336 - PLANT PHYSIOLOGY 



terminal forces frequently presents itself, and the necessity was 

 felt of looking for some intermediate mechanism situated in the 

 trunk, which might supplement 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 

 function as the cells of the root parenchyma, of pumping 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 contends that the water moves not through 

 the wood but through the live cells near the cambial region. 

 That water is in fact able to move only in wood layers that are 

 still alive seems to support the idea of the participation of the 

 living cells. In the dead inner layers of wood, 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 by 

 a number of experiments. Votchall (1897) has proved by his 

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

 zontally, water moves from the morphologically 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 that 

 could prevent 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 even 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 time, however, such killed por- 

 tions 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 in the properties '^f the cell walls under 



