419 
the Transpiration- Current. 
choked by the introduction of foreign substances (cocoa-butter, 
gelatine, air, in the experiments of other authors, or by 
paraffin and C0 2 in our own), and show that the freedom of 
lumina is necessary for the rapid transmission of water, yet 
that a slow current may pass through the wood even when 
the lumina are completely blocked. 
The Water transmitted in the Lumen is not in. the 
Form of Vapour. 
There appeared the possibility that the flagging 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 actual 
function of the lumina, although showing clearly that the 
freedom of them is essential to preserve the turgescence of 
the leaves. The well-known phenomenon of the equilibrium 
vapour-tension varying with the curvature of the meniscus 
suggested the possibility that a transport of vapour of 
considerable importance might occur in the conduits, the 
meniscuses high up in the trees possessing a lower equilibrium 
vapour-tension than the meniscuses lower down. By suc- 
cessive 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 ; the cut surface of the 
wood (which is cut at a sharp angle in order to expose 
the larger surface) being some 5 or 7 cm. raised above the 
surface of the water. A side tubulure to the vessel enables 
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 
