THE EEASON WHY. 287 



"And God said, Behold, I have given you every herb bearing seed, which is 



upon the face ef all the earth, and every tree, in the which is the fruit 



of a tree, yielding seed ; to you it shall be for meat." GKBTESIS I. 



1141. Why is it understood that some plants feel ? 

 Because the sensitive plant closes its leaves on being touched; 



the Venus 's fly trap closes its leaves upon flies that alight upon 

 them ; others close upon the approach of rain, and at sunset, and 

 open at sunrise, and turn towards the sun during its daily transit. 



1142. Why is it understood that some plants move ? 

 Because certain sea-weeds throw off undeveloped young plants, 



which move through the water by the aid of fine cilia, or mus- 

 cular hairs, until they find a suitable place upon which to attach 

 themselves. 



The roots of plants will penetrate through the ground in the 

 direction of water, and of favourable soil. 



1143. Of what elementary substances are plants composed? 

 Of carbon, oxygen, hydrogen, and nitrogen. 



1144. Whence do plants derive those substances ? 

 From the air, the earth, and water. 



1145. How do plants obtain carbon ? 



They obtain it chiefly from the air, in the form of carbonic acid 

 gas. The carbon, of the carbonic acid gas, which is thrown out 

 by the breath of animals, and by other processes in nature, is 

 absorbed by the leaves of plants, and the oxygen which had united 

 with the carbon to form the carbonic acid gas, is again set 

 free for the use of animals. 



1146. How do plants obtain oxygen ? 



They obtain it from the atmospheric air. But as they do not 

 require a large amount of oxygen for their own use, they throw off 

 the amount which is in excess, after having separated it from the 

 other elements with which it was combined when taken up by them. 

 From the humble blade of grass, to the stately tree of the forest, 

 plants operate to purify the air, and to correct and counteract the 

 corruption of the air, by the myriads of animals inhabiting the 

 earth. 



It has been generally stated that plants in rooms purify the air by absorbing 

 carbonic acid by day, and releasing a part of the oxygen ; but that, as the pre- 

 sence of light is necessary to produce this action, they do not restore oxygen to 



