ii ASCENT OF SAP IN STEMS 43 



actual function of the lumina, although showing clearly 

 that their freedom is essential to preserve the turgescence 

 of the leaves. The well-known phenomenon of the 

 equilibrium vapour-pressure varying with the curvature 

 of the meniscus suggested the possibility that a trans- 

 port of vapour of considerable importance might occur 

 in the conduits, the menisci high up in the trees possess- 

 ing a lower equilibrium vapour-pressure than the menisci 

 lower down. By successive condensations beneath and 

 evaporations above the pit-membranes, this current might 

 be maintained throughout the conduits unoccupied by 

 liquid water. 



This idea led to experiments in which cut branches were 

 fed entirely upon water- vapour in the following manner : 

 The branch had its cut extremity fixed in a short glass 

 vessel containing water at the bottom ; and the cut surface of 

 the wood (which was cut at an acute angle in order to expose 

 a larger surface) was raised some 5 or 7 cm. above the 

 surface of the water. A side tubulure to the vessel enabled 

 a vacuum to be maintained within by means of a Sprengel 

 pump. The vacuum was so complete that ebullition 

 occurred upon placing the hand round the lower part of 

 the vessel. Such experiments were made upon elm and 

 lime, using control branches, some of which were simply 

 left with their cut surfaces exposed to the air, others with 

 their ends sealed into tubes containing air, but no liquid 

 water. In no case was any result. obtained going to show 

 that the vapour-fed branch possessed any advantage over 

 the others. 



It appears then that the movement upwards of water in 

 the form of vapour through the lumina is insignificant, and 

 the imbibition theory cannot receive effective help from 

 this direction. 



It is needless now to go into the many arguments which 

 overthrew the imbibition hypothesis. It is enough to say 

 that the ingenuity of the theory, and the reputation of 



