TRANSLOCATION OF WATER 187 



the water used by the plant is supplied by its action. Only in 

 early spring, when the leaves have not yet expanded, or in case of a 

 very moist atmosphere, when transpiration falls to a minimum, is 

 the principal role in raising water assumed by the root cells. 



62. 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 of several centimeters and even of several meters in the 

 stems of herbaceous plants, shrubs, and even small 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 which will now be discussed. 



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

 water with sufficient rapidity to a considerable height. Only in 

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

 water by the undeveloped leaves is very low, it may be pumped 

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

 of the trunk and cut shoots. As soon as the leaves are completely 

 unfolded, the force of transpiration will be so much greater than 

 that of root pressure that all injured places will absorb water. 

 This circumstance permits the introduction of solutions into the 

 stem of plants, mentioned in Art. 48. The surface of a cut stump 

 usually does not exude water in summer, but on the contrary 

 absorbs it. 



Although, in the trunk of a tree the suction tension of the 

 leaves is transmitted downwards to a considerable distance, it is 

 known, however, that a suction pump is not able to lift water to a 

 greater height than about 30 ft. At this height the weight of the 

 water column is equal to the pressure of 1 atmosphere. No rare- 

 faction of the air will induce water to rise above this level. 



This, however, is only an apparent contradiction. The lifting 

 of water by a pump and the sucking of water by the cells are quite 

 different phenomena. The drawing of water by a pump is induced 

 by a decrease in density of the air in the cylinder situated above the 

 water surface. The atmospheric pressure exerted on the water in 

 the container into which the pump is inserted will cause the water 

 to rise to a height corresponding to the lowering of air pressure in 

 the cylinder. It will be understood, therefore, that only a maximal 

 rise corresponding to 1 atmosphere, or 10 m. of a water column, 

 can be attained. 



