;o8 



NATURE 



[July 29, 1897 



line ; if we differ from him, we are bound to seek for the miss- 

 ing evidence of the action of the living elements. 



Schwendener' s C/7/?V/j-/«. — Perhaps the best plan will be to 

 consider the most serious criticism that has been published of 

 Strasburger's work, namely Schwendener's paper " Zur Kritik," 

 &c. (A'. Preiiss. Akad. 1892, p. 911). 



Schwendener objects that although a continuous column of 

 water cannot be raised by air pressure to a greater height than 

 that of the barometric column, yet when broken into a number 

 of columns, as in the case of a Jamin chain, that a column con- 

 siderably over 10 m., even as much as 13 or 14 m., of water 

 can be suspended. This, though not fatal to Strasburger's con- 

 clusions, is no doubt a serious criticism. For if 13 m. can be 

 supported, some of Strasburger's experiments are inconclusive. 

 He finds that a branch can suck up a poisonous fluid to over 

 10 m.,and, as above explained, argues that all ascent above 

 that height, not being due to barometric pressure or to the 

 living elements (since the wood is poisoned), is for the present 

 inexplicable. But, if Schwendener is right, the effect above 10 

 m. may have been due to atmospheric pressure. Askenasy {loc. 

 cit. infra, 1895, p. 6) objects to Schwendener that the supposed 

 action cannot be continuous. By repeating the diminution of 

 air pressure at the upper end, the movement of water becomes 

 less and less, and sinks to almost nothing. Askena.sy adds, 

 moreover, that the amount of water which could be raised 

 according to Schwendener's theory would be very small. 



One difficulty about Schwendener's theory is that the result 

 depends on the length of the elements of which the chain is made 

 up (such element being a water column, pltis an air bubble.) In 

 his paper " Ueber das Saftsteigen" (K. Preiiss. Akad. 1886, p. 

 561), he finds that the elements of the chain in Fagus equal in 

 round numbers o'5 mm. In his paper {K. Preuss. Akad. Si(z. 

 1893, p. 842), " Wasserbewegung in der Jamin'schen Kette," 

 he finds the element in Acer pseiido-platamis = 0*9 mm., in 

 Acer plaianoides &r\di Ulimis effusa =:0"2. But the calculation 

 (1892, p. 934) is based on the existence of a chain in which the 

 water columns are each 10 mm. in length, a condition of things 

 which he allows does not occur in living trees. 



But even if we allow Schwendener to prove theoretically the 

 possibility of a Jamin chain being raised to a height much 

 greater than that of a barometric column, I do not think he 

 invalidates Strasburger's position. Schwendener's idea necessi- 

 tates the travelling of a Jamin chain as a whole, i.e. the transla- 

 tion not only of water, but of air bubbles. But this cannot (as 

 Strasburger points out) apply to his experiments on conifers, in 

 which the movement of air to such an extent is impossible 

 (" Ueber das Saftsteigen," Hist. Beiirdge, v. 1893, p. 50). 

 And for the case of dicotyledonous woods, Strasburger has 

 shown that the movement of air is excluded by the fact that 

 transverse walls occur in the vessels at comparatively short dis- 

 tances. In Aristolochia the sections maybe as long as 3 m., 

 but in ordinary woods, according to Adler (as quoted by Stras- 

 burger), we get: Alnus, 6 cm. ; Corylus, 11 cm. ; Betula, 12 

 cm. ; Quercus, 57 cm. ; Robinia, 69 cm. These facts seem 

 impossible to reconcile with Schwendener's views. 



Action of the Poisonous Fluids in Strasbttrger's Experi- 

 ments. — The question whether the living elements are killed in 

 Strasburger's experiments is of primary importance in the 

 problem. 



Schwendener does not criticise it at length ; he seems to 

 assume (Zur Kritik, loc. cit., 1892, p. 935) — as far as I can 

 understand — that since the death of the tissues extends gradu- 

 ally from the cut end upwards, there are living cells in the upper 

 part which may still be effective. He also doubts " whether the 

 cells were always killed at once." The first objection of 

 Schwendener's may or may not be sound, but in any case it does 

 not (as Strasburger points out) account for the experiment (jiT/jA 

 Beitr. v. p. 12) in which an oak stem was poisoned by picric 

 acid, and three days afterwards was placed in fuchsin-picric. 

 The second reagent had to travel in tissues already killed with 

 picric acid, yet a height of 22 m. was reached. 



The question whether the reagents kill the cells in Stras- 

 burger's experiments does not lend itself to discussion. It is 

 difficult to see how they should escape, and we have Stras- 

 burger's direct statement that the living tissues were visibly 

 killed. It must not be forgotten that in some of his experi- 

 ments the death of the tissues was produced by prolonged boil- 

 ing, not by poisons {Leitungsbahnen, p. 646). Thus the lower 

 12 m. of a Wistaria stem were killed in this way, yet liquid was 

 sucked up to a height of 108 cm. In \.\it Histolog. Beitr. v. p. 



NO 1448, VOL. 56] 



64, he has repeated his air-pump experiment, using a boiled yew 

 branch, and found that eosin was sucked up from a vessel in 

 which almost complete vacuum was established, so the action of 

 living elements and of atmospheric pressure was excluded. 



On the whole, the balance of evidence is, in my judgment, 

 against the belief that the living elements are necessary for the 

 rise of water. In other words, I think we should be justified, 

 from Strasburger's work, in seeking the cause of ascent in the 

 action of purely physical laws. 



Strasburger'' s general argument from the structure of wood. — 

 It seems sometimes to be forgotten that,, apart from the physio- 

 logical or experimental evidence, there is anolher line of argu- 

 ment founded on the structure of wood. Strasburger's un- 

 rivalled knowledge allows him to use this argument with 

 authority, and he seems to me to use it with effect. Thus (Hist. 

 Beitr. v. p. 17) he points out that though in coniferous wood 

 the action of the living elements in pumping water is conceiv- 

 able, yet this is fa> from being universally the case. He points 

 out that in the monocotyledons such theories meet with 

 almost unconquerable difficulties. This is, he says, especially 

 the case in Dracrena. He goes on to point to difficulties in the 

 case of such dicotyledons as Albizzia. The case may perhaps 

 best be put in the generalised manner that Strasburger himself 

 employs {/oc. cit. p. 20). If the living elements are of such 

 importance as Godlewski, Westermaier, and Schwendener hold, 

 we ought not to find these difficulties ; we ought rather to find 

 structural peculiarities pointing distinctly to the existence of 

 such functions. For instance, we ought to find the tracheal 

 water-path actually interrupted by living elements, which might 

 act like a series of pumping stations one above the other. It 

 should, however, be remembered that if we deny the import- 

 ance of the medullary rays and other living elements in raising 

 water, we ought to be able to point more clearly than we can 

 at present to the function of the medullary rays and to structural 

 adaptations to these functions. 



The work of Dixon and Joly and of Askenasy. — I now pass 

 on to the recent work in which Strasburger's indications to 

 search along a purely physical line have been followed. In the 

 paper of Dixon and Joly (Proc. Koy. Soc, vol. Ivii. 1894, No. 

 340), the suggestion was for the first time made that the 

 raising of water to the tops of trees depends on the quality 

 which water possesses of resisting tensile stress. To most 

 botanists the existence of this quality is a new idea. To believe 

 that columns of water should hang in the tracheals like solid 

 bodies, and should, like them, transmit downwards the pull 

 exerted on them at their upper ends by the transpiring leaves, is 

 to some of us equivalent to believing in ropes of sand. The 

 idea is more fully treated in the Plul. Trans, vol. clxxxvi., and in 

 the Annals of Botany, vol. viii. The same leading idea occurred 

 independently to Askenasy, who has published it in the Verhand. 

 a. d. naturkist. med. Vereins Heidelberg, N. F., Bd. v., 1895 ; 

 and N. F., Bd. v., 1896. 



Askenasy has earned the gratitude of his botanical readers by 

 giving some of the evidence which demonstrates the existence of 

 this property of water. ^ A tube a metre in length was filled by 

 Donny with water, and the remaining space was as far as possible 

 freed from air. When the tube was placed vertically, the water- 

 column at the upper end hung there, and could not be made to 

 break or free itself from the glass by violent shaking. Berthelot 

 filled a thick-wall capillary tube completely with water at 

 28°- 30° C. ; it was allowed to cool to 18°, so that the space left 

 by the shrinking of water was filled with air. It was then sealed 

 up and again warmed to 28°-30°, so that the air was dissolved in 

 the water. When it was allowed to cool again it retained its 

 volume, filling the tube completely. A slight shake, however, 

 allowed the water to break and return to its proper volume at 

 18° with the appearance of a bubble of air. In this experiment 

 the water contained air, yet it seems to have been until recently 

 assumed by some physicists that to show cohesion, water must 

 be air-free. If this were the case, the application of the principle 

 to plants would be impossible. Dixon and Joly have, however, 

 proved that this is not so, and this forms an important part of 

 their contribution to the subject. 



They also '- investigated the amount of tension which water 

 under these circumstances will bear, and found it about equal to 



1 He gives reference to Donny, Poggendorff's Annalen, 67 Bd. (143. Bd. 

 d. g. R.,\ 1S46, p. 562 ; Berthelot, Annates de Chiinie et de Physique, S. 3, 

 t. 30, 1850, p. 232 ; Worthington, Proc. Roy. Soc. vol. 1., 1892, p. 423. 



^ Phil. Trans, vol. 186, p. 570. With ethyl alcohol Worthington records 

 a tension of 17 atmospheres. See Proc. R. Soc, vol. 1. 



