October 21, 1922] 



NA TURE 



55i 



It is possible that the tension which causes these 

 contractions of the leaf-cells at the same time acts as a 

 stimulus to increase the permeability of the plasmatic 

 membranes of the cells ; and so one might imagine 

 that the development of a certain tension would auto- 

 matically release organic substances from the cells and 

 draw them through the tracheae downwards. Direct 

 experiment on this point presents difficulties, but it 

 may be worth recording that when the internal osmotic 

 pressure of the leaf-cells was overbalanced by an 

 external gas-pressure, the water pressed 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 experiment 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 favourable 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 bark 

 from the leaves out of the stem. 



Of course the conditions of this experiment are not 

 those obtaining in the normal 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 leaves. 



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 application 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, namely, that the 

 direction of the current might act as a stimulus regulat- 

 ing the permeability of the cells in 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 work 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 moved by 

 transpiration, expansion of the growing cells, or root 

 pressure. 



Physical considerations forbid us admitting that 

 sufficiently rapid transport can lie afforded by the bast 

 either for the observed upward or downward distribu- 

 tion of organic substance. 



The existence of downward as well as upward move- 

 ment of water in the tracheae of the wood may lie- 

 demonstrated by suitable experimental 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 permeability supply dissolved 

 organic substances to the distal ends of certain of the 

 filaments of tracheae. The tension developed by the 

 transpiration of other regions draws these along- 

 downward as well as upward channels in the wood. 



In thus ruling out the participation of the bast in the 

 longitudinal transport of organic substances in plants 

 one naturally is forced to speculate on its probable 

 function. Its distribution and conformation are such 

 that, while it possesses a very small cross-section, it 

 appears with the other living elements of the vascular 

 bundles, medullary rays, wood-parenchyma, etc., to 

 present a maximum surface to the tracheae. 



This large surface may find explanation in the 

 necessity of interchange between the living cells and 

 dead conduits. The colloidal contents of the former 

 render this process slow, hence the necessity for the 

 large surface of interchange to enable sufficient quan- 

 tities of organic substances to be abstracted from and 

 introduced into the tracheae to meet the needs of the 

 plant. 



Before concluding I would like to add that the 

 experimental work carried out on this matter would 

 have been quite impossible for me were it not for the 

 assistance and ingenuity ol Mr. N. G. Ball. He also 

 has contributed materially by his criticisms and 

 suggestions. 



Obituary. 



Colonel E. H. Grove-Hills, C.B.E., C.M.G., F.R.S. 



/^OLONEL EDMOND HERBERT GROVE-HILLS, 

 >— whose death occurred on October 2 at his residence 

 at Campden Hill. \Y., was the son of Herbert A. Hills of 

 High Head Castle, Cumberland. Born on August 1, 

 1864, he was educated at Winchester, whence in 1882 

 he passed into the Royal Military' Academy, Wool- 

 wich. There his abilities were recognised as giving 

 promise of a distinguished career, and he passed out 

 as the senior cadet of his term, receiving a com- 

 mission in the Royal Engineers in 1884. 



NO. 2764, VOL. I 10] 



Scientific subjects specially interested him, and in 

 1893 he was elected a fellow of the Royal Astro- 

 nomical Society; in the following year a paper by 

 him on the photographs of the spectrum of the eclipsed 

 sun taken at the solar eclipse of April 1893 was com- 

 municated to the Royal Society. The study of solar 

 physics strongly attracted him, and he also took part 

 in the eclipse expeditions of 1896 to Japan, of 1898 to 

 India, and in that of 1914 to Kieff, whence he was 

 recalled on the outbreak of war to military service. In 

 1898 he took up the appointment of instructor in 

 chemistry and photography at the School of Military 



