41 6 Stiles and j0rgensen. — Studies in Permeability. V. 
required, similar to that which has been attempted in the case of carbon 
assimilation, in which, even if our knowledge is still very imperfect, we have 
a considerable amount of information regarding the complexity of the 
processes involved and the factors influencing them. Without 'such an 
analysis the putting forward of theories is clearly of little help to the 
advancement of the subject, and undoubtedly, as with carbon assimilation, 
the development of the subject has been hampered by undue speculation. 
As to the water relation in permeability, after the extensive pioneer 
investigations of de Vries and Pfeffer, little work of any value was done, and 
no advance in the subject was brought about until the recent introduction 
by A. J. Brown (3, 4, 5) and by F. F. Blackman (6) of fresh methods of 
attack in which changes are measured quantitatively and are followed 
from time to time, thus rendering possible an investigation of the kinetics 
of the changes taking place. 
Method of Experiment. 
The observations on the passage of water between the cell and sur- 
rounding medium recorded in this paper have been made on potato and 
carrot. The changes with time have been followed quantitatively by weigh- 
ing the tissue experimented upon at intervals, as was done in the experi- 
ments of A. J. Brown (3, 4, 5) on the intake of water by seeds of barley. 
The method of experimentation is as follows : Cylinders of tissue 
having a diameter of i*8 cm. are cut from the potato or carrot by means of 
a cork-borer. These cylinders are then cut up by means of a hand micro- 
tome into a number of discs having a definite thickness. In all the experi- 
ments dealt with here the thickness was 2 mm. After they are cut, the 
discs are rapidly dried between blotting-paper and then weighed out into 
lots of io or 20 grm. It is not difficult to obtain such lots which contain 
the same number of discs and differ in weight by not more than 2 per cent. 
The weight of each such set is taken to the nearest centigramme; more 
accurate weighing is not justified on account of errors arising from other 
causes, namely drying of the discs and variability of the samples. Each 
lot of discs is transferred to a bottle or flask containing a comparatively 
large volume of liquid (ioo C.C.-700 c.c.) which is kept at constant tempera- 
ture in a thermostat. At intervals the discs are removed from the liquid, 
their surfaces are dried as rapidly as possible with blotting-paper, and they 
are weighed. They are immediately replaced in a fresh quantity of the . 
experimental liquid. 
Alterations in the weight of the discs are almost entirely due to the 
passage of water into or out of the tissue. The weight of the small 
quantity of dissolved substances which may pass into or out of the cells is 
negligible compared with the weight of water which passes across the 
tissue boundaries. 
