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TEXTBOOK OF PLANT PHYSIOLOGY 



been known for a long time, but usually misinterpreted. If on a 

 hot day the stem of a rapidly transpiring plant is cut, the air will 

 quickly enter the vessels. When placed in water such a stem will 

 soon wilt, as the air clogs the vessels, preventing water from rising. 



If the stem is cut under mercury, the 

 latter will enter forcefully into the 

 vessels and rise to a considerable 

 height. In order to keep plants fresh, 

 cutting must be made under water. 



The reason for this phenomenon is 

 that the vessels of the plant, being 

 compressed with the force of several 

 atmospheres, abruptly widen when the 

 strained water threads are cut. Air, 

 mercury, or water, depending on the 

 medium used, will rush into the empty 

 space. Yet for a long time this phe- 

 nomenon was explained in a different 

 way. It was supposed, that besides 

 water, the vessels contain highly rare- 

 fied air in the form of small bubbles. 

 When a cut was made this air contracted and drew after it mercury 

 or water. The recent investigations by Renner and his coworkers 

 have shown, however, that vessels which conduct water normally 

 do not contain air, but are filled with water throughout their 

 length. Air will enter only as a result of a mechanical injury. If 



Fig. 87. — MacDougal's dendro 

 graph. 



Fig. 88. — Curves showing diurnal fluctuations in the thickness of a tree trunk, 

 recorded by the dendrograph {after MacDougal). 



cutting is done carefully under mercury, the meniscus of the latter 

 will adjoin the water, no air bubbles being present in the vessels. 

 The supposition of the presence of air bubbles in the vessels has 

 prevented for a long time the acceptance of the cohesion theory, 



