V. H. Blackman 
i 14 
substance could diffuse out and a solution under pressure would 
be forced into any vessel attached at B. 
Hitherto normal osmosis has been considered but the question 
naturally arises as to whether these processes of exudation can be 
explained by the special properties of the membrane. It is known 
that some membranes will bring about a ‘ c negative osmosis” in which 
fluid will pass from a strong solution into a weak one just as it 
does in the case of Piloholus. There is, for example, the work of 
Bart ell (1914) who found that porcelain membranes with pores 
of large size showed an increase of pressure on the side of the weak 
solution, while similar membranes with finer pores showed a normal 
pressure. Bartell and Madison (1920) have examined the effect of gold¬ 
beaters’ skin used as an osmotic membrane. They found that with 
various solutions the normal osmotic tendency might be increased, 
decreased or reversed. The results can be explained by the electrical 
relations of the membrane; a difference of potential between the 
two faces of the membrane is developed if electrolytes are used and 
this electro-endosmosis may aid or retard the normal process of 
osmosis; we thus have an additional force superimposed on the 
ordinary osmotic relations as indicated earlier. For the production 
of negative osmosis energy must be available so that, as pointed 
out by Freundlich (1916), the phenomenon can occur if the membrane 
is permeable to the electrolyte as a whole, or if it is permeable to 
one ion only of the electrolyte, and the electrolytes on the two sides 
of the membrane are different and react with one another. The gold¬ 
beaters’ skin membranes are leaky membranes permeable to the 
electrolytes as a whole so they fulfil the first condition. 
The membranes of the living cell are generally more or less perme¬ 
able and they are bathed with electrolytes. Differences of electrical 
potential can be detected in living tissue, so that the negative 
osmosis to be observed, through such a membrane as gold-beaters’ 
skin, may be similar in origin to the negative osmosis which is 
characteristic of the exudation exhibited by the sporangiophore of 
Piloholus and by hydathodes of the type of those on the leaf of 
Phaseolus. It is, however, doubtful if exudation under the high 
pressures sometimes associated with root-pressure can be explained 
in this way. 
An attempt has been made to show that the claim of Lepeschkin 
that the osmotic pressure of the stronger cell contents is responsible 
for the exudation from the cell of a weaker solution cannot be 
substantiated. A number of mechanisms can, however, be suggested 
