which the green colour of the leaf is supposed to de- 

 pend, and of carbon, which alone gives firmness to the 

 wood. But when light is admitted, it enables the leaf 

 to convert the carbon of the air to its use, which as it 

 descends through the vessels of the bark, becomes al- 

 burnum, and at last forms perfect wood. It also ena- 

 bles the leaf to exhale its excess of fluids, and of oxy 

 gen, thereby increasing the proportion of that alkali, 

 on which its green and healthy colour depends. But 

 the beneficial influence of light upon leaves is confined 

 entirely to their superior surface, which always courts 

 the full influence of the sun, and turns as if to welcome 

 his congenial rays. Those who have cultivated plants, 

 must have observed how uniformly they mantain this 

 position, and if disturbed, how soon they resume it ; 

 and if a Vine leaf be suspended by a thread, it is still 

 susceptible of impression from the sun's light, and re- 

 tains the power to follow him in his course. 



The pinnate leaves are more under the dominion of 

 light than any others. They expand in the morning, 

 and at the approach of darkness close around each 

 other, and sometimes around the full blown flower, as 

 if for mutual protection and support. This is the sleep 

 of plants, but what analogy it bears to the sleep so es- 

 sential, and so refreshing to the animal kingdom, is left 



