42 TRANSPIRATION AND ASCENT OF SAP ch. 



stopped. The eosin being first removed, the surface to 

 which it had been applied was pared and dried. The 

 branch was then detached from the air-pump and allowed 

 to cool. On microscopic examination it was found that 

 the eosin-solution had passed 22 cm. up the wood, and at 

 this height was seen in cross-section as two irregular patches 

 occupying quadrants in the seventh and eighth rings. The 

 walls of these were uniformly coloured. At the level of 

 the mercury jacket, and throughout the 7 cm., where the 

 branch was immersed in mercury, the colouring was most 

 intense in the limiting membranes. At the end where the 

 eosin was applied, the walls were scarcely coloured, except 

 those adjoining the medullary rays and immediately round 

 the bordered pits. 



Small transmission in the walls. The simplest 

 interpretation of these results is that the coloured water 

 moved in the wall, while the lumen was occupied with 

 vapour ; the intenser coloration of the limiting membrane 

 is strongly in support of this view, for it is very probable 

 that for some distance from its surface the wall was so 

 far choked with vapour as to impede the motion of a 

 liquid. 



These experiments then, so far as they go, are in perfect 

 agreement with the previous set in which the lumina are 

 choked by the introduction of foreign substances (cacao- 

 butter, gelatine, air, in the experiments of other authors, 

 or by paraffin and carbon dioxide in our own) ; and they 

 show that the freedom of the lumina is necessary for the 

 rapid transmission of water, but that a slow current may 

 pass through the walls even when the lumina are completely 

 blocked. 



Negligible amounts transmitted as vapour. 

 There appeared the possibility that the nagging of the 

 branches having closed lumina might be due to the stoppage 

 of them as vapour-conduits, and not as water-conduits ; 

 that is, the experiments were not yet conclusive as to the 



