i.T" 



^ 



*k 



# 



I 



^ 



Wh 



1902] AVSE OF THE TRANSPIRATION STREAM 261 



Vesque (1884), who found the absorption by oleander roots to 

 be proportional to the pressure, both below and above one atmos- 

 phere. Vicia was found less sensitive to slight differences in the 

 pressure, \vhence it appears that osmosis plays a larger part in 

 its absorption of water. 



The atmospheric pressure theory says that the water-conduct- 

 ing structure of a tree is something like a large water barometer, 

 m water is withdrawn from the top of it to be evaporated, 

 the atmospheric pressure against the bottom forces water up to 

 make good the loss. The water column, instead of being con- 

 tinuous, is broken by bubbles, each supposed to have a tension 

 corresponding to its height in the column. The tension of these 

 bubbles will cause a local movement, and their elasticity will 

 make any adjustment of a disturbance gradual, so that the 

 dependence on the atmospheric pressure is not immediate, but 

 ultimate (and entire). 



The evidence for this theory, aside from Vesque's work, which 

 has never been adequately appreciated, is chiefly the presence of 

 rarefied air in the tracheae.^ We have already dwelt sufficiently 

 on the extent of this rarefaction. The theory demands that there 

 be an uninterrupted decrease in the tension of the air from below 

 upward; this must hold, however, only in each individual water- 

 path, and need not be at all true of the tree as a whole. R. 

 Hartig (Die Gasdrucktheorie) is quoted as reckoning the differ- 

 ence in tension sufficient to move water between adjacent tracheae 

 at 0.00004 atmospheres, and as making elsewhere an even lower 

 estimate, Manometric determinations of the tension are practi- 

 cally valueless unless it is certain that they are made in the same 

 path. All that has ever been proven (R. Hartig, Bot. Zeit. 1883, 

 among others) is that the tension is on the whole less near the 

 top of the tree, near where the water evaporates, than it is lower 

 down. There is no sound experimental evidence that in any 

 single water-path of any tree there is a constant increase in pres- 

 sure all the way down to the roots. But on the other hand, the 

 fact (Strasburger, 1893: 61-63) that in the crown of the tree 

 differences in tension depend upon the intensity of the transpira- 



\: 



'k^- 



