70 ELEMENTS OP BOTANY. 



sap there must be local transfers laterally through, the stem, 

 and these are at times of much importance to the plant. 



93. Bate 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 experimentally the fact that 

 it does rise, and to give an approximate idea of the time 

 required for its ascent. The best experiment for beginners is 

 one which deals with an entire plant under natural conditions. 



94. Experiment 19. Wilting and Recovery. — Allow a fuchsia 

 or a hydrangea i 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 approximate dis- 

 tance traveled, a calculation could be made of the number of 

 inches per minute which 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. 18 treated the twigs which 

 he was examining, and note carefully 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 vary- 

 ing from 34 inches per hour for the willow to 880 inches per 

 hour for the sunflower. A better method is to introduce the 

 roots of the plant which is being experimented upon into a 

 weak solution of some chemical substance which is harmless 

 to the plant and which can readily be detected anywhere in 

 the tissues of the plant by chemical tests. Proper tests are 

 then applied to portions of the stem which are cut from the 

 plant at short intervals of time. 



* Hydram^gea liortensis. 



