324 



SCIENCE 



[Vol. L,TI, No. 1447 



The main factors modifying the crystalliza- 

 tion of pure substances are: (1) Concentra- 

 tion; (2) Temperature; (3) Pressure; (4) 

 Agitation; (5) The presence of other sub- 

 stances, especially of colloids, which may pro- 

 foundly modify crystal forms by protective 

 action; (6) Iso-colloidism. Some substances 

 have the power of interfering with their own 

 crystallization, because a portion, which first 

 reaches the colloidal state, then protects the 

 balance. 



Deviations from normal crystalline forms 

 produced by the presence of colloids are usu- 

 ally symmetrical, but may not appear crystal- 

 line. Changes in the nature or degi-ee of dis- 

 persion of the colloid, or in its percentage, 

 mixtures of colloids, variations in salt or H-ion 

 concentration of the solution, must all have an 

 effect on the resultant quasi-crystals. Enzymes 

 may, of course, entirely change the nature of 

 the colloid. The species-specificity of proteins 

 seems to be maintained by degenerating food 

 protein to simpler forms (polypeptids and 

 amino-acids) and then building up the specific 

 proteins from these. 



Among the factors influencing diffusion, 

 especially in gels, are the chemical nature and 

 particle size of the gel, and the concentration 

 and nature of the diffusing solution. Liese- 

 gang has pointed out that "enzoon," which has 

 been considered to be the fossilized remains of 

 primitive organisms, is due to the phenomenon 

 that bears his name — the rhythmic banding re- 

 sulting from diffusion in gels. In his chapter 

 on "Growth, Metamorphoses and Development" 

 Bechhold ("Colloids in Biology and Medi- 

 cine," trans, by J. G. M. Bullowa, p. 252 et 

 seq., D. Van Nostrand Co., 1920) refers to 

 some of the remarkable diffusion figures and 

 osmotic foi-ms produced by T. E. Runge and 

 by Stephane Leduc, some of which resemble 

 algee, fungi, seaweed, etc., and even show a 

 cellular microstructure. While pointing out 

 the great differences between these formations 

 and the organized structures they simulate, 

 Bechhold says : "The physical forces which 

 produced these inorganic formations are the 

 same as those which produce the growth and 

 configuration of organized material mem- 

 branes, osmotic pressure, diffusion." 



Differences in diffusion speed mean varia- 

 tions in concentration that may affect the ac- 

 tion of the enzjTaes, for, as T. B. Robertson 

 showed, these may work analytically or syn- 

 thetically, depending on the concentration. 



A preliminary note of this character can not 

 consider all of the points above referred to, and 

 will therefore be limited mainly to a brief dis- 

 cussion of one of the most important factors 

 controlling the form of organisms, namely, the 

 influence of colloids on crystallization, and the 

 changes in form that may be expected when 

 the colloids are changed or the crystalizing sub- 

 stances varied. 



Perhaps the most familiar instance of modi- 

 fied crystallization is to be found in the delicate 

 frost tracery on window panes, the forms being 

 probably influenced by the glass (itself a col- 

 loid) or by substances adsorbed at its surface. 

 The writer has pointed out the powerful influ- 

 ence exerted by colloids such as gelatin, gum 

 arabi'c and albumin on crystallization (Kolloid 

 Zeit., 4, 86, 1909), and R. E. Liesegang, look- 

 ing at the question from the opposite stand- 

 point, has described the power of crystalloids 

 to give a form to colloidal jellies {Kolloid 

 Zeit., 7, 96, 1910). It may be said that with 

 different salts or combinations of salts, various 

 colloids or combinations of colloids, and varia- 

 tions in concentrations, temperature and speed 

 of evaporation, will produce characteristic and 

 generally reproducible forms on a microscope 

 slide. A characteristic form of sodium chloride 

 is a four pointed star with fern-like arms 

 which cross at a slight angle. 



A few slides made with solutions of common 

 salts such as NaCl, MgSO^, Na^SO^, etc., con- 

 taining from 0.5 to 50 per cent, of gum arable 

 or gelatin (figured on the basis of the dry salt) 

 will illustrate what is meant. When a drop of 

 the mixed solution is allowed to dry on the 

 slide without cover glass, changes of concen- 

 tration and temperature occur, giving a field 

 that changes progi-essively from rim to center 

 of the drop. A solution of one part sodium 

 chloride, one part sodium carbonate (dry) and 

 one tenth part gum arable or gelatin in ten 

 parts of water, when dried, shows in some part 

 of the field a "flowering plant," with graceful 

 stems and characteristic four-petaled flowers. 



