1902] RISE OF THE TRANSPIRATION STREAM 163 



figures must of course be doubled, the suction developed during 

 five days being then joi"^™. 



From the very beginning of the absorption of water from 

 the U tube this was a successful demonstration that a column of 

 water above lo"" high in a porous substance, even in the presence 

 of free air, will not break under its own w^eight. In addition to 

 this it was very soon evident from the lifting of the mercury 

 that maintaining its continuity did not strain it to near the break- 

 ing point. With the removal of water from the tube and the 

 elevation of the mercury at its low^er end, the tension of the air 

 in the tube of course diminished and the bubbles expanded. 

 During these five days, however, the expansion was not sufficient 

 to cause any general confluence of the bubbles, even in the upper 

 end of the tube. 



At 3:18 P. M., January i8, the leaf was removed and the 

 upper end of the tube closed tightly. The height of the mercury 

 thereafter was : 



January i8, 4:18 P. M., 151.5'""' January 19, 11:30 A. M., 155.5""° 

 January 18, g:io p. m., 155.0 January 19, 3:18 P. M., 157-0 



The cause of this rise must be sought in the elasticity of the 

 air in the tube. By virtue of this elasticity the suction of the 

 leaf is — to employ materialization as a figure of speech — stored 

 up, and only gradually transmitted to the root. Expressed in 

 terms used with natural trees, transpiration and absorption arc 

 not necessarily parallel. This stored up suction is probably rep- 

 resented purely by the rise of 5°"° before January 19, 11:30 A. M. 

 The subsequent more rapid rise I ascribe to the fact that the 

 temperature of the hall fell very decidedly that afternoon. 

 Such an effect of cooling, and the opposite result of warming, 

 appeared at some other times. 



• That the tube could take up more water if the pressure 

 one atmosphere ww7<rj 314™'° of mercury — at the bottom were 

 increased was evident ; still, 1 thought it worth demonstrating. 

 To this end mercury was poured into the free arm of the U tube 

 until it stood even in both arms. At the same time the T tube 

 4"° from the top was opened and connected with a capillary 



