Permeability 47 
data bearing on permeability, and so has not greatly helped towards 
a clear insight into the problems involved. 
It is to the second line of attack that we owe most of our know¬ 
ledge of permeability. The use of isolated cells or isolated pieces 
of tissue allows the employment of more exact methods and more 
careful control than can always be obtained in experiments with 
the whole organism. Such material as roots or stems and slices 
of storage tissues such as tubers or fleshy roots in the case of plants, 
blood corpuscles, eyes, pieces of muscle in the case of animals, have 
formed successful objects of experimentation. The elimination of 
error arising from variability among different individuals, the so- 
called “biological error/’ is possible with this mode of attack, and 
by a proper method of experimentation results can be obtained 
approximating in exactness more nearly to those of physics and 
chemistry than is possible when whole plants or animals form the 
experimental object, and such results are reproducible. 
As tissues differ in the form of the cells which compose them so 
they differ also in their functions, and it is reasonable to suppose 
there is no more uniformity in regard to permeability than in regard 
to other functions. By the use of isolated tissues we are thus on 
the way towards a physiological analysis which is not possible when 
the whole organism forms the experimental unit. 
The problems of permeability are problems of general physio¬ 
logy; they are common to all life, plants and animals, the lowest 
and the highest. They are problems of the cell and of the organism 
as a whole. While in this account of our present knowledge of 
permeability we shall deal mainly with plants, we shall thus have 
occasionally to refer to work on animals, as this may often be 
helpful in understanding the conditions in plants, while the results 
of investigations dealing with plants from very different groups of 
the plant kingdom and with both whole plants and parts of plants 
will have to be considered. 
