300 PHYSIOLOGY 
Imbibition. When a plant is placed in dry air, water evaporates 
from it and its various parts shrink and shrivel. A little shrinkage occurs 
when plants wilt on a hot, dry day. When water again enters in suffi- 
cient quantity, they swell and regain their fresh look. The water may 
even be driven out entirely from some plants, as certain mosses, and when 
again wetted, the parts swell and regain partly or wholly their original 
dimensions. The most obvious of these changes are due to the collapse 
or expansion of the cells; but that they are not limited to alterations 
in the dimensions of the cells may be shown by measuring a dry bit 
of cell wall or a dry starch grain under the microscope, and after 
wetting, remeasuring it. On examination it appears that almost every 
substance in the plant body is capable of imbibing water, and of swelling 
or shrinking as the proportion of imbibed water increases or diminishes. 
The smaller the quantity of water the more difficult, and the larger the 
amount the more easy it is to remove it. From the fully swollen gelati- 
nous body of a sea weed, Laminaria, some water maybe extracted by the 
pressure of the fingers, while the greatest pressure does not suffice to 
squeeze it all out, and even by heating it is most difficult to remove the 
last traces of water. 
Theoretical structures of organized bodies. A study of the phe- 
nomena of swelling by imbibition, and of the way in which cell walls and 
starch grains affect polarized light, permits some inferences either as to 
the form and position of the particles, or as to the existence of strain or 
tension between them, by which they are slightly deformed or displaced. 
These inferences lead to theories of the invisible structure of the cell 
parts. The particles of which wall and protoplast are composed, it 
seems probable, are surrounded by water. Whether these particles are 
the chemist's molecules, linked together in a tense network, or aggre- 
gates of molecules (micellae) having a crystalline form, which are 
features of the two prominent theories, is of only remote significance. 
In either case the water between them may increase or diminish in 
amount; correspondingly, the particles approach or recede from one 
another. When any water is present, it forms a connected whole, how- 
ever irregular its distribution may be. The particles of the swollen stuff 
also cohere, and remain so related to one another that when the water 
is all removed, they regain the form they had before it entered. 
Swelling and solution. In the recovery of the original form is a 
practical but only a partial difference between the behavior of merely 
swollen and of dissolved substances. In both cases water wanders in 
