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NATURE 



[October ii, 1900 



LETTERS TO THE EDITOR. 



ilTAe Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.^ 



Ascent of Sap. 



Prof. Vines, in his interesting address to the Botanical 

 'Section of the British Association, has referred to the problem of 

 ■the ascent of sap. 



We believe Prof. Vines is under a mistake when he states : 

 *' Now as to the force by which the transpiration-current is 

 •raised from the roots to the topmost leaf of a lofty tree. From 

 the point of view that the water travels in the substance of the 

 walls the necessary force need not be great, and would be amply 

 (provided by the transpiration of the leaves, inasmuch as the 

 weight of the water raised would be supported by the force of 

 imbibition of the walls. From the point of view that the water 

 travels in the lumina, the force required to raise and support such 

 vlong columns of water must be considerable." If we gather the 

 sense aright, this statement involves perpetual motion, as may 

 be seen by imagining both cell-walls and lumina filled with water. 

 According to Prof. Vines, water may be obtained from the cell- 

 walls in the higher parts of the plant with the exertion of a less 

 force than from the lumina. If now we establish a connection, 

 the lumina will draw from the cell- walls, and with a second 

 •connection below, an endless circulation will arise. The error 

 arises from supposing that water can be withdrawn from the cell- 

 walls and maintained in upward motion without opposition from 

 'the entire gravitational pull. Or, stating the matter in another 

 way, the force which is assumed to uphold the water will also 

 ract to resist its withdrawal from the walls. Indeed the with- 

 drawal of water from the cell-walls must be necessarily attended 

 by much higher frictional resistance than would obtain if the 

 supply were received from the lumina. The same objection 

 may, in our opinion, be urged against any exposition of the 

 •" imbibition " theory. The underlying fallacy is, in fact, essen- 

 tially the same as that on which the theory of capillarity and gas- 

 pressure is founded, and which Prof. Vines rejects as " quite 

 inadequate." 



Prof. Vines, in further discussing the question, speaks of a 

 tensile force of 360 lbs. to the square inch as being required to bring 

 the sap to the summit of a tree 120 feet high, and states that, not 

 ■ only is there no evidence for the existence of such a force, but 

 that it is even negatived by the indications of the experiments 

 of Hales and Boehm. Without discussing the validity of the 

 -supposition that such a force is anywhere required (beyond 

 stating, as our opinion, that the grounds upon which this estimate 

 •has been obtained are very doubtful), we would certainly like to 

 know to which of the experiments of Hales and Boehm Prof. 

 Vines refers. So far as we are aware, the indications of Hales' 

 and Boehm's experiments were of necessity limited by the 

 difficulty of putting the water (external to the branches 

 experimented upon) into a condition capable of bearing tension. 

 These investigators, however, did not clearly understand this 

 point. The experiments made by us in the same direction 

 certainly all failed from this cause. The most, then, that can 

 be admitted is that direct observation has never revealed the 

 full state of tension of the sap of a transpiring tree, although, 

 as in the case of some of Boehm's experiments, indications of 

 the existence of tension were conclusively obtained. This is a 

 very different thing from assuming that negative indications 

 •have been experimentally obtained. It is hard to see why Prof. 

 Vines should consider the existence of a transpiration force of 

 20 atmospheres as improbable. It has, indeed, been shown by 

 experiment that the turgescence of the cells of the leaves of 

 many trees is capable of exerting a tractional force of over 20 

 -atmospheres on the water in the conduits. 



Prof. Vines dismisses the tension theory as offering no solution 

 to the problem. But how does the matter stand ? The more 

 important points may be stated in a very few words. 



In the theory of the tensile sap we find full reason for the 

 subdivided structure of the water-conduits and for the structure 

 of their lignified cell-walls (especially as seen in the ingenious 

 mechanism of the bordered pit) ; stability is conferred on a liquid in 

 tension and liable to the evolution of gas-bubbles by the first, and 

 3. minimum of resistance with safety against rupture is secured 

 to the wall by the second. To raise the water through this system 



NO. 1615, VOL. 62] 



the turgescence of the leaf-cells is fully adequate, even were the 

 tension greater than what Prof. Vines demands. Again, in the 

 light of this theory, the advantage of the periodic recurrence of 

 root pressure becomes apparent, as a safeguard against the multi- 

 plication of functionless lumina destroying the continuity of the 

 system. On the other hand, those who have discussed this 

 theory have as yet brought to light no fact in vegetable physiology 

 or anatomy opposed to its validity ; while many points, e.g. 

 the collapse of protoxylem elements, and the occurrence of year- 

 rings, have received in it an explanation. 



From the physical point of view, the theory is not only 

 adequate to meet all the requirements of the plant, but the 

 existence of tension in a system of minute chambers having 

 walls at once permeable to water and impermeable to free gas, 

 whether altogether or partially filled with dust-free water, is 

 inevitable. The onus of proof does not here lie with the up- 

 holders of the tension theory merely because it has come late 

 upon the scene, but its opponents must show how the tensile 

 state is evaded before they can dismiss the existence of the 

 tensile stress in the sap at such times as root-pressure is not the 

 uplifting force. 



If, then, the sap is in tension from the nature of the conditions 

 and the leaves active in withdrawing water from above, why deny 

 the adequacy of the explanation ? 



With regard to the date of publication of our theory. Prof. 

 Vines is slightly in error. Our paper was communicated to the 

 Royal Society in Nov. 1894, and an abstract of it appeared in 

 Nature in the same month. Henry H. Dixon. 



Trinity College, Dublin. J. Joly. 



Homochronous Heredity and the Acquisition of 

 Language. 



The question raised by Mr. Stuart-Menteath in Nature of 

 September 27 (page 524) is one of such general interest to all 

 students of heredity that it is to be hoped that some authorita- 

 tive expression of opinion will be forthcoming. Even in its 

 present form the query ; involves the subject of the heredity of 

 acquired characters, and it would be of the greatest importance 

 to have experimental systematic observations carried out if such 

 observations have not already been recorded. So far as my very 

 limited acquaintance with the subject extends, I know of no 

 such experiments. It would be desirable perhaps to widen the 

 scope of the question, and to put it in this form : Take children 

 of different nationalities, say German, French and English ; 

 allow them from infancy to hear all three languages indiscrimi- 

 nately. Is there any reason for believing that each child would 

 show a predilection or greater facility for acquiring the language 

 of its country ? R. Meldola. 



October 8. 



Autotomic Curves. 



British mathematicians have usually employed the phrase 

 " non-singular curve " to designate a curve which has no double 

 points. This phrase is an exceedingly infelicitous and mislead- 

 ing one, since a point of inflexion is just as much a singularity ~ 

 as a double point. 



The word autotomic (self-cutting) has occurred to me as a 

 suitable one to designate a curve which has double points ; but 

 the objection to this word is that the phrase " an anautotomic 

 curve " is somewhat offensive to the ear. In the case of media 

 which are not isotropic, mathematicians have evaded a similar 

 difficulty, which would be caused by the use of the word 

 anisotropic, by employing the term aeolotropic. 



Perhaps some of your readers, who have kept up their . 

 classics, may be able to suggest a suitable word to convey the ,• 

 idea of a " not-self-intersecting-curve." A. B. Basset. 



Fledborough Hall, Holyport, October 5. 



THE OPENING OF THE MEDICAL SCHOOLS. 



THE subject-matter of the studies comprising the 

 medical curriculum lends itself exceptionally well 

 to the delivering of inaugural addresses. Kvery October 

 produces its crop of young men and women beginning 

 the study of medicine, and to these are addressed with 

 never-failing regularity an almost constant number of 

 introductory lectures. To those who watch from a dis- 

 tance the perhaps somewhat monotonous rhythm of 



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