CAUSES OF MOVEMENTS OF WATER IN THE STEM 77 



low ; still it is of some use. It has given results varying 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 hi the tissues of the 

 plant by chemical tests. Proper tests are then applied to por- 

 tions of the stem which are cut from the plant at short intervals 

 of time. 



Compounds of the metal lithium are well adapted for use in 

 this mode of experimentation, if a spectroscope is available to 

 test for its presence. 



90. Causes of movements of water in the stem. Some of the 

 phenomena of osmosis were explained in Sees. 48-51, and the 

 work of the root hairs was described as due to osmotic action. 



That portion of the sap pressure which originates in the roots 

 (Sec. 38), being apparently able to sustain a column of water 

 only 80 or 90 feet high at the most (and usually less than half 

 this amount), would be quite insufficient to raise the sap to the 

 tops of the tallest trees, since many kinds grow to a height 

 of more than 100 feet. Our California "big trees," or Sequoias, 

 reach the height of over 300 feet, and an Australian species of 

 Eucalyptus, it is said, sometimes towers up to 470 feet. Eoot 

 pressure, then, may serve to start the soil water on its upward 

 journey, but some other force or forces must step in to carry it 

 the rest of the way. What these other forces are is still a matter 

 of discussion among botanists. 



The slower inward and downward movement of the sap may 

 be explained as due to osmosis. For instance, in the case of 

 growing wood cells, sugary sap descending from the leaves into 

 the stem gives up part of its sugar to form the cellulose of 

 which the wood cells are being made. 



This loss of sugar leaves the sap rather more watery than 

 usual, and osmosis carries it from the growing wood to the 

 leaves, while at the same time a slow transfer of the dissolved 



