ANATOMY OF THE CELL. 5 
There are some organic substances whose molecules do not 
unite themselves into groups of any special character. When 
these are placed in water, solution takes place. The generally 
accepted explanation of this process is that the molecules of the 
substance have a stronger attraction for the water molecules 
than for each other and hence are torn apart and mingled with 
the water molecules till a homogeneous mixture results. 
In the greater number of organic substances, however, the 
molecules are supposed to unite themselves into groups named 
micellae, which are considered the units of organized substances. 
A micella is a group of molecules so closely united that it is 
claimed by some authorities that water molecules are not able 
to force their way between them. The micellae themselves, on 
the other hand, are surrounded by thin films of water. At 
least, this is their condition when the substance which they 
compose is in a fresh and active state. If this substance is 
allowed to become dry by evaporation it shrinks in size; if 
then again placed in contact with water, the micellae compos- 
ing it possess so strong an attraction for the water molecules 
that it sucks up the water; the films surrounding the micellae 
thus become thicker, and the substance increases in size. ‘This 
process of taking in water with corresponding increase in size 
is called imbibition, and the quality belongs only to organized 
substances. 
A process somewhat similar to this, and often mistaken for 
it, is known as capillarity, which consists of the penetration of 
water molecules into narrow channels already existing and filled 
with air. The water simply drives out the air and takes its 
place, and this process is therefore not accompanied by increase 
in size. A common unglazed earthen jar placed in contact 
with water shows this action of capillarity in unorganized sub- 
stance. The same process may take place in organized matter, 
as, for example, in a piece of wood containing ducts or tracheae. 
If this is placed in water, the air may be driven out of the small 
