X. EXCHANGES AND MOVEMENTS OF FLUIDS 



243. Physical Constitution of Protoplasts. Vegetable proto- 

 plasts, except in the lowest forms, are invested by more or less 

 rigid walls composed of a mixture of substances which may be 

 included under the term cellulose. The periphery of the proto- 

 plast forms a plasmatic membrane in contact with the wall, and 

 also a second membrane enclosing the spaces filled with fluid 

 after the formation of vacuoles in its interior. Membranes also 

 surround the nucleus and the several kinds of plastids. All of 

 these parts of the protoplast, including the plasma itself, are capable 

 of imbibing water and swelling in different degrees. In regard to 

 solutions of other substances however, these membranes exhibit 

 the most diverse reactions and are by no means permeable to the 

 same substances. The cell walls are permeable to the greatest 

 number, but their capacity decreases when impregnated with waxy 

 and oily substances, as in cork and cuticle. The imbibing power 

 of the plasmatic membranes is regulated by the protoplasm and 

 may be varied from time to time. When two solutions of unequal 

 concentration, or two substances which attract each other, occur on 

 opposite sides of one of these membranes they will diffuse through 

 the separating membrane with a rapidity dependent upon the ease 

 with which they are imbibed by the membrane. If the mem- 

 brane is permeable to one of the substances alone it will pass 

 through alone, and the other will remain stationary. 



244. Imbibition. The imbibition of a fluid by a solid is due to 

 the energy of surface tension, or attraction, existing between the 

 particles of the two substances. This causes the fluid to pene- 

 trate between the particles of the solid and separate them as far 

 as their cohesion will allow. The beginning of the process is ac- 

 companied by a display of enormous energy which decreases as 

 the expansion of the solid proceeds. 



