Turgescence and the Absorption of Water. 113 
walls, we have 
I = p [=P—T] = C. 
The colloidal protoplasm and its various constituents can be 
regarded not merely as constituting a semipermeable membrane, 
but also as a separate phase (or phases) capable of developing an 
imbibition force, or as another osmotic phase separated from the cell 
sap by the bounding layer of the vacuole, without invalidating the 
general statement symbolised in the equation. 
The same relation also applies to the equilibrium of living cells 
at all points of the transpiration stream with the water under tension 
in the vessels. It is not a necessary consequence of the Cohesion 
Theory that the sap of cells at the top of a tree should be osmotically 
stronger than the sap of cells lower down. On the other hand the 
water absorbing power must be greater at higher levels as it has to 
balance a greater tension, due to the greater weight of the longer 
water column and the added resistance to passage through a greater 
length of the water channel. 
A corresponding relation governs the dynamic equilibrium in a 
shoot transpiring at a steady rate. At any point in the water 
stream the rate of flow is directly proportional to the gradient of 
water-absorbing power and inversely proportional to the resistance 
per unit length, so that for a steady flow we have 
= constant 
-a 
where p! and p 2 are the values of the water absorbing power (or, 
alternatively, the imbibition force or the cohesion tension) at points 
1 and 2 respectively, and R T - 2 is the resistance to flow between 
these two points. 
The writer hopes to deal in subsequent papers with other 
aspects of the problem of water absorbing power. The structure 
of the transpiring tissues of xerophilous plants, for instance, requires 
reconsideration from the mechanical point of view. It is conceivable 
that in times of drought the water content of the cells may diminish 
past the point at which the cell walls are completely relaxed. The 
walls will then be exposed to a tension tending to collapse 
them and any mechanical resistance they may offer would 
increase the water absorbing power of the cells over and 
above the osmotic pressure of their sap. The prevailing cylindrical 
form of the cells in such tissues, the peculiar ingrowths which 
characterise the assimilating cells of pines, and the distribution of 
sclereids in certain leaves are suggestive in this connexion. 
Botanical Department, 
University of Manchester. 
February, 1918. 
