202 
Walter Stiles 
somewhat permeable to solutes on the side towards the pore or 
xylem vessel. Using the same nomenclature as before and assuming 
the suction pressure is uniform over the whole surface of a cell, if 
S x > S 2 water will enter the exuding cells and will only cease to do 
so when S t = S 2 . But in the meantime solutes are diffusing out from 
the exuding cells into the pore and the liquid in the pore will thus 
develop a suction pressure of S 0 equal to the osmotic pressure P 0 , 
since the turgor pressure is zero. This suction pressure will continue 
to increase until no further exosmosis of osmotically active solutes 
takes place from the exuding cells, while at the same time the suction 
pressure of the exuding cells is decreasing. A time will therefore 
arrive when S 0 (= P 0 ) increases to a value greater than S x (= P x — Tj), 
at which point exudation of water will take place. If this is the 
mechanism of water exudation the conditions for this to take place 
are thus P 0 > (P 1 - T 1 )> (P 2 - T 2 ). 
One explanation offered by Pfeffer was to suppose that the 
exuding cell possessed a higher osmotic concentration in that part 
of it bordering on the neighbouring cells than in the part adjacent 
to the water pore (or water conducting element). The suction pressure 
will thus be different in different parts of the cell. We can then under¬ 
stand that water might be absorbed by the exuding cell both from the 
neighbouring cell and the pore. This will result in increased turgor of 
the exuding cell and consequently the wall pressure will soon balance 
the osmotic pressure on the side of the pore where the osmotic concen¬ 
tration is low, and absorption of water from the pore will stop. Owing 
to the higher suction pressure on the side of the neighbouring cell, 
however, water will continue to enter the exuding cell on that side, 
the turgor pressure will be further increased and in consequence 
water will be forced out from the exuding cells into the pore or xylem 
vessel. As V. H. Blackman (1921) rightly points out, if this process 
is to continue, there must be some mechanism in the cell to prevent 
mixing of the solutes in the exuding cells and to maintain the differ¬ 
ence in concentration in different parts of the cell. 
Ursprung and Blum (1921), as already mentioned, consider that 
a non-homogeneous suction pressure is exerted over the surface of 
endodermal cells in the absorbing zone of the root, and they suppose 
that the same may be the case with the parenchymatous cells border¬ 
ing on the water-conducting elements. They relate the difference in 
suction pressure in different parts of the same cell rather to differ¬ 
ences in the quantity of water imbibed in the cell wall than to differ¬ 
ences in osmotic concentration in the cell as suggested by Pfeffer. 
