July 29, 1897] 



NATURE 



309 



even atmospheres. If, therefore, the leaves at the top of a tall 

 iree can exert the requisite upward pull on the water in the 

 trunk, it seems certain (if no other condition in the problem 

 interfere) that the pull can be transmitted to the level of the 



I'lund. This opens up the question whether the leaves can 



ort this traction on the water in the tracheals, and what is 



iUally important, Are there any factors in the problem incom- 

 patible with the theory? 



(l) The sucking force of the leaves. — In Dixon and Joly's first 

 jiaper (/V/z7. Tians. pp. 563, 567) they assume that tractional 



ice is given by the meniscuses " formed in the membranous 

 can of the evaporating cell-walls," as well as possibly by the 



inotic action of the cells of the mesophyll. We shall take 

 iicse theories in order. Our knowledge of the cell wall does 

 ;i. it allow us to believe in the existence of pores visible with even 

 iie highest powers of the microscope. Dixon's more general 



pression {Proc. Roy. Irish Acad. Jan. 13, 1896, p. 767) "sur- 

 c tension forces developed in the substance of the walls of the 



iporating cells," is therefore preferable. But Askenasy seems 



me to state the matter much more conveniently by using the 



:m '•imbibition" [loc. cit. 1895, p. 10). The force with which 

 vegetable membranes, e.g. the thallus of Laminaria, absorb 

 water, has been demonstrated by Reinke and others, and the 

 existence of such a force is familiar to botanists. 



Both Askenasy {loc. cit.) and Dixon and Joly {Annals of Bot., 

 September 1895) have pointed out that the force of imbibition, or 

 the surface tension forces, as the case may be, can exert a trac- 

 tional effect on the water in the tracheals, when the turgescence of 

 the mesophyll has been destroyed. But Askenasy in his original 

 paper (1895), Dixon in the January 1896 paper, and again 

 Askenasy in his second paper (March 1896) have also considered 

 the imbibitional or surface tension forces in connection with the 

 turgescent cell. It must clearly be understood that this does not 

 remove imbibition from the problem. The sun's heat causes the 

 evaporation of the water with which the walls of the mesophyll 

 cells are imbibed ; this water is replaced by imbibition from the 

 cell-sap. The concentration of the cell-sap so produced main- 

 tains the osmotic force of the cell, which again exerts suction on 

 the water on the tracheals.^ 



I have now given, in its simplest form, the modern theory of 

 the rise of water. Apart from the main idea, it combines the 

 points of several familiar views. Imbibition becomes a factor of 

 paramount importance, though not in the way that Sachs employs 

 it. The suspended threads of water remind us of Elfving's 

 capillary theory, while the living element factor is represented 

 by the turgescent mesophyll cells. 



Resistance. — It is not possible to discuss the question whether 

 the tractional forces in the leaf are sufficient for the work imposed 

 on them until we know what is the resistance to the passage of 

 water through wood. For it is clear that the work done by the 

 leaf includes not only the lifting of a given column, but the over- 

 coming of the resistance to its flow. 



The resistance to the flow of the transpiration current is in 

 want of further investigation. Janse {PringsheinC s Jahrb. xviii., 

 1887, p. i) has discussed the question, and points out {loc. cit. 

 p. 36) that two kinds of resistance must be reckoned with. The 

 first (which he calls statical) is illustrated by means of a cylinder 

 of Pinus wood fixed to tlie short arm of a J tube filled with 

 water, when it was found that in five days the level of water in 

 the long arm was only one mm. above that in the short arm. J 

 That is to say, when time enough is given, the resistance is 

 practically nothing. Janse has also investigated the resistance 

 to the passage of water flowing through wood at the rate of an 

 ordinary transpiration current. His method seems to me open 

 to criticism, but this is not the place to give my reasons. His 

 experiments give a wide range of results. With Pinus strobus 

 a pressure of water equal to ten times the length of the wood 

 was required to force water through at a pace equal to the 

 transpiration current. In Ginkgo the pressure was twenty-one 

 times the length of the wood. Strasburger {Leittingsbahnen, 

 p. 779) has repeated Janse's experiment, and finds a column 

 "several times the length of the object" necessary. Nageli 

 ("Das Mikroskop," 2nd edit. p. 385) found that 760 mm. of 

 mercury were needed to force water through fresh coniferous 

 wood at the rate of ^'y mm. per second, i.e. at 180 mm. per hour. 

 If we allow one metre per . hour as a fair transpiration rate 

 (Sachs' "Arbeiten," ii. p, 182), we get a pressure of 5 atmo- 



1 Sachs' " Text Book," edit, iv., Eng. Tr., p. 679, describes evaporation 

 taking place in the cell wall, which makes good the lo.ss bjr imbibition. 



2 Strasburger {Leitungsbaknen, p. 777) observed equilibrium established a 

 good deal quicker. 



spheres required to produce such a flow. To return to Janse's 

 experiments : even if we assume that the resistance (expressed 

 in water) = 5 times the length, it is clear that with a tree 40 m. 

 in height, the resistance of 20 atmospheres has to be overcome. 

 This would not be a pressure greater than that which osmotic 

 forces are able to exert, but when we come to a tree of 80 m. 

 in height, and a resistance of 40 atmospheres, the thing becomes 

 serious.^ A great difficulty in the question of resistance is that 

 the results hitherto obtained are (though here I speak doubt- 

 fully) much greater than those obtained by physicists for the 

 resistance of water flowing in glass capillaries. Until this dis- 

 crepancy is explained, it is rash to argue from our present basis 

 of knowledge."'' 



Is the osmotic suck sufficient ? — The osmotic force of a tur- 

 gescent cell is usually measured by its power of producing hydro- 

 static pressure within the cell. Thus, De Vries (" Untersuch- 

 ungen liber d. mechanischen Ursachen der Zellstrecken," 1877, 

 p. 118) investigated the force necessary to extend a plasmo- 

 lysed shoot to its original length; \^e?,\.Qtma.\ex {Deutsch. Bot. 

 Ges. 1883, p. 382) the weight necessary to crush a tissue of given 

 area ; Pfefifer {Abk. k. Sachs. Ges. 1893) the pressure exerted by 

 growing roots ; Krabbe {K. Akad. Berlin {Abhandlungen), pp. 

 57, 69, 1884) the pressure under which cambium is capable of 

 maintaining its growth. 



The figures obtained by these naturalists have a wide range ; 

 it may be said that the hydrostatic pressure varies between 3 

 and 20 atmospheres. 



Another method is to ascertain the osmotic strength of the 

 cell-sap in terms of a KNO;j solution, and calculate the pressure 

 which such a solution can produce. According to Pfefifer 

 (Pfeffer, Phys. i. p. 53), i per cent. KNO3 with artificial mem- 

 brane gives a pressure of 176 cm. =2*3 atmospheres. De 

 Vries {Pringsh. Jahrb. xiv. p. 527) calculates that in a cell, 

 ao"i equivalent solution (practically =1 per cent. ) gives a pres- 

 sure of 3 atmospheres. We may therefore take it as between 

 2 '5 and 3 atmospheres. Now, De Vries found that beetroot 

 requires 6-7 per cent. KNO3 to plasmolyse it ; this would mean 

 15-21 atmospheres. I do not know what is the greatest pres- 

 sure which has been estimated in this way. Probably Wieler's 

 {Pringsh. Jahrb. xviii. p. 82) estimate of the pressure in the. 

 developing medullary ray cells of Pinus sylvestris at 21 atmo- 

 spheres is the highest. It is clear that investigation of the 

 osmotic capacity of leaves for high trees is wanted, also inves- 

 tigations of the variation in osmotic power produced by varying 

 resistances in the flow of the current. The experiments of 

 Pfeffer and others ^ show that the osmotic strength of cell-sap 

 is capable of great adaptation to circumstances — cells respond 

 by increased turgescence to various stimuli. Whether they can 

 respond sufficiently to account for the ascent of water is another 

 question. 



My own opinion is that the question of resistance to the flow 

 of water is a difficulty which the authors of the modern theory 

 have not sufficiently met. Unless it can be shown that the 

 resistance to the flow of water in wood is less than that indi- 

 cated by existing researches, we must face the fact that we do 

 not at present know of osmotic forces which we can suppose 

 capable of raising water to a greater height than 40 metres. 



Continuity of the water in the tracheals. — The theory we are 

 considering apparently requires that there shall be continuous 

 columns of water from leaf to root, because a break in the 

 column means a collapse of the machinery. This seems at first 

 sight a fair assumption, though I doubt its complete correct- 

 ness. It is in any case worthy of discussion. It has been con- 

 stantly insisted on by Sachs and others that at the time of most 

 active transpiration the vessels contain air, and not water. It 

 is therefore a violent disturbance of our current views to believe 

 in continuous columns of water. 



For evidence on this point we are chiefly indebted to Stras- 

 burger. It is a remarkable fact that he should, without any 

 theory to encourage such a view, have come to the conclusion 

 that approximate continuity of water columns is a condition of 

 primary importance, and that he should have made out the cog- 

 nate fact that the whole of the alburnum need not be simul- 



1 Schwendener's experiments, K. Preuss Akad. 1886, p. 579, do not 

 particularly bear on this question. 



* It is possible that the rate of the ascending water is much less than is 

 usually assumed. Thus Schwendener (K. Preuss. Akad. 1886, p. 584) cal- 

 culates from an observation of v. Hahnel that the transpiration current in the 

 stem of a tall beech was only 2 metres per day. 



3 Pfeffer, " .\bhand. der k. Sachs." Ges. xx. p. 300 ; Eschenhagen, l/nter- 

 SHchungen aus d. Bot. Inst. z. Tiibingtn, 1889 ; Stange, Bot. Zeit., 189J. 



NO. 1448, VOL. 56] 



