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. 



