244 PRACTICAL BOTANY. 



tubing a glass tube arranged vertically. See that the 

 joints are perfectly tight. The sap soon rises into the 

 glass tube. Observe its height from day to day. It is 

 forced up by what is known as root pressure. This ex- 

 periment is most successful in the spring. 



c. Fill a thistle tube, having a bladder diaphragm over 

 its mouth, with water, and stand it, mouth up, with the 

 lower end of the stem dipping into mercury. Observe 



FIG. 168. Apparatus to show the lifting effect of evaporation through a 



membrane. (After Oels.) 



FIG. 169. Apparatus to show the effect of root pressure. (After Detmer.) 

 FIG. 170. Apparatus to show the lifting effect of transpiration from the 



leaves. (After Detmer.) 



that the water evaporates through the membrane and the 

 mercury rises in the stem of the tube. Keep it under 

 observation for some days. 



d. Arrange a glass tube so that one end dips in mer- 

 cury. Fill the tube with water. Fasten the top of the 

 Geranium used in b by means of a rubber stopper so that 

 its end will dip in water, making the joints air tight. 

 Because of the transpiration of the water from the leaves, 



