the Ascent of Water in Trees. 639 
Until this discrepancy is explained, it is rash to argue from our present 
basis of knowledge l . 
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 2 investigated the force necessary to 
extend a plasmolyzed shoot to its original length; Westermaier 3 the 
weight necessary to crush a tissue of given area ; Pfeffer 4 the pressure 
exerted by growing roots ; Krabbe 5 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 KN 0 3 solution, and calculate the pressure which such 
a solution can produce. According to Pfeifer 6 , 1 per cent. KN 0 3 
with artificial membrane gives a pressure of 176 c.m. = 2*3 atmo- 
spheres. De Vries 7 calculates that in a cell, a o-i 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. KN 0 3 to plasmolyze it; 
this would mean 15-21 atmospheres. I do not know what is the 
greatest pressure which has been estimated in this way. Probably 
Wider’ s 8 estimate of the pressure in the developing medullary ray 
cells of Pinus sylvestris at 2 1 atmospheres is the highest. It is clear 
that investigation of the osmotic capacity of leaves for high trees is 
wanted, also investigations of the variation in osmotic power produced 
by varying resistances in the flow of the current. The experiments 
of Pfeffer and others 9 show that the osmotic strength of cell-sap 
is capable of great adaptation to circumstances — cells respond by 
1 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) calculates from an 
observation of v. Hohnel that the transpiration-current in the stem of a tall beech 
was only two metres per day. 
2 Untersuchungen liber d. mechanischen Ursachen der Zellstreckung, 1877, p. 118. 
3 Deutsch. Bot. Ges. 1883, p. 382. 4 Abh. k. Sachs. Ges. 1893. 
5 K. Akad. Berlin (Abhandlungen), pp. 57, 69, 1884. 
6 Pfeffer, Phys. i. p. 53. 7 Pringsh. Jahrb. xiv. p. 527. 
8 Pringsh. Jahrb. xviii. p. 82. 
9 Pfeffer, Abhand. der k. Sachs. Ges. xx. p. 300 ; Eschenhagen, Untersuchungen 
aus d. Bot. Inst. z. Tubingen, 1889 ; Stange, Bot. Zeit. 1892. 
