Physiological Observations 199 



Moreover, the extent to which albuminoids in affected roots 

 are saturated with copper — only 7.99 per cent for peas and 4.96 

 per cent for wheat — indicates a maximum effectiveness upon 

 roots of small amounts of the metal. 



Reactions of Copper with Growing Points 



Coi'n seedlings fifteen .days old were fixed with cotton, in 

 tall 50-c.c. graduated Nessler tubes containing different strengths 

 of copper sulphate in pure distilled water. The strengths of 

 solution employed' were 20, 10, 5, 2.5, and 1.25 p. p.m. There was 

 a check culture with no copper. After three days, in all cases 

 except the check, the roots were flaccid, showing contraction on 

 graduations and giving biuret and ferrocyanide tests, increasing 

 in strength from w^eaker to stronger concentration. 



An experiment with pea seedlings gave similar results, but 

 when the quantity of pea roots was increased and weak solutions, 

 2.5 and 1.25 p. p.m., were employed in small quantities (20 c.c), 

 the tests became much fainter. 



Severed roots of corn, also, were observed to give as good 

 tests as roots of living plants. A large number (seventy) of 

 severed root tips placed in a small quantity' (20 c.c.) of weak 

 solution (5 p.p.m.) gave only a faint ferrocyanide test. These 

 observations indicate that the concentration of copper in growin^^j 

 points is due to ionic dissociation and migration through the 

 semi-permeable membranes of the root systems,'* rather than to 

 transpiration. The fainter test for copper in large quantities of 

 root material indicates lessened toxicity of dilute solution-; of 

 copper in presence of excess of root materials. 



Mature wheat, corn and pea plants in nutrient solutions, but 

 not growing actively, were treated with gradually increasing 

 amounts of copper from January 21 to February 2, as follows : 



Wheat, Corn, and Pka Plants, Thirty-seven Days Old, Treated with 



Copper in Nutrient Solution 



Jan. 21-25; nutrient sol. w. 2 p.p.m. Cu. 

 Jan. 25-28; nutrient sol. w. 4 p.p.m. Cu. 

 Jan. 28 to Feb. 5, nutrient sol. w. 10 p.p.m. Cu. 



s See Bibliography, p. 237, references 35, 36, 37, 38, 39, 40, 41, 42, 

 43. 44. 



