WORK OF THE STEM 



81 



Since tlie 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. ^ — Some 

 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- 

 ward movement of the sap may 

 be explained as due to osmosis. 

 For instance, in the case of grow- 

 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 



Fig. 47. A Cutting girdled 

 and sending down Roots 

 from tlie Upper Edge of 

 the Girdled Ring. 



