PATH OF MOVEMENTS. 55 



temperature at night this activity would cease and there might result a 

 lessened pressure in the cells. Since the specific activities of cells are 

 known to varj' in different plants, and in the same plant at different 

 seasons, some support is gained for this theory, though it is generally 

 believed that the activities of the living cells in the stems are not necessary 

 for the upward flow of water. It must bo admitted, however, that at 

 present we know very little about this interesting problem. 



