BIIIEF OUTLINE OF VEGETABLE rUYtilOLOGT 233 



in spring, before tlie lea.ves appear, the sap may piress up into tlie 

 trunk and on toward the buds witli considerable force. Or again, if 

 in an lierljaceous plant evaporation of water from tlie leaves is checked, 

 tlie sap may press into the leaves so strongly that drops exnde from 

 the leaf tips or from tlie marginal teeth — usually in those cases from 

 definite water p)ores. 'J'he drops seen at the tips of grass blades after 

 a warm, damp night, are of this sort. In all these cases the rise of 

 water in the plant is due to what is termed root pressure. 



546. The phenomenon of root pressure may be observed when the 

 stem of a plant, such as the Sunflower, is cut off near the ground. 

 After a time water (sap) begins to run from the cut. If now an effort 

 is made to stop the outflow, a considerable force must be used before 

 the pressure of the sap — the so-called root pressure — is neutralized. 

 Hales, the earliest of exact p)hysiological botanists, wlio, about 1731, 

 made some measurements of the root pressure of the Grapevine, found 

 it to be equal to the downward pressure of a colnnrn of water forty- 

 three feet high. A pressure of sap, equal to tlie pressure of eighty-five 

 feet of water, has been observed in a Birch. Root jjressure falls to 

 nothing, however, wlien the loss of water at the leaf is going on with 

 any rapidity. Eoot pressure, therefore, cannot continuously supply 

 the leaves with the water they need. 



547. The ascent of water in the stem lias been the subject of many 

 investigations and much discussion. The path followed by the cur- 

 rent is the cavities of the ducts and fibers of tlie wood. The force 

 working to raise the water in these cavities is not, to any considerable 

 extent, capillarity, as was once supposed. The ultimate cause is 

 doubtless the evaporation of water from the leaves ; but how this 

 works to raise water through the stem is still a disputed question. 



548. Evaporation of water from the shoot; transpiration. — Land 

 plants ai'e perpetually giving off water vapor from their piarts aliove 

 ground, in greater or smaller quantities according to external circum- 

 stances or internal peculiarities. Even in winter tlie twigs of trees 

 transpire a little. In desert plants transpiration is reduced to almost 

 nothing in the dry season. 



549. Leaves are tlie especial organs of transpiration in ordinary 

 cases. 'J'hongli their surfaces are covered with an epidermis that jiro- 

 vents too great loss of water, the pores or sloiimles :\\]o\\ a regnlaled 

 escape of vapor which is of great importaiici' to Ihe }ihint. The inter- 

 cellular passages of tlip spongy tissut" furnish communication lietween 

 the leaf cells, saturated with water, and the atmosphere withont. As 

 long as the stomates riMiiain open, tlierefore, vapor given off by the 

 moist walls of the cells escapes from the leaf. AVlien tlie stomates 

 close from any cause, the exit of vapor is checked. Even then, how- 

 ever, some evaporation takes place through the cuticle, which is 

 imperfectly waterproof lit most Dlants. 



