16 PLANT PHYSIOLOGY 



True solutions on the one hand and mechanical suspensions 

 on the other differ from colloidal solutions in the following par- 

 ticulars : 



True solutions Colloidal solutions Mechanical suspensions 



Particles are: 



1. of molecular size 1 /*-l m/* in diam. larger than 1 /a in diam. 

 less than 1 m/x. in 



diam. 



2. invisible visible under the visible with the micro- 



ultramicroscope. scope or unaided eye. 



3. able to pass through able to pass through able to pass through 

 filters and parchment filters but not through neither filters nor 

 membranes. parchment. parchment. 



4. in molecular motion, in Brownian movement, in gravitational motion. 



The system : 



5. shows high osmotic shows low osmotic shows no osmotic pres- 

 pressure. pressure. sure. 



6. is transparent. shows the "Tyndall is generally opaque. 



effect." 



Of the various combinations mentioned above, the most im- 

 portant from a physiological point of view are the emulsoids and 

 suspensoids. Protoplasm under normal conditions seems to be a 

 watery solution with liquids and solid particles contained within it. 

 It is hence a colloidal solution of suspensoids and emulsoids. 



Emulsions. — In addition to these various types of colloidal 

 suspensions, mention should be made of emulsions, which consist 

 of drops of oily material dispersed in water or of drops of water 

 dispersed in oily material. Mayonnaise, milk, and egg yolk are 

 examples of such mixtures. If the particles are of colloidal size, 

 the system then becomes a colloidal one, although this is seldom 

 the case. Protoplasm contains droplets of oily material; it hence 

 has some of the properties of an emulsion and is commonly spoken 

 of as such. Under certain circumstances the inner phase of an 

 emulsion may become the outer one, in which case the phases are 

 said to be reversed. In cream, fat particles are dispersed in a 

 watery liquid, but in butter the fat is on the outside and the water 

 has become the dispersed phase. 



Lyophobe and Lyophile Colloids. — If the colloidal, dispersed 

 phase has little affinity for the material of the continuous phase, 

 the colloid is said to be lyophobic; if there is much attraction be- 

 tween the two, it is called lyophilic. Since in biological studies, 

 the chief dispersing medium is water, we may use the terms hy- 

 drophobic and hydrophilic. Most of the colloids of interest to 

 biologists, such as proteins, starches, etc., are hydrophilic. 



