198 SECTIONAL ADDRESSES. 



were but repetitions of Perault's earlier work. In these experiments 

 Hales showed that water applied at the top of a branch, which is 

 severed from the tree, is drawn back into the branch and supplies its 

 leaves, and makes good their transpiration losses. A tree inarched into 

 two adjacent trees may continue to grow even after its connection with 

 the ground is severed, drawing its supplies from its neighbours. Finally, 

 a forked branch removed from a tree will remain fresh, and continue to 

 transpire water for many days if one limb of the fork with its leaves 

 is immersed in water. There is, of course, recent work also showing 

 this reversed current. 



By means of an eosin solution this reversal of the transpiration 

 current may be very easily demonstrated. If the tip of a leaf of a 

 growing potato-plant is cut under eosin solution, the coloured solution 

 is very quickly drawn back into the trachefe of the conducting tracts 

 of the leaf; from there it passes into those of the petiole, and makes 

 its way not only into the upper branches and leaves, but also passing 

 down the supporting stem may completely inject the tracheae of the 

 tuber, and from thence pass up into the wood of the remaining haulms 

 of the plant. Its passage is entirely in the tracheae of the wood of the 

 conducting tracts. 



Another very striking experiment may be carried out with the 

 imparipinnate leaf of Sambucus nigra. Its petiole is split longitudinally 

 for a few centimetres and half removed. The remaining half is set in 

 a solution of eosin. The solution is rapidly drawn up the wood-capil- 

 laries of the intact half-petiole, and soon appears in the veins of the 

 pinnae on the same side of the leal, beginning with the lowest, and 

 gradually working up into the upper ones. Finally it appears in the 

 terminal pinna. All this while the veins of the pinnae on the other 

 side remain uncoloured. Now, however, the eosin begins to debouch 

 into the base of the uppermost of these pinnae and spreads tlu'ough its 

 veins ; finally it makes its way down the offside of the rachis to the 

 bases of the lower pinnae, and from thence spreads into their veins. 

 In this case we see very clearly how transpiration actuates an upward 

 current on one side and a downward current on the other. It is 

 interesting to note that if the terminal pinna and its stalk is removed 

 the eosin does not appear in the pinnae of the second side, or only 

 after a considerable time when the small anastomosing conducting 

 tracts are utilised. 



Luise Birch -Hirschf eld recently also describes many experiments 

 with herbaceous and woody plants, tracing the path of the reversed 

 current by means of lithium nitrate and eosin. 



In all these cases the tension of the sap determines the flow from 

 a source wherever situated, and transpiration from the leaves, or parts 

 of leaves, which are not supplied with liquid water from without draws 

 the water through the plant along the channels of least resistance. 

 Hence it is that if the cut vein of a lateral pinna provides the point of 

 entry, the solution may pass backwards in some of the conducting 

 tracheae, leaving others quite uncoloured, so that only some of the veins 

 of the pinna are injected. The injected tracts bring the solution down 

 the rachis and petiole into the stem, while a few or many, as the case 



