Stiles and J ' ergensen.— Studies in Permeability . V. 419 
In the majority of cases the differences recorded are many times larger 
than any that could be accounted for by sampling error, and they may 
therefore be regarded as having a real significance. 
By this method it is only those experiments conducted at the same 
time with material from the same general sample of mixed discs that can 
be directly compared, as otherwise differences due to previous history and 
inherent causes may become evident. An example of this we shall have an 
occasion to quote in the section dealing with the influence of temperature. 
We may take this opportunity of calling attention to the necessity of 
determining the error of experiment in plant physiological work. Probably 
little plant tissue is so uniform as potato and carrot, and it is shown here 
how necessary it is, even with this tissue, to determine the probable error 
of experiment before drawing conclusions from observed differences in 
behaviour in relation to different conditions. In very few plant physio- 
logical researches has this very necessary calculation been made, with the 
result that conclusions are constantly drawn for which there is really no 
experimental evidence. 
Experimental Results. 
(a) The Swelling of Potato and Carrot in Water . 
When living tissue of potato or carrot is immersed in distilled water 
there is a passage of water into the cell. The rate of this process, rapid at 
first, gradually slows down until a condition of equilibrium is reached. 
The curves marked Pd and 
Cd in Fig. 1 illustrate the 
general course of this swelling 
in distilled water at 13 0 C. It 
will be observed that, in the 
case of carrot, 90 per cent, of 
the total swelling takes place 
during the first two days. The 
maximum swelling is reached 
after about six days ; little 
alteration takes place during 
the next six days, at the ex- 
piration of which time the 
experiments were discontinued. 
Fig. 1. Swelling of potato and carrot in distilled 
and in 'tap water. C D , carrot in distilled water; Cr, 
carrot in tap- water; P D , potato in distilled water; 
Pt, potato in tap-water. 
The course of swelling with the potato is similar, but the initial rate of 
swelling is less rapid (see Fig. 2). This fact we have noticed throughout our 
experiments. Equilibrium is, however, reached in about the same time as 
with carrot, and is maintained for a considerable time just as with that 
tissue. 
