HYDROPHILIC SOLS AND GELS 155 



or, more likely, the rhythmic precipitation is an example of the 

 fact that banding may occur in liquids when it takes place in 

 capillaries; this qualification is fulfilled by the living cell. The 

 capillary forces present there are sufficient to hold the rhythmi- 

 cally produced precipitates in place. This is nicely illustrated 

 by Lloyd in artificially produced banding in the hairs (trichomes) 

 of plants (C, Fig. 92). 



The applicability of the Liesegang phenomenon to biological 

 processes is speculative, but there is no apparent reason why it is 

 not the cause of banding so common in animals, such as the 

 markings on butterfly wings and the banded colors of fish and 

 the feathers of birds (pheasants). When Hatschek spoke of 

 Liesegang rings before the seminar at Bateson's laboratory near 

 London, Bateson asked if biologists could assume that the stripes 

 of a zebra were a Liesegang phenomenon. The remark was taken 

 humorously at the time, but Lloyd has since considered it seri- 

 ously. He says that whatever the cause of the parallel lining 

 on the zebra may be, it looks most suspiciously like rhythmic 

 precipitation. The similarity is especially marked when certain 

 irregularities occur in gel banding (D, Fig. 92). 



The banded colors of agates, fish, and birds, and the concen- 

 tric rings in certain fossil remains indicate that the Liesegang 

 phenomenon is rather common in nature. The anatomist Sweet 

 has made an interesting collection of gallstones, which, on cutting 

 and polishing, show in some cases remarkable rhythmic markings 

 (G, Fig. 92). Their production is probably a typical Liesegang 

 phenomenon due to the gradual infiltration of bilirubin (the 

 surrounding pigment) into the calcium-cholesterol (a gel-forming 

 monoatomic alcohol) matrix of the gallstone. The precipitate 

 formed, in concentric rings, is calcium bilirubinate. The cho- 

 lesterol is then presumably shifted from a colloidal to a crystal- 

 loidal state, the final structure of the gallstone being crystalline. 



Fig. 92. — A, Liesegang rings of silver chromate precipitated by silver nitrate 

 diffusing from the center into the gelatin gel containing potassium dichromate 

 (Liesegang's original experiment) ; B, silver chromate bands formed in capillaries; 

 C, Liesegang rings artificially produced in the living hair (trichome) of a plant 

 (from F. E. Lloyd); D, rings produced as in A, illustrating irregularities; E, a silver 

 chromate spiral precipitate in an agar gel; F, fungus (Cephalothecium) growing in 

 culture and forming concentric rings of fruiting bodies which have nothing in 

 common with Liesegang rings except rhythm (from G. G. Hedgcock); G, gallstone 

 (twice natural size) showing Liesegang rings of calcium bilirubinate produced by 

 bilirubin diffusing into the calcium-cholesterol base of the gallstone {from J. E. 

 Sweet) . 



