13° 



OUTLINES OF PLANT LIFE. 



the largest loss of water by evaporation (^ 177)- From the 

 root hairs the water passes inward through the cortex, and 

 reaches the stele. The forces which determine this move- 

 ment and its direction are not fully understood. They are 

 comprehended under the general phrase root pressure. 



173. Root pressure. — The action of root pressure may be 

 demonstrated by severing a suitable stem close to the ground 

 and observing that water flows out, 

 after a short time, from the cut end. 

 Careful examination of the cut surface 

 shows that the water is forced out chiefly 

 from the woody parts of the stele, and 

 this continues for a considerable time. 

 The force with which water is extruded 

 may be measured by attaching to the 

 stump, by means of a rubber tube, a 

 pressure gage (fig. 113). In this way 

 it may be ascertained that in woody 

 plants, such as the birch, the pressure 

 sometimes becomes great enough to 

 sustain a column of mercury about two 

 meters high (2.5 atmospheres). 



EXERCISE XXIV. 



Fig. 113. — Apparatus for Demonstration. To show root pressure as a 

 measuring root pressure. j- ^ ■ ^i j^ ^ ■ ._t \ 



For explanation see Ex. factor m the movement of water tn plants. 



ercise XXIV. p. 131.— Cut off the stem of an actively growing plant 

 (plants of castor bean and tomato 25-30 cm. 

 high are especially recommended) a short distance above the soil and 

 fasten tightly to the stump, by means of rubber tubing, a, piece of glass 

 tubing a meter long, and a,bout the diameter of the stump. Wrap joint 

 with tire or electric tape to prevent stretching of rubber and leakage. 

 Add enough vi'ater to rise 10 cm. above the rubber connection. Keep 

 roots well watered and mark the height of the water in tube from time 

 to time until it reaches the top or begins to fall. Does the water rise 

 from the first ? 



