680 REPORT—1896, 
a pace equal to the transpiration current. In Ginkgo the pressure was 
twenty-one times the length of the wood. Strasburger’ has repeated 
Janse’s experiment, and finds a coluinn ‘several times the length of the 
object’ necessary. Nigeli? found that 760 mm. of mercury were needed 
to force water through fresh coniferous wood at the rate of 4 mm. per 
second, 7.¢., at 180 mm. per hour. If we allow one metre per hour as a 
fair transpiration rate,? we get a pressure of 5 atmospheres 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 doubtfully) much greater than those 
obtained by physicists for the resistance of water flowing in glass capil- 
laries. Until this discrepancy is explained, it is rash to argue from our 
present basis of knowledge.® 
Is the osmotic suck sufficient ?—The osmotic force of a turgescent cell 
is usually measured by its power of producing hydrostatic pressure within 
the cell. Thus, De Vries® investigated the force necessary to extend 
a plasmolysed shoot to its original length ; Westermaier’ the weight 
necessary to crush a tissue of given area ; Pfeffer’ the pressure exerted 
by growing roots ; Krabbe ® 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, solution, and calculate the pressure which such a solution 
can produce. According to Pfeffer,!° 1 per cent. KNO, with artificial 
membrane gives a pressure of 176 cm. = 2°3 atmospheres. De Vries!! 
calculates that in a cell a 0:1 equivalent solution (practically=1 per cent.) 
gives a pressure 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. KNO; to plasmolyse it ; this would mean 15—21 atmo- 
spheres. Ido not know what is the greatest pressure which has been 
estimated in this way. Probably Wieler’s '? estimate of the pressure in 
the developing medullary ray cells of Pinus sylvestris at 21 atmospheres 
is the highest. It is clear that investigation of the osmotic capacity of 
' Leitungsbahnen, p. 779. 
® Das Mikroskop, 2nd edit. p. 385. 
% Sachs, Ardeiten, ii. p. 182. 
‘ Schwendener’s experiments, K. Preuss. Ahad., 1886, p. 579, do not particularly 
bear on this question. 
5 Tt is possible that the rate of the ascending water is much less than is usually 
assumed. ‘Thus Schwendener (K. Preuss. Ahad., 1886, p. 584) calculates from an 
observation of v. Héhnel that the transpiration current in the stem of a tall beech 
was only 2 metres per day. 
® Untersuchungen iiber d. mechanischen Ursachen der Zelistrechen, 1877, p. 118. 
7 Deutsch. Bot. Ges. 1883, p. 382. 
8 Abh. kh. Stichs. Ges. 1893. 
® K. Ahad. Berlin (Abhandlungen), 1884, pp. 57, 69. 
10 Pfeffer, Phys., i. p. 53. 
1 Pringsh. Jahrb., xiv. p. 527. 
2 Thid., xviii. p. 82. 
