THE WORK OF STEMS 



239 



level of the water in the short tube) should be the 

 same in both cases. Mark carefully the height of the 

 water- column in all the tubes at the beginning of the 

 experiment, and at the end of twenty- four hours 

 compare the amount of water that has run through 

 the two stems, as indicated by the rise of the water- 

 columns in the short tubes. 



In order to gain some idea of the energy required to 

 force the water needed for transpiration through the 

 stem in each of these trees, we may first ascertain the 

 normal rate of flow and then the amount of energy 

 required to force water through the branch at the 

 same rate. Cut vigorous leafy branches, one of Oak 

 and one of Pine, three or four feet long, and place 

 them in jars of water, on the surface of which we 

 pour a little oil (cottonseed or olive) to prevent 

 evaporation. Having done this, we weigh each jar 

 with its contents and, after exposing it for twenty -four- 

 hours to conditions favorable to transpiration, weigh 

 again. 



We now take a section of a lamp-chimney or of 

 glass tubing, at least an inch in diameter, fit it at 

 each end with a stopper of rubber or paraffined cork, 

 and insert air-tight in one of these corks a glass tube 

 (one-eighth to one-fourth inch in diameter), about 

 three feet long. Now heat the tube and bend it till it 

 comes in contact with the large tube (as shown in Fig. 

 141), and secure it firmly in this position by wire. 



