JO TIMBER AND SOME OF ITS DISEASES [CHAr. 



pressed : Sachs assumed that they were closed entirely^ 

 and that the fact that the leaves above still transpired 

 proved the truth of his assumption — the water moves 

 in the substance of the cell-walls, because there is here 

 no other passage open to it. If a branch is cut and 

 allowed to dry, the power of conducting water is 

 lost — a dry stick placed in water may become 

 s^radually saturated, but it has lost the power of con- 

 ductnigihQ water. Sachs explained this as due to some 

 molecular change in the substance of the cell-walls. 

 To the same cause he attributes the gradual loss of 

 conductivity noticed when the cut end of a fresh 

 branch is immersed for some time in water. Those 

 familiar with the literature will have noticed that 

 Sachs did not regard several facts then known, and 

 bearing more than indirectly on his hypothesis. 



For instance, if a shoot is cut and placed with the 

 cut end in water in one leg of a U tube, and allowed to 

 droop, it is often possible to restore the turgid condi- 

 tion of the upper tissues, by forcing water in under the 

 pressure of mercury poured into the other leg of the 

 U tube. 



Again, at the time when transpiration is most active 

 in the summer, it was found by Von Hohnel that if 

 a shoot is bent gently down, and cut through under 

 water, or a coloured solution, or even mercury, the 



