July 29, 1897] 



NA TURE 



307 



viscera ; while the widespread contraction of the muscle 

 system has probably also a good effect. How pronounced 

 are the dynamic effects induced by completely abandon- 

 ing oneself to a fit of crying is shown by the exhaustion 

 which it entails. It is partly through this exhaustion that cry- 

 ing induces sleep; we hear of "crying oneself to sleep," 

 though this must be but a very crude explanation of the 

 phenomenon. The tendency of women to cry should, of course, 

 be kept within proper bounds, but certainly harm may result 

 from its complete suppression, as Tennyson recognises in the 

 line — 



" She' must weep or she will die." 

 It is said that women who are able to find relief in tears keep 

 their youth longer than those who repress them. The internal 

 cankering action " like a worm i' the bud " of pent-up emotion 

 is not only a beautiful poetic conceit, but a profound physio- 

 logical truth. In short, strong emotion should receive expres- 

 sion — " give sorrow words." 



Sighing. 

 ■' A sigh is a deep thoracic respiration, with retraction of the 

 abdomen." ^ The retraction of the abdominal muscles leads to 

 a compression of the splanchnic veins. This compression is 

 probably increased by slight descent of the diaphragm. The blood 

 is thus pressed out of those veins into the right heart, and 

 the flow into this chamber is further favoured by the deep 

 inspiration which aLo aids the circulation through the lungs. 

 X more common cause of sighing I believe to be shallow 

 breathing, however induced. Thus sadness and a sense of 

 weariness or boredom are wont to be attended by shallow 

 breathing, and in all of them sighing is frequent. In conse- 

 quence of this shallow breathing, blood-aeration lags behind, 

 and the blood tends to accumulate in the right heart and 

 systematic veins. The sigh benefits by promoting the aeration 

 of the blood and quickening the pulmonary circulation, and it 

 is for similar reasons that sighing is apt to occur during a state 

 ■f "breathless attention" — when the attention, i.e., is so 

 -trained that one forgets, as it were, to breathe adequately. 



There can be little doubt that one of the objects of yawning 

 is the exercise of muscles which have been for a long time 

 quiescent, and the acceleration of the blood and lymph flow 

 which has, in consequence of this quiescence, become sluggish. 

 Hence its frequency after one has remained for some time in the 

 same position — e.g. when waking in the morning. Co-operating 

 with this cause is sleepiness and the shallow breathing which it 

 entails. This factor, as well as muscle-quiescence, is apt to 

 attend the sense of boredom which one experiences in listening 

 to a dull sermon. Hence it is that the bored individual is apt 

 to yawn. As in the case of sighing, the deep breath which 

 accompanies the act of yawning compensates for the shallow 

 breathing, which is so apt to excite it. 



ON THE ASCENT OF WATER IN TREES.'- 

 ■\ VITHIN the last few years the problem of the ascent of 

 water has entered on a new stage of existence. The re- 

 searches which have led to this new development are of such 

 weight and extent that they might alone occupy our time. It 

 , will be necessary therefore to avoid, as far as possible, going 

 into ancient history. But it will conduce to clearness to recall 

 some of the main stepping-stones in the progress of the subject. 

 The two questions to be considered are: (i) What is the 

 path of the ascending water? (2) What are the forces which 

 produce the rise ? 



1(1) The first question has gone through curious vicissitudes. 

 The majority of earlier writers assumed "that the water travelled 

 in the vessels. This was not, however, a uniform view. Cresal- 

 pinus, 1583, .seems (Sachs' "History of Botany," English 

 Trans., p. 451) to have thought that water moved by imbibition 

 in the "nerves." Malpighi and Ray held that the vessels 

 serve for air, and the wood fibres for the ascent of water. 

 Hales ("Vegetable Staticks," p. 130), who believed in the 

 " sap-vessels " as conduits, speculated on the passage upwards 

 of water between the wood and the bark. Also {/oc. cit. p, 19), 



J L. Hill : Journal of Physiology, vol. xv. p. 48. 



- A paper read before Section K of the British Association at the Liver- 

 pool meeting, by Francis Darwin, F.R.S. (Revised January 20, 1897). 



NO. 1448, VOL. 56] 



that water may travel as vapour not in the liquid state. In the 

 present century Treviranus (Sachs' "History"), 1835, held 

 that water travelled in vessels ; De Candolle, 1832, that the 

 intercellular spaces were the conduits. In Balfour's " Manual 

 of Botany," 1863, vessels, cells, and intercellular spaces are 

 spoken of as transmitting the ascending water. 



The change in botanical opinion was introduced by the great 

 authority of Sachs, ^ who took up Unger's view- that the tran- 

 spiration current travels in the thickness of the walls as water 

 of imbibition. 



Then followed the reaction against the imbibitionists— a re- 

 action which has maintained its position up to the present time. 

 Boehm, who had never adopted the imbibition theory, must 

 have the credit of initiating this change ; his style was confused 

 and his argument marred by many faults, but the reaction 

 should in fairness be considered as a conversion to his views, 

 as far as the path of the travelling water is concerned. Never- 

 theless, it was the work of others who principally forced the 

 change on botanists— £.^. von Hohnel {Pringsheim's fahrb. xii., 

 1879), Elfving {Bot. Zeitting, 1882), Russow [Bot. Centr. 

 xiii., 1883), R. Hartig (" Ueber die Vertheilung," &c., Unter- 

 sitchungen aus dem Forst. Bot. Inst, zti Miinchen, ii. and iii. ), 

 Vesque ^Ann. Sc. Nat. xv. p. 5, 1883), Godlewski {Prings- 

 heiitCs Jahrb. xv., 1884), and others. 



(2) The second question has a curious history, and one that 

 is not particularly creditable to botanists generally. It has been 

 characterised by loose reasoning, vagueness as to physical laws, 

 and a general tendency to avoid the problem, and to scramble 

 round it in a mist of vis a tergo, capillarity, Jamiii i/iai/ts, 

 osmosis, and baroinetric pressure. 



An exception to this accusation (to which I personally plead 

 guilty) is to be found in Sachs' imbibition theory, in which, at 

 any rate, the barometric errors were avoided, though it has 

 difficulties of its own, as Elfving has pointed out. 



But the most hopeful change in botanical speculation began 

 with those naturalists who, concluding that no purely physical 

 causes could account for the facts, in%'oked the help- of the 

 living elements in the wood. To Westermaier (Deutsch Bot. 

 Ges. Bd. i., 1883, p. 371) and Godlewski {Pringsheini s [ahrb. 

 XV., 1884) is due the credit of this notable advance, for whether 

 future research uphold or destroy their conclusions, it claims 

 our sympathy as a serious facing of the problem by an ingenious 

 and rational hypothesis. ■* 



We may pass over the cloud which arose to witness for and 

 against these theories, and proceed at once to Strasburger's 

 great work {Leitnngsbahitcu, 1891), in which, with wonderful 

 courage and with the industry of genius, he set himself to work 

 out the problem de novo, both anatomically and physiologically. 

 In my opinion it is difficult to praise too highly this great effort 

 of Strasburger's. 



Strasburger's general conclusion is now well known. He 

 convinced himself that liquid can be raised to heights greater 

 than that of the barometric column in cut stems, in which the 

 living elements have been killed. Therefore, the cause of the 

 rise could not be (i) barometric pres.sure, (2) nor root pres.sure, 

 (3) nor could it be due to the action of the living elements of 

 the wood. His conclusions may be stated as follows : — 



{a) The ascent of water is not dependent on living elements, 

 but is a purely physical phenomenon. 



{b) None of the physical explanations hitherto made are 

 sufficient to account for the facts. 



Strasburger has been most unjustly depreciated, because his 

 book ends in this confession of ignorance. I do not share such 

 a view. I think to establish such distinct, though negative, 

 conclusions would be, in this most nebulous of subjects, an 

 advance of great value. Whether he has established these con- 

 clusions must of course be a matter of opinion. To discuss 

 them both would be to go over 500 pages of Strasburger's book, 

 and will not here be attempted. Conclusion (a) that the ascent 

 is not dependent on living elements must, however briefly, be 

 discussed, because it is here that the roads divide. If we agree 

 with Strasburger, we know that we must seek along the physical 



1 Physiol. Vcgi'tale (French Trans.), 1868, p._23S, and more fully in the 

 Lehrbuch. Sachs also p.irtially entertained Quincke's well-known sugges- 

 tion of movement of a film of water on the surface of vessels. 



^ Sitz. k.k. Akad, Wien, 1868. Dixon's and Joly's paper in the Annals 

 of Botany, September 1895, gives evidence in favour of a certain amount 

 of movement of the imbibed water. 



3 It is of interest to note that Hales, in speaking of the pressure which he 

 found to exist in bleeding trees, says : "This force is not from the root only, 

 but must also proceed from some power in the stem and branches." (" Veg. 

 Staticks," 1727, p. no.) 



