94 
W. J. V. OSTERHOUT 
confirmed by measurements of electrical conductivity. The writer 
laid emphasis on these facts in order to call attention to the misleading 
character of Overton's conclusions and to indicate that the lipoid 
theory does not rest on a firm foundation. 
The writer, however, has nowhere stated, as Hober se ems to think, 
that the penetration in pure NaCl represents the normal state of 
things. He has, on the contrary, emphasized the fact that in sea water 
(and in other solutions in which life can be maintained for a long time) 
the permeability is much less than in pure NaCl because CaCl2 (and 
other salts) are present to antagonize the action of NaCl. 
On the other hand the permeability in sea water (which the writer 
regards as the normal permeability of Laminaria), while less than that 
in NaCl, is by no means the minimum permeability, nor is the resistance 
in sea water the maximum resistance, as Hober seems to suppose. 
The net resistance of a cylinder of tissue, which in sea water is 800 
ohms, may rise to over 1,600 ohms in a solution of La 2 (N03)6 which 
has the same conductivity as sea water. In other words the maximum 
resistance is at least twice as great, and the minimum permeability 
at least 50 per cent less than that found in sea water. 
Under these circumstances we are not justified in speaking (as 
Hober does) of the "normal impermeability" of protoplasm to salts, 
unless it is understood that we mean by "impermeability" merely a 
rate of penetration considerably lower than that found in dead cells. 
In the present instance it was found that the resistance of the cylinder 
of dead tissue was about 100 ohms, which was approximately the same 
as that of a cylinder of sea water of the same size. We can not, 
however, conclude that the resistance of the living protoplasm (in sea 
water) is just eight times as great as that of dead cells, for in the living 
tissue a part of the current passes between the protoplasts, travelling 
in the intercellular substance in which protoplasmic masses are 
imbedded. 
It is evident that much confusion wil' disappear when such terms 
as' permeability and impermeability can be quantitatively defined in 
all cases. The writer has sought to formulate quantitative expressions,^ 
not only for permeability but also for such conceptions as injury and 
vitality. It is to be hoped that the study of permeability may before 
long be placed on a quantitative basis. 
Harvard University, 
Laboratory of Plant Physiology 
^ Cf. Science, N. S. 40: 488. 1914. 
