202 SECTIONAL ADDRESSES. 



the leaf -cells was overbalanced by an external gas-pressure, the water 

 jjressed from the cells and forced out of the tracheae of the supporting 

 stem was found to be practically pure, and if it contained carbohydrates 

 they were in such small quantities that no reduction could be detected 

 with Benedict's solution either before or after inversion. This experi- 

 ment was repeated several times with branches of Sambucus nigra and 

 Tilia americana. The cut branch, well supplied with water, was first 

 exposed for several hours to conditions favom'able to photosynthesis, 

 and then either immediately or after a sojourn in darkness subjected 

 to the gas-pressure. A pressure of thirteen atmospheres was found 

 sufficient to drive water back from the leaves out of the stem. 



Of course the conditions of this experiment are not those obtaining 

 in the noi'mal plant, where during transpiration the volume of a leaf, 

 or part of a leaf, changes. In the transpiring plant we can also imagine 

 the accumulation of a substance or an ion which would give rise 

 to an alteration of the permeability of the plasmatic membranes of the 

 lea.ves. 



When, in order to imitate these conditions, the cells of the leaves 

 in the foregoing experiment are rendered permeable by the introduction 

 of a little toluene into the pressure-chamber, the aiDplication of a 

 smaller pressure is sufficient to press the cell-contents into the water- 

 channels, and liquid emerges from the base of the stem which readily 

 reduces Benedict's solution. 



In the same way, if a pinna of Sambucus nigra is surrounded with 

 toluene vapour, transpiration from the adjacent pinnae draws back the 

 cell-contents of the toluened pinna, and afterwards their track in 

 the wood of the vascular bundles of the rachis may be traced by the 

 browning of this tissue. 



Another possibility presented itself — ^viz., that the direction of the, 

 current might act as a stimulus regulating the permeability of the cells ! 

 m contact with the tracheae. To test this, short lengths of stem set | 

 in their normal position were supplied, first through their lower and! 

 afterwards through their upper end, with distilled water. In neither] 

 case could carbohydrates be detected in the issuing stream. 



The foregoing short consideration of some recent physiological workj 

 leads us, then, to the following conclusions: — 



The transport of the organic substances needed in the distal growing 

 regions is effected through the tracheae of the wood. The substances 

 travel dissolved in the water filling these channels, which is m.oved b^ 

 transpiration, expansion of the growing cells, or root pressure. 



Physical considerations forbid us admitting that sufficiently rapid 

 transport can be afforded by the bast either for the observed upwan 

 or downward distribution of organic substance. 



The existence of downward as well as upward movement of water 

 in the tracheae of the wood may be demonstrated by suitable experi-j 

 mental means, and may be inferred by the transport of hormones in the 

 wood. 



The occurrence of local contractions in leaves suggests that local| 

 increases of peiTneability supply dissolved organic substances to the 

 distal ends of certain of the filaments of tracheae. The tension de- 



