CHAPTER V. 



ROOT PRESSURE. 



66. 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 con- 

 siderable. 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 supplying the water from the soil. 



67. Boot 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(i5 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. 



68. Experiment to demonstrate root pressure. By a very simple method 

 this power of root pressure may be demonstrated. During the summer season 

 plants in the open may be used if it is preferred, but plants grown in pots are 

 also very serviceable, and one may 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. The stem is cut off rather close to 

 the soil and a long glass tube is attached to the cut end of the stem, still con- 

 nected with the roots, by tne use of rubber tubing as shown in figure 32, and a 



31 



