OCTOBKK 26, 1905] 



NATURE 



645 



necessary to remark that the problem of the rise of sap 

 is one of mechanics, in so far as concerns the mode of 

 the flow and the propelling power. 



Contrary to the view above referred to, it seems not 

 unreasonable to consider that the weight of the sap in 

 each vessel is sustained in the main by the walls and base 

 of that vessel, instead of being transmitted through its 

 osmotically porous base to the vessels beneath it, and thus 

 accumulated as hydrostatic pressure. 



We could in fact imagine, diagrammatically (as happens 

 in ordinary osmotic arrangements), a vertical column of 

 vessels, each provided, say, with a short vertical side-tube 

 communicating with the open air, in which the pressure 

 is adjusted from moment to moment, and yet such that 

 the sap slowly travels by transpiration from each vessel 

 to the one ne.\t above, through the porous partitions 

 between them, provided there is an upward osmotic 

 gradient, i.e. if the dissolved substances are maintained 

 in greater concentration in the higher vessels.' This 

 difference of density must be great enough, between 

 adjacent vessels, to introduce osmotic pressure in excess 

 of that required to balance the head of fluid in the length 

 of the upper one, into which the water has to force its 

 way. Thus, in comparing vessels at different levels, the 

 sap must be more concentrated in the upper ones by 

 amounts corresponding to osmotic pressure more than 

 counteracting the total head due to difference of levels, 

 in order that it may be able to rise. As osmotic pressure 

 is comparable with gaseous pressure for the same density 

 of the molecules of the dissolved substance, the concen- 

 tration required on this view is considerable, though not 

 very great. 



.Such a steady gradient of concentration could apparently, 

 on the whole, become self-adjusting, through assistance 

 from the vital stimuli of the plant, for concentration in 

 the upper vessels is promoted by evaporation. Yet 

 pressures in excess or defect of the normal atmospheric 

 amount might at times accumulate locally, the latter 

 giving rise to the bubbles observed in the vessels, through 

 release of dissolved gases. 



It may be that this assumes too much concentration of 

 dissolved material in the sap, as it exists inside the vessels 

 of the stem, to agree with fact. In that case the capillary 

 suction exerted from the nearest leaf-surface might be 

 brought into requisition, after the manner of Dixon and 

 Joly, to assist in drawing off the excess of water from 

 the vessels. The aim proposed in this note is not to 

 explain how things happen, which is a matter for observ- 

 ation and experiment, but merely to support the position 

 that nothing abnormal from the passive mechanical point 

 of view need be involved in this or other vital phenomena. 



As regards estimating the amount of flow, at first sight 

 it may not appear obvious, a priori, that the transpiration 

 through a porous partition or membrane, due to osmotic 

 gradient, is equal or even comparable in amount to what 

 would be produced, with pure water, by a hydrostatic 

 pressure-head equal to the difference of the osmotic 

 pressures on the two faces of the partition. But more 

 e.xact consideration shows that, on the contrary, osmotic 

 pressure is defined by this very equality ; " it is that 

 pressure-difference which would produce such an opposite 

 percolation of water as would just balance the direct 

 percolation due to the osmotic attraction of the salt- 

 solution. 



^ Thus, in an ordinary osmotic experiment with a U-tube, the percolation 

 of water through the plug gradually firoduccs a difference of hydrostatic 

 pressure on its two faces, which is sustained by the fixity of the plug itself, 

 but would he at once neutralised if the plug were free to slide in the tube. 

 This increase of volume of the salt-solution, by the percolation of pure water 

 into it, is on the van 't HotT analogy correlated with the free expansion of 

 the molecules constituting a gas. It goes on with diminished speed under 

 opposing pressure, until a definite neutralising pressure is reached, inaptly 

 called the osmotic pressure of the molecules of the solute, which just stops 

 it, while hieher pressures would reverse it. The stoppage is due to the 

 establishment of a balance between the amounts of water percolating one 

 way under osmotic attraction, and the opposite way under hydrostatic 

 pressure. The pressure established, e.g. in an organic cell immersed in 

 salt-solution, is thus really the reaction which is set up against the osmotic 

 process. That process itself is perhaps more directly and intelligibly 

 described as the play of osmotic affinity ur attraction, even though it mu« 

 be counted as of the same nature as the affinity of a gas for a vacuum. C/. 

 Proc. Camii. Phil. Sac, J^nad^ry, 1S97, or Whetham's "Theory of Solu- 

 tion," p, 109, 



2 .See preceding footnote. 



NO. 1878, VOL. 72] 



It would, however, appear that the great resistance 

 to flow offered by what botanists call Jamin-tubes, viz. 

 thin liquid columns containing and carrying along numerous 

 broad air-bubbles, is conditioned inainly by the viscosity 

 of the fluid, and involves only indirectly the surface-tension 

 of the bubbles. In fact, the resistance to flow may be 

 expected to remain much the same if each bubble were 

 replaced by a flat solid disc, nearly but not quite fitting 

 the tube. Its high value arises from the circumstance 

 that the mass of liquid between two discs moves on nearly 

 as a solid block when the flow is steady, so that the 

 viscous sliding has to take place in a thin layer close to 

 the wall of the tube, and is on that account the more 

 intense, and the friction against the tube the greater. 

 The increased curvature of the upper capillary meniscus 

 of the bubble is thus merely a gauge of the greater 

 intensity of the viscous resistance instead of its cause, and 

 modification of the surface-tension cannot be involved as 

 a propelling power. The experimental numbers given by 

 Dr. Ewart show that, even where the vessels are largely 

 occupied by bubbles, the greater part of the resistance to 

 active transpiration still resides in the partitions between 

 them. 



If the osmotic gradient, assisted possibly by capillary 

 pull at the leaf-orifices, is insufficient to direct a current 

 of transpiration upward, capillary alterations inside the 

 vessels, arising from vitally controlled emission and absorp- 

 tion of material from the walls, cannot be invoked to 

 assist : rather it must be osmotic alterations from one 

 vessel to the next, of, so to speak, a peristaltic character, 

 that might thus come into play. But any such alteration 

 (of either kind) will involve local supply of energy. Is 

 there a sufficient fund of energy, latent in the stem, to 

 provide permanently the motive power for the elevation 

 of the sap? In what form could this energy get trans- 

 ported there? The energies of the plant-economy come 

 from the sunlight absorbed by the leaves. The natural 

 view would appear to be that the work required to lift 

 the sap is exerted at the place where the energy is re- 

 ceived, and that it operates through extrusion of water 

 by evaporative processes working against the osmotic 

 attraction of the dissolved salts; while the maintenance 

 of equilibrium along the vessels of the balanced osmotic 

 column, with its semi-permeable partitions, demands that 

 an equal amount of water must rise spontaneously to take 

 the place of what is thus removed. 



The subject might, perhaps, be further elucidated by 

 observation of the manner in which the flow is first 

 established at the beginning of the season, or possibly by 

 experiments on the rate at which water would be absorbed 

 by a wounded stem high above the ground. 



EXPERIMENTS WITH THE L.iNGLEY 

 AERODROME.' 

 T^HE experiments undertaken by the Smithsonian 

 Institution upon an aerodrome, or flying machine, 

 capable of carrying a man have been suspended from 

 lack of funds to repair defects in the launching apparatus 

 without the machine ever having been in the air at all. 

 As these experiments have been popularly, and of late 

 repeatedly, represented as having failed on the contrary, 

 because the aerodrome could not sustain itself in the air, I 

 have decided to give this brief though late account, which 

 may be accepted as the first authoritative statement of 

 them. 



It will be remembered that in 1896 wholly successful 

 flights of between one-half and one mile by large steam- 

 driven models, unsupported except by the mechanical 

 effects of steam engines, had been made by me. In all 

 these the machine was first launched into the air from 

 "ways," somewhat as a ship is launched into the water, 

 the machine resting on a car that ran forward on these 

 ways, which fell down at the extremity of the car's 

 motion, releasing the aerodrome for its free flight. 



In the early part of 1898 the Board of Ordnance and 



Fortification of the War Department allotted 50,000 dollars 



for the development, construction, and test of a large aero- 



1 Abridged from a paper by Dr. S. P. Langley in the Smithsonian Repoit 



for 1904. 



