Permeability 199 
the tissues when the external pressure is no longer acting, it is clear 
that the suction pressure so found gives no measure of the suction 
pressure of the cell in the uninjured organ. It is therefore necessary 
to confine the determinations to cells which are under no appreciable 
external pressure in the intact plant, or else to take account of such 
pressure. 
The Magnitude and Range of Suction Pressures 
in Plants 
Ursprung and Blum (1916 £) t have investigated the range of suc¬ 
tion pressures in roots and shade leaves of Fagus sylvatica. Their 
chief results are as follows. 
In all the leaves examined Ursprung and Blum find differences 
in the full suction pressures of the cells of different tissues, the order 
being under epidermis, upper epidermis, guard cells, spongy tissue, 
palisade. In all these tissues the suction pressure was found to in¬ 
crease with increasing distance from the ground. The values found 
for root apices 40 cms. below the level of the ground were the lowest 
of all, but root apices 10 cms. below ground gave abnormally high 
values. Apart from these last, which are completely unexplained, 
the mean values given by Ursprung and Blum’s determinations are 
summarised in Table XXIV. 
Table XXIV 
Full Suction Pressures in Cells of Fagus sylvatica 
(Data from Ursprung and Blum) 
Suction pressures in atmospheres 
Distance 
r 
-A 
A 
from root 
apex in 
metres 
Root 
apex 
Upper 
epidermis 
Under 
epidermis 
Guard 
cells 
Spongy 
paren¬ 
chyma Palisade 
o-o 
5*3 
— 
— 
— 
— 
— 
2-7 
— 
5*9 
7*5 
8-i 
ii-i 
15-0 
8-7 
— 
8-4 
9‘3 
9-9 
12-4 
15*6 
ii-i 
— 
9*3 
9-9 
9-9 
14*3 
17-1 
13-0 
9-9 
io-5 
10-5 
14-3 
17*1 
There 
appears 
to be here definite evidence of 
a gradient 
in the 
suction pressure of cells of the beech in proceeding from lower to 
higher levels. This is obviously of the greatest importance with 
regard to the questions of transpiration and the ascent of water 
in trees, questions which, however, cannot be discussed in this 
place. 
