II 



ASCENT OF SAP IN STEMS 



4i 



intensity, for the method of observation is very delicate. 

 However, the method is beset by the doubt involved in 

 the evident ice-pressure upon the walls. The large 

 increase in the viscosity of water at the lower temperatures 

 would also greatly reduce or stop the flow. 



Lumen blocked with vapour. Experiments in 

 which the wood of Taxus was exposed to high tem- 

 peratures above 100- appear to show that water, carry- 

 ing a dye in solution, can be drawn through the wood 

 when this is at a temperature so high as 125, and 

 very certainly filled with water-vapour everywhere in 

 its lumina. 



Fig. 10 shows and explains the arrangement of the 

 experiment. The vessel into 

 which the branch dips con- 

 tains mercury heated from 

 beneath. A glass tube sur- 

 rounds the branch, the space 

 between branch and glass 

 being filled with mercury. 

 To resist the pressure of the 

 vapour evolved from the 

 surface of the wood at this 

 temperature, it was necessary 

 to bind the stick into the tube with air-tight rubber 

 rings overlaid with wire. The following experiment was 

 made : 



A small branch of Taxus baccata, 24 cm. long, having 

 a woody cylinder of 5-6 mm. in diameter, and com- 

 posed of nine annual rings, was jacketed with mer- 

 cury at 125-130 for eight minutes, while its basal 

 end was attached to an air-pump so that the atmospheric 

 pressure forced through water supplied to the distal end 

 of the branch. The water was then replaced by a strong 

 solution of eosin, and the whole, still kept at 125- 

 130, was left for two hours. Then the experiment was 



Fi.;. 10. 



