The Processes of Roots 195 



Plants wilted in the daytime sometimes recover at night, because 

 of the reestablishment of the capillary water columns in the soil 

 surrounding the root. 



Before we leave the subject of absorption, attention must be 

 called to the fact that the permeability of the epidermal cells 

 determines what dissolved salts in the soil water will pass into 

 the plant. These cells are more permeable to some salts than to 

 others. Hence, some salts diffuse in more rapidly than others. 

 But the root does not in any sense select the salts it needs and 

 retard the salts it does not need. Neither does it prevent the 

 entrance of poisonous substances. Salts of zinc, lead, copper, 

 and arsenic readily pass into and accumulate in plants growing in 

 the vicinity of smelters, and ultimately kill them. 



Root pressure and sap 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 be forced out with pressure sufhcient to raise water 30 to 

 40 feet. In most plants, however, a rise of only a few inches is 

 obtained. This pressure is called root pressure. When such 

 pressures exist in plants, they probably aid in the lifting of water 

 in stems. Extensive experiments have shown, however, that 

 root pressure is intermittent. It may exist at one time and not 

 at another, and when transpiration is most active and the largest 

 volumes of water are being raised in a plant, root pressure is 

 wanting entirely. Because of all these facts, it is generally be- 

 lieved that root pressure is not a necessary, or important, factor 

 in the raising of water in tall stems. 



Imbibition and osmosis sometimes lead to the development of 

 high sap pressure, and they are partly responsible for the flow of 

 maple sap. Grapevines pruned in the spring exude water or 

 " bleed " for days afterward. On a small scale the same thing 

 may be seen when well-watered geraniums, begonias, and fuchsias, 

 are cut off near the soil. There is evidence, however, that in 

 " bleeding " only the cells near the cut surface are involved. 



