LIVrbTG PAETS OP THE STEM. 71 



Compounds of the metal lithium are well adapted for use 

 in this mode of experimentation. 



95. Cannes of Movements of Water in the Stem. — Some of 

 the phenomena of osmose were explained in §§ 60-54, and 

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

 action. 



Eoot pressure (§ 55), 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 raige the sap to the tops of the tallest trees, since many 

 kinds grow to a height of more than a hundred 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. 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 dis- 

 cussion among botanists. 



The slower inward and downward movement of the sap 

 may be explained as due to osmose. 



For instance, in the case of growing wood-cells, sugary sap 

 from the leaves gives up part of its sugar to form the cellu- 

 lose of which the wood-cells are being made. 



This loss of sugar would cause a flow of rather watery sap 

 to take place more rapidly than usual from the growing wood 

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

 dissolved sugar will be set up from leaves to wood. The 

 water, as fast as it reaches the leaves, will be thrown off in 

 the form of vapor, so that they will not become distended 

 with water, while the sugar will be changed into cellulose 

 and built into new wood-cells as fast as it reaches the region 

 where such cells are being formed. 



Plants in general'^ readily change starch to sugar, and sugar 



' Not including most of tlie flowerless and very l0TV,and simple kinds. 



