404 THE COLLOIDAL STATE 



many concentric ring structures or other alternating deposits 

 found in nature will be obvious ; in illustration, the formation 

 of starch grains may be mentioned. According to Kiister * 

 many plant structures such, for example, as the banded pith 

 of Magnolia grandiflora, the calcium oxalate sacs of Ficus 

 carica, the regular alternation of crystal-bearing zones with 

 those containing no crystals found in the bark of the Pome- 

 granate and the zebra-like pigmentation of the succulent 

 leaves of Hai&orthia fasciata and Aloe varigata may be due 

 to similar causes ; these structures at any rate may have their 

 origin in some analogous internal rhythmic stimulus. 



The peculiar concentric growth of certain moulds resulting 

 in structures closely resembling the Liesegang rings have been 

 studied by Munk.f 



THE NATURE OF GELS. 



As already pointed out above, emulsoids are regarded as 

 two-phase systems in which the disperse phase is a more con- 

 centrated solution, and the continuous phase a relatively dilute 

 one. When such a solution gives a gel, the roles of the two 

 phases are assumed to be changed, resulting in a sort of net- 

 or sponge-Hke structure, of concentrated solution representing 

 the continuous phase, whereas the disperse phase is repre- 

 sented by a dilute solution filHng up the interstices. 



Evidence for the existence of some such sponge-like or 

 honeycomb structure has been obtained by Hardy % in study- 

 ing under the microscope the formation of a gel. 



It is only by postulating some analogous structure that it 

 is possible to understand how i gram of agar can cau.se 99 

 grams of water to set to a stiff jelly just as the organized cell 

 structure of many plants enables them to maintain a rigid 

 form while consisting of practically 90 per cent of water. 



* Kiister : " Beitrage z. Entwicklungsmechanischen Anatomic d. 

 Pflanzen, Jena," 191 3. 



t Munk : " Centralblatt fur Backteriologie," 1912, 32, 353, and 34, 561; 

 also " Biol. Centralbl.," 1914, 34, 621. See also Liesegang, " Naturwis- 

 sensch. Wochenschr.," 1913, [xii], 25. 



t Hardy : " Proc. Roy. Soc," 1912, A., 87, 29. 



