364 BOTANICAL GAZETTE [may 



water equal in volume to the cultures. The amount of oxidation in 

 these blanks is subtracted from what is observed in the plant cultures. 



Plant cultures usually show striking results at the end of 10 to 

 20 hours, depending somewhat upon the temperature and amount 

 of root surface. At the end of the experiment the plants are removed 

 from the cultures, and all are rendered distinctly alkaline with sodium 

 hydroxid solution, and thus the red phenolphthalein color appears. 



The great advantage in the use of phenolphthalin to demonstrate 

 the oxidizing power of roots lies in the fact that it is capable of yielding 

 quantitative results. After the colors have been developed in the 

 alkaline solution, their intensities may be estimated by the aid of a 

 colorimeter. In the work reported below the color intensities were 

 estimated by means of the colorimeter previously described, 25 which 

 permits of rapid and accurate readings. The colored solutions may 

 be read against a standard phenolphthalein solution or against a 

 standard Lovibond red glass slide. 26 The readings of the colon- 

 metric tubes are inversely proportional to the color intensity and are 

 easily reduced to their relative values. 



Aloin is a substance which may be used to demonstrate the oxi- 

 dizing power of roots in the same way as phenolphthalin is usea 

 Aloin, or barbaloin, is the active principle of Barbadoes aloes, and 

 is obtained in the market in the form of a yellow powder, fairly soluble 

 in water and serving well as an indicator of the oxidizing power oi 

 plants. At the concentrations used in our work it was not oun 

 to exert any toxic action upon plants. As a result of a limited im e> 

 gation of the chemistry of aloin, it seems that its value as an indicator 

 of the oxidizing power of plants depends largely upon the conten 

 of iso-barbaloin. , 



When oxidized by the plant roots, the aloin solution is change^ 

 from a pale yellow color to a permanent deep wine-red color, sun 

 to that given by Klunge's reaction for iso-barbaloin. KL ^ 

 reaction 2 » consists in dissolving aloin (containing iso-barbaloin) 1 



Chem.Soc. 27:1102. iocs. Bur. Soils, U. S. Dept. Agric Bull- 31. 



Amer 



1906. 



^Loviboxd, Jour. Soc. Chem. Ind. 13:308. 1894; see also Schreine** 



Rev. 19:61. 1901. 



2 ? Schweizerische Wochenschr. Pharm. 21:1. 1883; also Leger, Comp 

 Acad. Sci. Paris 131:55. 1900. 



Rend. 





