CHAPTER IX. 



HOW WATER MOVES THROUGH THE PLANT. 



I. ROOT PRESSURE, OR OSMOTIC PRESSURE. 



89. Flow of water from pruned vines. It is a very common 

 thing to note, when certain shrubs or vines are pruned in the 

 spring, the exudation of a watery fluid from the cut surfaces. 

 In the case of the grape vine this has been known to continue 

 for a number of days, and in some cases the amount of liquid, 

 called " sap," which escapes is considerable. In many cases it 

 is directly traceable to the activity of the roots, or root hairs, 

 in the absorption of water from the soil. For this reason the 

 term root pressure is used to denote the force exerted in supply- 

 ing the water from the soil. 



90. Root pressure may be measured. It is possible to 

 measure not only the amount of water which the roots will raise 

 in a given time, but also to measure the force exerted by the 

 roots during root pressure. It has been found that root pressure 

 in the case of the nettle is sufficient to hold a column of water 

 about 4.5 meters (15 ft.) high (Vines), while the root pressure 

 of the vine (Hales, 1721) will hold a column of water about 10 

 meters (36.5 ft.) high, and the birch (Betula lutea) (Clark, 

 1873) has a root pressure sufficient to hold a column of water 

 about 25 meters (84.7 ft.) high. 



Demonstration 1 7. 



91. To demonstrate root pressure. Use a potted begonia or balsam, the 

 latter being especially useful. The plants are usually convenient to obtain 

 from the greenhouses, to illustrate this phenomenon. Cut off rather close to 



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