328 



PLANT PHYSIOLOGY 



76. The Rise of Water in the Trunk of a Tree. Water Tension 

 in the Vessels. The Theory of Cohesion. — The action of the 

 upper and lower terminal mechanisms readily explains the rise 

 of water through several centimeters and even through several 

 meters in the stems of herbaceous plants, shrubs, and even small 





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FiG. 105. — Transpiration pull shown by rise of mercury in the tube {after 



Molisch) . 



trees. In extending this explanation to the lifting of water in 

 large trees to a height of 100 ft. or more above the soil surface, 

 difficulties are met with that will now be discussed. 



In the first place, root pressure is too feeble a force to drive 

 water with sufficient rapidity to great heights. Only in the 

 spring when root pressure is especially high and the loss of water 

 from the undeveloped leaves is very low, may it be pumped with 

 such force by the roots that the sap will exude from wounds of 



