EXPERIMENTS WITH CUT-OFF SHOOTS. 245 



of conduction, even when transpiration was energetic, restored to such an extent 

 that after the pressure of the column of mercury {q q') had become cquaHsed, 

 the shoot nevertheless continued to absorb water, and indeed with such force 

 that the level q of the mercury was raised many centimetres high above q. De 

 Vries, who investigated this phenomenon further, found now that the withering 

 of cut-off shoots only occurs very late, or does not occur at all, if the shoot-axis, 

 before being cut, is bent down into a bowl of water, and the portion under water 

 then cut through. This proves that the contact of the cut surface with air is one 

 of the essential causes of the diminished conductivity at the transverse section ; and 

 that the alteration concerns not exclusively the section, but also parts of the con- 

 ducting tissue higher up, may be concluded from the fact that the conductivity is again 

 increased when a shoot, which has been cut off in the air and then placed in water, 

 is cut through under the level of the water, but several centimetres above the first 

 section. It is still questionable on what this injurious influence of the air, even 

 on only momentary contact with the section, depends, and perhaps the sudden 

 entrance of the air into the cavities of the wood filled with very rarefied gases plays 

 a part in the matter^. That the absorption of water at a cut surface of wood 

 gradually diminishes, even apart from these circumstances, is shown by the fact that, 

 by cutting off daily a small piece from the lower end, shoots standing in water may 

 be maintained fresh for a much longer time. Evidently the absorbing section 

 becomes altered and diseased by slimy substances and colonisations of Bacteria ; 

 and even when this is not the case, since the cut surfaces of the wood cell-walls 

 gradually absorb large masses of water, the fine dust particles always contained in 

 the latter must collect and cover the section with an impenetrable layer. This 

 circumstance also brings it about that, when apparently pure water is filtered through 

 fresh wood, it goes through at first with great ease, after which the filtration becomes 

 slower and slower; if an extremely thin cross lamella is then cut off from the wood, 

 the filtration again proceeds more rapidly for a short time. 



* With respect to the drooping of the apices of cut-off shoots, cp. Hugo de Vries (Arb. des bot. 

 Inst, in Wzbg. I, p. 287). 



