LIVING PARTS OF THE STEM. 71 
Compounds of the metal lithium are well adapted for use 
in this mode of experimentation. 
95. Causes of Movements of Water in the Stem.— Some of 
the phenomena of osmose were explained in §§ 50-54, and 
the work of the root-hairs was described as due to osmotic 
action. 
Root 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 raise 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 
500 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 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 
1 Not including most of the flowerless and very low and simple kinds. 
