HOW PLANTS OBTAIN WATER. 33 



heat, and weigh again. It has lost in weight. This has been brought about 

 by driving off the moisture which still remained in the soil after the plant 

 began to wilt. This teaches that while plants can obtain water from soil 

 which is only moist or which is even rather dry, they are not able to with- 

 draw all the moisture from the soil. 



65. " Root pressure " or exudation pressure. It is a very com- 

 mon 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 has been used to denote the force exerted in sup- 

 plying the water from the soil. But there are some who object 

 to the use of this term "root pressure." The principal objec- 

 tion is that the pressure which brings about the phenomenon 

 known as "bleeding" by plants is not present in the roots alone. 

 This pressure exists under certain conditions in all parts of the 

 plant. The term exudation pressure has been proposed in lieu 

 of root pressure. It should be remembered that the movement 

 of water in the plant is started by the pressure which exists in 

 the root. If the term "root pressure" is used, it should be 

 borne clearly in mind that it does not express the phenomenon 

 exactly in all cases. 



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. 



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

 this lifting of water by root pressure is shown. During the summer season 



