268 BOTANICAL GAZETTE Loctober 



have been made by Hansen, Weber, Janse (1887 :2i), Bohm 

 (Bot. Centralb. 1890; 1892 II; 1893), Vesque (1891:583), 

 and Strasburger (1891 1645 seq,). Strasburger boiled the lower 

 12"" of a 15"" high Wistaria, after which water rose at least 10.8" 

 from the base. When part of a stem has been boiled, the living 

 part above it usually wilts after a few days. Weber found 

 and Janse confirmed it — that the living part above, or both 

 above and below, the boiled part had become impervious by the 

 growth of tyloses, or other healing devices, such as would be 

 used to cut off an injury arising in any other way. It is, then, 

 the activity of the live part, and not any inactivity of the killed 

 part which is the immediate cause of the stoppage of the trans- 

 piration stream. There is no direct evidence that any activity 

 of living cells is necessary, even to keep the dead conducting 

 elements in lasting proper condition. 



It was never more difficult than it is now to say safely that 

 any performance is not possible by living cells. But it has been 

 'most thoroughly proven that their assistance is in no way neces- 

 sary in the ascent of sap; it is therefore exceedingly improbable. 

 More than this, it is evident that if water be withdrawn from any 

 vessel by living cells and returned to the same vessel higher up, 

 or to any other vessel in easy communication with it, the water 

 will run dowm again unless it be prevented by friction or by some 

 force outside the living cells. Friction is not available, since 

 water will sink in wood by its own weight ; and without friction, 

 a force which can hold it from running down can also draw it 

 up, without any help from the living cells. The work of living 

 cells in lifting water can be effective only w^hen these cells form 

 a closed layer without a leak. Root pressure is made possible by 

 the presence of such a layer of cells. But an unbroken layer of 

 living cells across the entire path of the transpiration stream, or 

 even across so much of it as supplies water to a single leaf, is 

 unheard of in the stem of any plant. It is the tracheae w^hich 

 are uninterrupted. It is precluded by its structure, then, that 

 the stem of any plant should serve in the elevation of sap as the 

 root does in setting up root pressure. 



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