Movement of Materials in Stems 



i6i 



there is pressure from below. It is known also that there may 



be currents moving downward in one layer of the wood and 



upward in another, although the 



general direction of water transport 



is upward to the leaves. The 



movement of water in the tracheae 



and tracheids is a mass movement, 



similar to the flow of water in a 



pipe, in spite of the fact that it 



frequently must pass through the 



cross-walls which divide the vessels 



at intervals. It is certain that the 



roots alone do not force water up 



into the tops of trees. 



The primary factor, then, in the 

 rise of sap is transpiration ; the 

 second factor is the movement of 

 water from the water-conducting 

 tissue to the mesophyll cells, re- 

 placing that lost through transpi- 

 ration ; the third factor is the cohe- 

 sion of water columns in the long 

 strands of water-conducting tissue, 

 which makes it transmit the pull 

 from the mesophyll cells all the 

 way down to the roots. In Chap- 

 ter XXI we shall learn how the 

 water passes from the soil into the 

 roots, and to what extent the roots aid in the lifting of water. 

 (See Fig. 95.) 



The pulling up of water by transpiration is exempHfied when 

 cut flowers are placed in a vase containing water. That water is 

 drawn into the flowers maybe shown by placing the stems of white 

 flowers in water colored with red ink. Try this experiment with 



Fig. 95. Experiment to show the lift- 

 ing power of transpiration and evapo- 

 ration. Both tubes were filled with 

 boiled water and placed in a dish of 

 mercury. In C the mercury has been 

 drawn up by transpiration from a 

 branch of arbor- vitae (A) ; in D, by 

 evaporation from a porous cup (B). 



