432 Stiles and j0rgensen. — Studies in Permeability. V. 
a colloid in contact with a crystalloidal solution. The difficulties of this 
theory (see e. g. Bayliss, 1) are so great, and it has explained so little, that 
it has found practically no support. Martin Fischer (7), as a result of 
an extensive series of experiments with gelatine, blood fibrin, frog’s muscle, 
and sheep’s eyes, has also rejected the osmotic theory and attempted to 
explain swelling of animal and plant tissue and related phenomena as due 
to the colloidal nature of the tissue. In view of the fact that the causes of 
the swelling of colloids are not by any means settled, and especially when 
it is considered that the swelling may be an osmotic phenomenon, this 
explanation of the swelling of living tissue is not so far very helpful. 
Probably the most striking result of Fischer is the exact similarity in the 
effect on acids in increasing swelling of gelatine and blood fibrin on the one 
hand, and muscle and sheep’s eyes on the other. This may very well be 
the same phenomenon, and the indication of an increased swelling in dilute 
acid in our experiments may also be due to the colloidal nature of the 
protoplast, but it scarcely justifies Fischer’s conclusion that in all cases the 
swelling in water is due to the production of acid as a result of the 
conditions of experiment. In our experiments a very low hydrogen ion con- 
centration is sufficient to kill the cell, which would appear to dispose com- 
pletely of the view that the total swelling of plant tissue in water is due to 
the formation of acid in the protoplast. 
In such a complex organization as the cell it is unlikely that any one 
single explanation will explain all the phenomena of permeability. We 
know that there are at least three structures of which account must be 
taken : the cell wall, the protoplast, and the vacuole. We also know that at 
the junction of two immiscible phases the properties of the material may be 
different from those in the body of either phase, and indeed we find that de 
Vries postulated two limiting membranes to the protoplast, an outer one 
surrounding the protoplast (the plasma membrane or ectoplast), and an 
inner one surrounding the vacuole (the vacuole wall or tonoplast). 1 It 
would for instance* be- reasonable to suppose that the protoplast behaved 
towards exter^l^^^^^in much the same way as gelatine or blood 
fibrin, while at the^^Swfe time it formed a semi-permeable membrane 
between the vacuole a^hd external solution. 2 Such a theory might be 
elaborated to explain the permeability of the cell, but we feel, as we have 
already emphasized in previous papers in this series, that we know so little 
1 The arguments for the existence of plasmatic membranes with diosmotic properties dis- 
tinct from those of the body of the protoplast have been stated by Pfeffer (13). They cannot 
be regarded as very convincing. 
2 ‘ The general principles previously laid down as governing the exchanges of substance 
taking place between the protoplast and the external world would still hold good, even though 
no differences existed between the diosmotic properties of the general mass of the protoplast and 
the limiting membranes which enclose it ; if, for example, the entire thickness of the primordial utricle 
behaved like a single very thick plasmatic membrane’ (Pfeffer, 14, p. 107 ). 
