1902] RISE OF THE TRANSPIRATION STREAM 193 



I detected it in the thin strip; then it had traveled the whole 

 length of it on one side of the pith, nearly the whole on the 

 other. It was absolutely confined to the spiral (primary) ves- 

 sels, next the pith, and was about equally distributed in three to 

 five layers of these. Twenty minutes later, isolated vessels out- 

 side of these (not the pitted ones) were injected. It was 

 exceedingly evident in these isolated vessels, as I had been able 

 to make out quite unsatisfactorily in the inner spiral ones where 

 the whole group was red, that columns of colored water were 

 separated by bubbles. With a high magnification I w^as able in 



a few places, where the thickness of the wood did not interfere 

 too much, to see a very distinct red film lining the wall and con- 

 necting the indices of red liquid. It must be remembered that 



ghborheo 



carr}' the eosin. It seems to me that the conclusion is warranted 

 that the bubbles are not an appreciable obstacle to the passage 

 of the water. Strasburger describes exactly the same case. 



To summarize the discussion of the cohesion theory: It is 

 applicable only in explaining the transmission of negative ten- 

 sions ; but negative tensions do not occur, nor do the positive 

 tensions present approach zero ; the movement of water is pre- 

 vented by artificially removing the normal positive tensions; 

 if negative tensions could arise they would be immediately 

 released as a result of the movement of the water and the agita- 

 tion of the trees ; more than an insignificant negative tension is 

 impossible in the presence of free gas or a vacuum, as the only 

 bar to the enlargement of the bubble is its surface tension, and 

 this decreases with the pressure ; when the tension is low and 



transpiration is active, there are bubbles in all water conducting 

 elements. 



\_To be concluded^ 



