252 THE SECOND BOOK OF BOTANY. 



sequent transformations may go on in all parts of 

 the living structure to which the substances are con- 

 veyed by circulation. It is only in daylight that the 

 initial step is taken in the green leaf; but at all times, 

 by night as well as by day, the internal elaborations 

 and the growth of parts may go on. 



From this it will be seen that, so far as the air 

 is concerned, plants and animals perform opposite 

 offices. The lung and the leaf antagonize each other. 

 Animals absorb oxygen from the air, and return car- 

 bonic acid to it ; and, as carbonic acid is a poison, if 

 there were no plants in the world, animals would, in 

 sufficient time, contaminate it so that it would be 

 unfit to breathe. But the poisonous exhalations of 

 animals are absorbed by leaves, and destroyed, so 

 that the entire vegetable kingdom acts as a vast 

 purifier of the air. 



You may very easily observe the powerful influ- 

 ence which light exerts upon plants. Remove a 

 sprouting potato from the dark cellar into the sun- 

 shine ; its pale, watery shoots will quickly begin to 

 turn green. The first effect of light is thus to pro- 

 duce chlorophyll, and this chlorophyll becomes the 

 medium of subsequent changes. Observe whether 

 it is the upper or under surface of leaves which is 

 exposed to the light. Whichever it be, reverse it, 

 and note whether the leaf resumes its former posi- 

 tion. Place a movable plant one growing in a box 

 or pot with an erect stem, in a window, where the 

 sunshine will fall upon it. After a little time observe 

 the attitude of the stem. If you find it bent over 

 toward the light, turn it round, and see if it will 

 bend back again. 



