T902] RISE OF THE TRANSPIRATION STREAM 171 



removed ; but in living cells, when the gas higher up in the 

 stem is not more dilute than that outside, it must exert a similar 

 resistance of at least an atmosphere to the rising sap. In the 

 spring when root pressure is most active, manometers inserted at 

 ■different heights show pressures corresponding to columns of 

 Avater of the same differences in height (Clark ; Hofmeister, 

 Plora, 1858:4.) So water in the tree, when it practically fills the 

 lumina, will at most be shoved up as far as the same pressure 

 would drive it in a glass tube. As we will see later, this is not 

 necessarily the case when the wood contains a great deal of 

 rarefied air. 



III. Let us now turn to the force of attraction. It is a mat- 

 ter of common understanding that the leaves have available 

 practically unlimited energy for the elevation of water if only it 

 can be so applied. Of the radiant energy absorbed by the 

 chloroph}'!! but a small fraction is used in photosynthesis ; and 

 evaporation, by cooling the leaf, enables it to absorb more heat 

 from the air. This energy is nearly all used for evaporation. 

 Whether we say with Mayer (1895 -366) that the cause of 

 •evaporation is '* die lebendige Kraft der Wassermolekiile durch 

 welche diese geneigt sind Dampfform anzunehmen," or with 

 Nageli (1860:38) that the energy is drawn from the tissue 

 itself, it ultimatel}^ comes as heat and light from without. 

 Whatever force has been used in drawing water up to the leaves 

 must be overcome when it has evaporated ; which slightly 

 •decreases the amount evaporated. At the same time, the same 

 force^becomes available for the elevation of more water. The 

 energy thus indirectly used in lifting is relatively inconsid- 

 erable. In a former note (1900), figuring the latent heat of 

 -evaporation, which varies with the temperature, at 536 calories, 

 the mechanical equivalent of a calorie being 424, I pointed out 

 that the energy used in vaporizing water would lift it about 142 

 miles. At ordinary temperatures the latent heat of vaporization 

 is enough greater to raise this to perhaps 156 miles. Of course, 

 no large part of this could be used in lifting (Rodewald, 1892). 

 Nageli (i860 : 39) says that the evaporation uses in general 131 



