1G2 PLANT SOCIOLOGY 



fact thai they do not dialyze. An important role is played by the 

 surface phenomena, especially by adsorption. 



However, crystalloids as well as colloids, while maintaining their 

 chemical composition, may change their physical condition; many 

 crystalloids may be transformed into colloids, and vice versa. They 

 pass into one another, often without distinct boundary lines, and are 

 really the same substances in different degrees of division. The degree 

 of division of crystalloids is greater than that of colloids. In colloids 

 the individual particles have a diameter of 1 to lOO^tju,^ while the indi- 

 vidual particles of crystalloids are molecular or ionic and have a 

 diameter of less than I^m- Crystalloids, although highly dispersed 

 systems made up of molecules and ions, do not exhibit active surface 

 phenomena. 



The phenomena of adsorption, the most important phenomena 

 of colloid chemistry, are due to the surface action of the extremely fine 

 particles. 



Saturated and Unsaturated Colloids. — With progressive division 

 of the particles the effect of the surface forces increases. Colloidal 

 particles are therefore capable, to a high degree, of attracting foreign 

 substances, i.e., of adsorption. A distinction is made between satu- 

 rated colloids, which have gathered a maximum of foreign substances, 

 and unsaturated colloids, which have not reached their limit of adsorp- 

 tion. Important changes of properties occur when saturation is 

 reached. In the case of soils, these changes are clearly reflected in the 

 vegetation. The behavior of the humus colloids in the soil is of special 

 importance. According to their more minute division they are char- 

 acterized by greater capacity for adsorption. If soil-alkali cations are 

 present (magnesium, calcium) in ample quantities, they bring about the 

 coagulation of the humus substances; the product is a saturated humus 

 (r/. p. 250). 



In the absence of bases, in high moors and upon sandy forest soils, 

 the humus colloids remain adsorptively unsaturated and contain 

 exchangeable hydrogen ions. Then the soil is densely packed, poorly 

 aerated, poor in food substances and in electrolytes, and, because of its 



1 The classification of dispersed systems used in colloid chemistry is based upon 

 the size of the particles {cf. Wiegner, 1926, p. 9): 



1. Coarsely dispersed systems (dispersions); diameter of particles over IOOmm 

 (microns). 



2. Colloid-dispersed systems (dispersoids) ; diameter of particles 1 to 100^/1 

 (ultra-microns). 



3. Maximal-dispersed systems (true solutions); diameter of particles less than 

 \)iti (amicrons, molecules, ions). 



