The Structures and Processes of Roots 



177 



Root pressure. If a number of well- watered plants are cut 

 off just above the soil, some of them will exude water for a 

 day or two. Experiments have 

 shown that the sap may in some 

 cases be forced out with pres- 

 sure sufficient to raise water 30 

 or 40 feet. This pressure is 

 called root pressure. When such 

 pressures exist in plants, they 

 probably aid in the Hfting of 

 water in stems. Under these cir- 

 cumstances transpiration pulls 

 on the columns of water in the 

 water-conducting vessels from 

 the top, and root pressure pushes 

 on them from below. Extensive 

 experiments have shown, how- 

 ever, that root pressure is inter- ->■----'- - " '--""-- 

 mittent. It may exist at one f^^" ^°f A plant with its stem cut in 



•^ two and connected again with a tube 



time and not at another, and similar to that shown in Figure 102. 

 when transpiration is most active ^^ ^^' ^^'^ ^^^ '■°°^' ^'^ absorbing 



water more rapidly than the leaves are 

 and the largest volumes of water transpiring it, since the mercury at D is 



are being raised in a plant, root Pushed away from the plant. By set- 



, ting the plant in bright sunshine, the 

 pressure is Wantmg entirely, transpiration may be increased. The 

 Because of all these facts, it is mercury is then almost immediately 



generally believed that ^^^^ ^^--'^ ^ov..r^ ^^- ^^--^■ 



pressure is not a necessary factor in the raising of water in 



stems. 



Imbibition and osmosis lead to the development of root 

 pressure, and they are partly responsible for the flow of 

 maple sap (page 135). Grapevines pruned m the spring exude 



