356 Mr F. Darwin and Mr R. W. Phillips, [Nov. 23, 



The incisions were sawn at 17 cm. and 19 cm. from the base on 

 opposite sides, and to the depth of over half the diameter. 



It will be seen that there is a general resemblance between 

 the effects obtained with the pump and those obtained when the 

 transpiring branch supplies the force, though the effect would 

 seem to be greater in the pump-experiments. Thus in both- we 

 get a depression in rate when the first cut is made and a much 

 greater depression following the second incision. 



§ 6. Discussion on a point of difference between the pump and 

 transpiration-results. 



In the pump — or siphon — experiments no recovery takes place 

 after the depression. This is exactly what we should expect — the 

 force exercised by a pump is constant; whereas the force exercised 

 by the transpiring plant can increase when anything occurs to block 

 the passage of the stream. As soon as the current of water is 

 diminished, the transpiration of the leaves will begin to empty 

 the xylem elements of water, and this will necessarily increase the 

 sucking power and will increase the current through the blocked 

 place. This effect may be clearly seen in cases such as Sachs has 

 described where the absorbing power of a cut branch has been 

 lessened by lengthened immersion in water. If a fresh surface 

 is cut and the branch quickly fitted into the potometer the current 

 will be found to be extremely rapid. It is found that this quick 

 rate of absorption is not permanent but is rapidly falling. 



Exp. 32. A branch of Portugal laurel which had become 

 very dirty at the cut end, and which was absorbing water slowly, 

 gave the following readings : 



July 24, 1884. Time Rate 



4.27 28-6 

 5.11 27-8 



A clear surface was cut and the branch refitted to the apparatus : 



5.18 

 18} 

 23 

 32 



333 

 333 



222 

 161 



The same result may be obtained if a branch is cut off above 

 the place where two opposite incisions have been made, and this 

 shows once more that the double sawing of a branch acts as a 

 serious block to the transpiration-stream. 



