WORK OF THE STEM 81 
Since the liquid building material travels straight down 
the stem, that side of the stem on which the manufacture of 
such material is going on most rapidly should grow fastest. 
95. Causes of Movements of Water in the Stem. 
of the phenomena of osmosis were explained in Sect. 60, 
and the work of the root-hairs was 
described as due to osmotic action. 
Root-pressure (Sect. 61), being 
apparently able to sustain a col- 
umn of water only eighty or 
ninety 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; 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 down- yy¢. 47. A Cutting girdled 
ward movement of the sap may and sending down Roots 
be explained as due to osmosis, *0™ the Upper Edge of 
: . the Girdled Ring. 
For instance, in the case of grow- a 
ing 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 would leave the sap rather more 
watery than usual, and osmosis would carry it 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 will be thrown off in the form of 
vapor as fast as it reaches the leaves, so that they will not 
Some 
