BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 233 



in spring, before the leaves appear, the sap may press up into the 

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

 in an herbaceous plant evaporation of water from the leaves is checked, 

 the sap may press into the leaves so strongly that drops exude from 

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

 definite water pores. The 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 physiological botanists, who, about 1731, 

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

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

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

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

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

 any rapidity. Root pressure, therefore, cannot continuously supply 

 the leaves with the water they need. 



547. The ascent of water in the stem has 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 the 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 are perpetually giving off water vapor from their parts above 

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

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

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

 nothing in the dry season. 



549. Leaves are the especial organs of transpiration in ordinary 

 cases. Though their surfaces are covered with an epidermis that pre- 

 vents too great loss of water, the pores or stomates allow a regulated 

 escape of vapor which is of great importance to the plant. The inter- 

 cellular passages of the spongy tissue furnish communication between 

 the leaf cells, saturated with water, and the atmosphere without. As 

 long as the stomates remain open, therefore, vapor given off by the 

 moist walls of the cells escapes from the leaf. When the 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 in most plants. 



