302 PHYSIOLOGY 
the observed facts of structure and of the migration of substances into 
the plant. The continuous cell wall determines that only substances 
soluble in water can enter the body. But according to this picture a 
continuous waterway is provided along which water-soluble substances 
may travel. Now in order to conceive how this migration occurs, one 
must have a mental picture of the behavior of watery solutions. To 
get such a picture it is necessary to bring to mind certain ideas of physi- 
cists regarding matter in its various states. 
2. DIFFUSION AND OSMOSIS 
For convenience, matter is said to exist in three states : gaseous, liquid, 
and solid. 
Gases. One characteristic of gases is that their particles tend to 
separate and to occupy to its utmost limits any receptacle in which the 
gas is placed. If unconfined by impermeable walls on one side, they 
form no free surface, but show unlimited capacity for diffusion, and their 
particles may become so dispersed among the other gases constituting 
our atmosphere as to be unrecognizable by any means at our disposal. 
This distribution of the particles is independent of any mixing by mass 
movements, such as those which show as currents or arise by jarring or 
stirring. On the contrary, it is assumed to be due to the energy of the 
gas molecules themselves, being hastened by any means which imparts 
energy, as by the application of heat. 
Liquids. The molecules of liquids are much less mobile than those 
of gases. When placed in a container, they shape themselves to it and 
form a free surface that is horizontal under the action of gravity, from 
which particles may fly off as vapor into the air. In volatile liquids 
this takes place at ordinary temperatures to such an extent that the 
process is easily measurable; in others, called non- volatile, the move- 
ment is too slight to be observed, or is masked by other changes. In- 
creasing the molecular energy of the liquid, as by heating it (unless it 
dissociates too rapidly), hastens its conversion into vapor, which behaves 
nearly or quite as a gas. 
Solids. The particles of solids are still less mobile than those of 
liquids, so that solids retain more or less perfectly their own shape, except 
under stress. Some solids, like ice and iron, can be liquefied and then 
vaporized; others, like camphor, may vaporize without passing through 
the liquid state. 
