Permeability 
201 
Exudation and Root Pressure 
The exudation of liquid water or a solution from superficial cells 
of a plant on to the outside surface of the cells takes place in certain 
cases, as in water pores (water stomata) and nectaries. What appears 
to be an essentially similar thing is met with in the bleeding of cut 
plants and in the phenomenon of root pressure, except that in the 
latter exudation takes place into dead xylem elements instead of on 
to the surface of the plant. 
Several explanations of this exudation of liquid from plant cells 
have been offered by Pfeffer and subsequent workers, but what is 
the true explanation must be regarded as still remaining in doubt. 
The cells immediately concerned are the exuding cells themselves 
with an osmotic concentration C x corresponding to an osmotic pres¬ 
sure P lt and the cells internal to, and bordering on, these with an 
osmotic concentration C 2 corresponding to an osmotic pressure P 2 . 
The suction pressures of the exuding and neighbouring cells respec¬ 
tively are S x and S 2 . Then if S x > S 2 water is absorbed from the 
neighbouring cells. This will proceed until the increased turgor of 
the excreting cells reduces S x until water is no longer absorbed. But 
if the outer wall of a superficial cell is permeable to water, water will 
continually pass through this wall on account of evaporation if for 
no other reason, so that 5 X will be continually increased and there 
will be in consequence a continual passage of water into the excreting 
cells from the neighbouring ones. The difficulty is not to account for 
the movement of water, but for a movement of water sufficiently 
rapid to bring about an exudation of liquid. 
One theory of the mechanism of the exudation of water supposes 
the protoplasm is more permeable to dissolved substances on the side 
towards the pore (or xylem vessel) than on the side bordering on 
neighbouring cells. This was supposed to be the case by Lepeschkin 
(1906) in the exudation of water by the hydathodes of Phaseolus 
multiflorus, and has been applied by Priestley (1920) to explain the 
existence of root pressure. It is not proposed to deal here in detail 
with Lepeschkin’s theory of water excretion, for it involves fallacies 
which have already been noted in these pages and in other writings; 
it may, however, be worth while to consider briefly the theory of 
differential permeability of the different parts of the protoplasm (or 
plasma-membrane) of exuding cells. Let us suppose we have the 
extreme case in which the protoplasmic membrane is completely 
impermeable to solutes on the side of the neighbouring cells, but is 
Phyt. XXI. IV. 
14 
