112 FOUNDATIONS OF BOTANY 



hour for the willow to 880 inches per hour for the sun- 

 flower. A better method is to introduce the roots of the 

 plant which is being ^perimented 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. 



Compounds of the metal lithium are well adapted for 

 use in this mode of experimentation. 



120. Causes of Movements of Water in the Stem. — Some 

 of the phenomena of osmosis were explained in Sect. 62, 

 and the work of the root-hairs was described as due to 

 osmotic action. 



Root-pressure (Sect. 66), being apparently able to sus 

 tain 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 Californian "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. 

 Root-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 othei 

 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 uade. 



