110 TRANSLOCATION IN PLANTS 



pressures or tensions in the water and air columns will be 

 altered as a result of the cutting incident to injection. 

 For example, where a column of water may have actually 

 been moving upward under reduced pressure or under 

 tension, the opening of the column accompanied by the 

 introduction of a solution at a different pressure may 

 result in an actual reversal of the flow over part at least 

 of the system. On the other hand, the injected solution 

 may flow into a channel in which there was originally 

 only gas at reduced pressure. An interpretation based 

 on the movement of the introduced solution may therefore 

 be completely erroneous. The reversals observed by 

 Birch-Hirschfeld (1920), DLxon and Ball (1922) and others 

 and the erratic behavior of injected solutions reported by 

 MacDougal (1926) can probably be explained as resulting 

 from cutting into such a closed system with different 

 parts at varying pressures or showing varying resistances 

 to flow. 



I am not convinced that the conclusions arrived at by 

 IMacDougal, Overton, and Smith (1929) are based on 

 indisputable evidence, that is, that specific regions of 

 annual rings of woody plants are concerned in water 

 transport and others are normally filled with gases. The 

 seasonal fluctuations in gas and water contents also, as 

 reported by these investigators (pp. 50 to 52) as well as 

 by many others previously, may not really exist or may 

 be greatly exaggerated because of the method of investi- 

 gation. If, during a season of high transpiration or deficient 

 water, one carefully exposes the wood without cuttmg 

 into it, the presence of water in the conducting vessels is 

 made evident by the somewhat translucent appearance 

 which is almost instantaneously lost when the vessels are 

 cut across.* The abrupt change in appearance is obviously 

 due to the displacement of water by air or a partial vacuum 

 when the columns under tension are ruptured. Little or 

 no evidence is available as to the extent of this displace- 

 ment. In all probability when such ruptures take place, 



* This behavior was brought to my attention by Dr. Crafts. 



