250 TEXTBOOK OF BOTANY 



influence upon the air, and the members of each group supply 

 the particular gas that the members of the other group need. 

 During a bright day, green plants give off more oxygen to 

 the air than they take in, because carbohydrate manufacture 

 goes on then much more rapidly than respiration. During 

 the night, green plants respire, but do not make carbo- 

 hydrates ; therefore, they take oxygen from the air and give 

 off carbon dioxid. 



262. Movements of Leaves : The Sensitive Plant. If 

 we turn a leaf until its lower surface is upward and its upper 



A 



FIG. 148. A , a sensitive plant whose leaves are in the ordinary expanded 

 condition. B, the same plant, after its leaves and leaflets have responded 

 to the stimulus of a blow. 



surface downward, and fasten it in the new position, we shall 

 find that in the course of some hours it has turned (usually 

 by a twisting of the leaf -stalk) until the leaf -blade faces in the 

 same direction as before. This is one of the ways in .which 

 leaves can adjust their position in relation to light. A differ- 

 ent and more rapid sort of leaf movement is seen in certain 

 plants, one of which is the familiar " sensitive plant." This 

 plant, like the bean, is a member of the pulse family ; it is a 



