674 REPORT—1896. 
On the Ascent of Water in Trees. By Francis Darwin, F.R.S. 
[Ordered by the General Committee to be printed in extenso.] 
Wiruin the last few years the problem of the ascent of water has entered 
on a new stage of existence. The researches 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—(1) What is the path of the 
ascending water? (2) What are the forces which produce the rise ? 
(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. Czsalpinus, 1583, seems! 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,” who believed in the ‘sap-vessels’ as conduits, speculated 
on the passage upwards of water between the wood and the bark. Also,* 
that water may travel as vapour not in the liquid state. In the present 
century Treviranus,‘ 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 transpiration current travels 
in the thickness of the walls as water of imbibition. 
Then followed the reaction against the imbibitionists—a reaction 
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. 
Nevertheless, it was the work of others who principally forced the change 
on botanists—e.g., von Hohnel,’ Elfving,® Russow,® R. Hartig,!® Vesque,!! 
Godlewski,!? and others. 
(2) The second question has a curious history, and one that is not 
particularly creditable to botanists generally. Jt has been characterised 
' Sachs, Hist. of Bot. (English Trans.), p. 451. 
* Vegetable Staticks, p. 130. 
3 Loe. cit. p. 19. 
4 Sachs, History. 
5 Physiol. Végétale (French Trans.), 1868, p. 235, and more fully in the Lehrbuch. 
Sachs also partially entertained Quincke’s well-known suggestion of movement of a 
film of water on the surface of vessels. 
* Sitz. kk. Ahad. Wien, 1868. Dixon 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. 
7 Pringsheims Jahrb. xii. 1879. 
8 Bot. Zeitung, 1882. 
® Bot. Centr. xiii. 1883. 
‘0 «Ueber die Vertheilung,’ &c., Untersuchungen aus dem Forst. Bot. Inst. zu 
Miinchen, ii. and iii. 
n Ann. Se. Nat. xv. 1883, p. 5. 2 Pringsheims Jahrb. xv. 1884. 
