LIVING PARTS OF THE STEM 111 



Plant-food is made out of the raw materials by the leaves, 

 and so the more leafy side of a tree forms thicker rings 

 than the less leafy side, as shown in Fig. 79. 



119, Rate of Movement of Water in the Stem. There 

 are many practical difficulties in the way of ascertaining 

 exactly how fast the watery sap travels from the root to 

 the leaves. It is, however, easy to illustrate experimen- 

 tally the fact that it does rise, and to give an approximate 

 idea of the time required for its ascent. The best experi- 

 ment for beginners is one which deals with an entire 

 plant under natural conditions. 



EXPERIMENT XXII 



Wilting and Recovery. Allow a fuchsia or a hydrangea 1 which 

 is growing in a flower-pot to wilt considerably for lack of watering. 

 Then water it freely and record the time required for the leaves to 

 begin to recover their natural appearance and position, and the 

 time fully to recover. 



The former interval of time will give a very rough idea 

 of the time of transfer of water through the roots and the 

 stem of the plant. From this, by measuring the approxi- 

 mate distance traveled, a calculation could be made of the 

 number of inches per minute that water travels in this 

 particular kind of plant, through a route which is partly 

 roots, partly stem, and partly petiole. Still another 

 method is to treat leafy stems as the student in Exp. XXI 

 treated the twigs which he was examining, and note care- 

 fully the rate of ascent of the coloring liquid. This plan 

 is likely to give results that are too low, still it is of some 

 use. It has given results varying from 34 inches per 



1 Hydrangea hortensia. 



