1918] STILES & JORGENSEN—PERMEABILITY 531 
three heads: (1) which part of the system it is, the permeability of 
which he intends to measure; (2) how far the values he obtains 
for the electrical conductivity of plant tissues are true measures of 
this conductivity; and (3) whether it is legitimate to assume that 
the electrical conductivity is a measure of the permeability. 
In regard to the first question it is perhaps significant that when 
discussing the statements of the writers OstERHOUT should speak 
of permeability in reference to the passage of substances through 
“the plasma membrane (or other surface),’’ while when discussing 
his own he should refer to the “‘permeability of the protoplasm.” 
It is therefore not at all clear what it is that OsteERHOUT considers 
he is measuring, whether he is dealing with the whole cell content 
or part of it, or only the limiting layer of the protoplasm. 
We come then to OsterHouT’s method of measuring the 
electrical conductivity of living tissues. The essential of this 
method (7) is that a pile of disks of Laminaria thallus is immersed 
in sea water or other medium between two electrodes. These are 
separated by a length of 20 mm. of sea water and the resistance 
between them measured. This resistance is called the resistance 
of the apparatus. The electrodes are then separated so that the 
roll of Laminaria disks is inserted between the electrodes in such 
a position that between each end of the roll of disks and the electrode 
isa length of 1omm. of sea water. The resistance is again measured 
and the increase in resistance is taken to be the resistance of the 
tissue. Now whether the resistance of the tissue can be determined 
in this way depends entirely upon the form of the apparatus used, 
for the 20 mm. of sea water and the tissue must be strictly in series 
and there must be no surrounding conductor through which current 
might pass. As OsTERHOUT has never published any details 
regarding the arrangement of his apparatus, it is impossible to 
accept his results when their correctness is highly dependent upon 
the details of the experimental arrangement. Indeed, certain 
facts given in OsTERHOUT’S very inadequate description suggest an 
incorrect arrangement; for instance, why, if the sea water and 
Laminaria are arranged in series, should the resistance of 2 cm. of 
sea water be 305, while the resistance of 2 cm. of sea water plus a 
cylinder of sea water of the same transverse dimensions as the 
