TRANSLOCATION OF WATER 195 



the role of the air entering separate vessels were considerably 

 overrated. Frequently, for this reason, the idea of the insufficiency 

 of the terminal forces presented itself and the necessity was felt of 

 looking for some intermediate mechanism which might support the 

 terminal ones. Most frequently the living cells of the wood were 

 regarded as such an intermediate mechanism, especially the cells of 

 the medullary rays and those of the wood parenchyma, which 

 adjoin the vessels. It was supposed that these cells have the same 

 faculty as the cells of the root parenchyma, of pressing water 

 through them in one direction only. Lately this view has been 

 strongly supported by the Indian scientist Bose. This author tries 

 to explain not only the translocation of water in the stem, but also 

 the loss of water in transpiration, by the pulsation of the living 

 cells. He even contends that the water moves not through the 

 wood but through the bark, where the living cells are much more 

 numerous. That water is indeed able to move only in wood layers 

 that are still alive seems to favor the idea of the participation of the 

 living cells. In the dead inner layers the vessels are filled either 

 with resinous substances, forming the so-called heartwood of the 

 tree, or simply with air. They no longer participate in water 

 conduction. 



This theory of intermediate mechanisms has been refuted 

 through a series of experiments. Votchall (1897) has proved by 

 his comprehensive work that in a living tree trunk, placed hori- 

 zontally, water moves from the lower end to the upper one at the 

 same rate as from the upper to the lower. This clearly shows 

 the absence of any valves in the wood, preventing the downward 

 movement of the water. But without such valves the supposed 

 intermediate mechanism would not be able to work. Moreover, 

 Strasburger (1893) and other investigators have shown that when 

 poisonous solutions, such as picric acid, are introduced through cut 

 vessels, they will rise unchecked in the trunk to a height of several 

 meters, or clear to the upper leaves, though they certainly poison 

 all living cells on their way. Likewise, the unchecked rise of water 

 through portions of a herbaceous stem, killed by heat, freezing, or 

 toxins, has been observed. 



After some period, however, such killed portions of a stem 

 cease to conduct water and their vessels become filled with air. 

 The exact reason for this behavior has not been established as yet. 

 Some investigators suppose that it results from changes occurring 



