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 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. 
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 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 from the leaves 
gives up part of its sugar to form the cellulose of which 
the wood-cells are being made. | | 
