Walter Stiles 
4 6 
of the objects of research to do so. While therefore we must aim 
at a stricter use of the term permeability so as to conform to its 
usage in physical chemistry, at present our knowledge simply does 
not allow us to do so when we are dealing with the problems of 
the cell. 
Research on the problems involved has proceeded along two 
rather distinct lines. In one, the whole living organism has been the 
unit of experimentation, while in the other isolated cells and tissues 
have been employed. In the case of unicellular and other small 
organisms the difference between the two groups of methods dis¬ 
appears. The difference between the two lines of attack is, however, 
very obvious in the case of work on higher plants where the methods 
of the first mode of attack are for the most part those of pot culture 
and water culture. In the hands of a number of plant physiologists 
from Woodward in 1699 onwards, research by these methods led to 
fundamentally important discoveries in regard to plant nutrition, 
and in the hands of Sachs and Knop about i860 it was used to 
demonstrate successfully the elements essential for the nutrition 
and development of plants, the absorption of water and dissolved 
substances from the soil and the absorption of gases from, and their 
excretion into, the air. The methods as employed to-day have 
provided a quantity of empirical information on the relation between 
the amount of growth of plants and the constitution of the medium 
external to their roots; as far as permeability problems are con¬ 
cerned they have not led us much further than the experiments of 
Sachs, Knop and other workers of their time, who showed by water- 
culture experiments that plants were capable of absorbing certain 
substances through their roots, while ash analyses showed that the 
constituents of these substances were capable of passing through the 
tissues to remote parts of the organism. These methods, in short, 
afford no data in regard to the intake of salts by the plant and 
the subsequent movement of the absorbed substances, beyond the 
information that these take place in certain cases. Nevertheless in 
the past they yielded results of the first importance for our subject, 
and there is no reason to suppose that, with suitable modification, 
their period of usefulness is over. 
In the animal organism similar considerations hold. From con¬ 
siderations based on the whole organism as a unit it early became 
clear that some substances could penetrate through certain cells 
and become absorbed into the animal, while others could not. Rut, 
generally speaking, this line of attack has not afforded quantitative 
