424 LSSSONS WITS PLANTS 



may take place more freely in darkness. So it 

 comes that most of the growth in plants is made 

 at night. 



542a. Since the plant cannot directly use, in the making of tis- 

 sue, the materials which are taken in from the soil and air, it has 

 been proposed that the term food be not applied to them ; but the 

 application of the terms food and plant-food to these raw materials 

 is so thoroughly established in scientific literature that it seems to be 

 unwise to attempt to change it. 



543. ' The trunk of a young tree girdled by a 

 label wire is shown in Fig. 431. It is larger 

 above the girdle. There has been more rapid 

 growth at that point, and we suspect that the 

 cutting of the bark by the wire prevented the 

 distribution of the elaborated food to the part 

 below the girdle. In time, then, the root must 

 starve, even though it collects food. The crude 

 materials taken in by the root pass upwards in 

 the young wood, — the sap-wood, — by a process 

 which is not yet well understood. The elaborated 

 materials are redistributed through parts of the 

 inner bark. There is no circulation in plants in 

 the sense in which there is in animals, — through 

 definite tubes or openings. 



5430. There are many good and recent books upon the physi- 

 ology of plants, treating the subject froiii various points of view. 



