122 THE PRINCIPLES OF AGRICULTURE 



the outside of the old wood, and to a new layer 

 of bark on the inside of the old bark. Thus the 

 heart -wood is the oldest wood, and the outside 

 bark constantly breaking off is the oldest bark. 

 The interior wood takes less and less part in the 

 activities of the plant, and the heart -wood of 

 trees is nearly useless except as a support to 

 the plant. 



4. Irritability 



208. Growing parts are sensitive or responsive. 

 This responsiveness or irritability may be called 

 forth by diverse external forces, and is manifest 

 in definite movements, in growth reactions, and 

 in complex internal changes. 



209. Some plants make visible movements, 

 and may even be sensitive to shocks. The 

 sensitive -plant suddenly closes its leaves and 

 droops when touched ; the leaves of sun -dew 

 and other insectivorous plants close upon their 

 prey; and the tendril of the gourd gradually 

 bends around the object it touches. 



210. Green parts turn towards the light, and 

 assimilation is thereby increased. Plants in 

 windows turn the broad surfaces of their leaves 

 perpendicular to the incoming rays of light ; 

 and a seedling grown under a box into which 

 light is admitted through a single slit will grow 



