441 



facts warrant considering this injury the exciting agent in the formation of 

 intumescences. Other adjacent blades which have not been similarly 

 injured, do not develop the excrescences. The assumption can be very 

 easily made that, given an abundant supply of water and nutritive sub- 

 stances, the turgescence in the stem would be great, while the evaporation 

 from the node covered wath soil would be slight and an injury from grazing 

 cattle which would remove part of the tissue, w^ould so increase the turgor 

 that intumescences would be formed. 



I had already observed similar correlation i)henomena in the action of 

 copper sprays on potato leaves\ In vigorously growing varieties a number 

 of leaves were found injured by the spray; near the dead spots in the tissue, 

 the intumescences later appeared. Still other causes may have similar 



9 . -g 



Fig-. 78. Intumescence on the lower node of an oat plant. 



results, since small warts have been observed on potato leaves when the 

 copper solution had not been used". Von Schrenck^ has reported more 

 recent results in this connection. A few days after cabbage plants in green- 

 houses had been sprayed with copper ammonium carbonate, pale knots, 

 gradually becoming white, developed on the under side of the leaves. They 

 proved to be intumescences. Unsprayed plants in the same house showed 

 no eruptions. Spraying with weak solutions of copper chlorid, copper 

 acetate, copper nitrate and copper sulphate did cause some distensions. Von 

 Schrenk, however, considered these intumescences a reaction of the leaf 

 tissue to the chemical stimulus of the poisons, not correlative phenomena. 



1 Sorauer, P., Einige Beobachtungen bei der Anwendung- von Kupfermitteln 

 geg-en die Kartoffelkrankheit. Zeitschr. f. Pflanzenkrankh. 1893, p. 32. 



^ Masters, Leaves of potatoes with warts. Gard. Chron. 1878, I, p. 802. 



3 Schrenk, H. v., Intumescences formed as a result of chemical stimulation. 

 Sixteenth ann. report Missouri Bot. Gard. May, 1905. 



