91 



elusion that the beautiful autumn colors of leaves are due to this 

 same process, when the slowing up of the metabolic processes 

 of the plant by the frost tends to hasten the formation of the 

 oxidized pigments. It should be noted that in many cases the 

 tannins act in this manner when oxidized, after being set free 

 from their glucoside form. In a very recent study of the role 

 of the glucosides in the plant, Weevers'^'^ concludes that these 

 substances may be considered as reserve foods, the cyclic com- 

 pounds being attached to glucose-yielding substances of low 

 dififusibility, thus serving to accumulate sugar, etc., for future 

 use. 



Besides this coordinated action of the hydrolytic and oxidizing 

 enzymes just described, there also seems to be an antagonistic 

 action between the oxidases and the reducing substances in the 

 cell ; this antagonism tending to keep each sort from getting the 

 upper hand during life, but after death when the production of 

 reducing substances ceases for a time, the oxidases run riot, and 

 blackening as well as colorations of various sorts result. The 

 blackening of the foliage of many plants after a frost, and 

 the production of the red and gold of our autumn forests, are 

 doubtless due to the killing of the leaves or to an interference 

 with their metabolism by the low temperature, and consequent 

 excessive activity of the oxidases upon tannins and other sub- 

 stances. 



Finally, Czapek^'' has brought to light a most interesting 

 example of the part played by oxidases in the life of the plant. 

 He found that geotropically and phototropically stimulated plant 

 organs always contained more reducing substances and also 

 showed weaker tests for oxidases than similar organs unstimu- 

 lated. Later he proved that the reducing substance which ac- 

 cumulated after stimulation was homogentisic acid, and that, after 

 stimulation, it did not seem to be destroyed by the oxidases as it 

 had been before. What caused this accumulation of easily 



52Weevers. Die physiologische Bedeutung einiger Glycoside. Recueil. Trav. 

 Bot. Neerland. 7: i. 1910. 



s^Czapek: (a) Ueber einen Befund an geotropsich gereizten Wurzeln. Ber. Bot. 

 Gesell. 15: 516. 1897. (b) Stoffwechselprocesse in der geotropisch gereizten 

 Wurzelspitze, etc. Ber. Bot. Gesell. 20: 464. 1902. 



