PROTECTION 



247 



The yellowness of leaves is not due to a newly manu- 

 factured substance. It is due to a substance called xan- 

 thophyll which is present in chlorophyll all the time. The 

 yellowness of xanthophyll does not show in active leaves. 

 It is obscured by the green. But as the chloroplast begins 

 to break down in the dying leaf, its greenness disappears, 

 and the color of the xanthophyll begins to show. 



B. Movements of Leaves. — The most striking case of 

 the movement of leaves is the case of the sensitive plant. 

 Perhaps you have seen 

 this plant. It is a 

 native of the tropics, ,^l^||§i?\8^1l^ 



but it is common in 

 greenhouses. It is a 

 small creeping plant 

 having somewhat the 

 habit of growth of the 

 strawberry. Its com- 

 pound leaves are com- 

 posed of many small 

 opposite leaflets. If 

 the plant is touched 

 or affected by vibra- 

 tion, first these leaflets 

 fold together, and then 

 the whole leaf bends 



down as On a hinge p IG . S6. — Movement of the leaves of the sensitive 

 at the base of the plant {Mimosa pudica) ; m, pulvinus, s, stipule. 

 . See context. 



petiole. (See Figure 



86.) These same movements are produced, but more 



gradually, by drought. 



