66 ILLINOIS ACADEMY OF SCIENCE 



age of protective colloids for holding certain insoluble sub- 

 stances in suspension. 



Hofmeister showed that certain anions of alkali salts de- 

 crease the amounts of water absorbed by gelatin gel, while 

 others increase it. The sulfate, tartrate, citrate, and acetate 

 anions cause shrinkage, while bromides, nitrates and rhodo- 

 nates cause swelling. Starting with this determination a great 

 amount of later work has led to a rather definite arrangement 

 of the anions of alkali salts in relation to this as well as other 

 characters of colloids and to a number of physical and chemical 

 processes in homogeneous solutions (1: pp. 309-326). The 

 following are interesting colloidal applications of this series. 

 In salting out hydrophyllous colloids, the sulfate-tartrate end is 

 most effective, while certain anions of the opposite end of the 

 series cause dissolution. The same order holds in the produc- 

 tions of gels from sols. The cations show less range in ef- 

 fectiveness, but they too can be arranged especially in regard to 

 precipitation of colloids, in definite order reversed, however, 

 with a reversal in the charge. We generally consider tropic 

 movements in olants, so far as their underlying- causes and 

 processes are concerned, as amongst the most complex of plant 

 phenomena. Porodko (33) shows in a recent publication that 

 the effectiveness of the anions of salts of alkali metals in pro- 

 ducing positive chemotropism follows almost exactly this 

 series: Tartrate > citrate> sulfate> acetate> chlorate> 

 chloride> nitrate> iodide> cyanide. The effectiveness of 

 their cations for positive chemotropism also approximated 

 their effectiveness for the precipitation of colloids in an acid 

 medium, thus: rubidum> caesium > potassium > lithium > 

 sodium. Bromide does not cause positive chemotropism. 

 These and a number of other facts discovered by 

 Porodko indicate that dehydration or flocculation of colloids 

 bears a fundamental relation to chemotropism. Work by 

 Fischer and by Schley indicates similar significant relations 

 between geotropic response and water absorption by colloids 

 (12, 34). To date, however, the results on both chemotropism 

 and geotropism in this line are only suggestive and open the 

 possibility of attacking these complex phenomena of plants on 

 a definite basis of the physics of colloids. 



X CONCLUSIONS 



In closing let us attempt to make a few summary state- 

 ments : In discussing the topic colloidal chemistry in rela- 

 tion to biology, one faces a relatively new science with a still 

 newer application to another science. The application to date 



