1904] NEWCOMBE & RHODES: CHEMOTROPISM OF ROOTS 29 
retarding growth there, and thus causing the tip to swing around 
to make the positive curve. This result would be the same as 
the effect of mechanically wounding a root in the elongating 
_ zone, as determined by Spalding.’® It is to be noted, however, 
that to produce a positive curve by injury the sodium phosphate 
must act traumatically on the elongating zone, whereas we should 
expect it to act first on the more sensitive ‘root apex, and pro- 
duce a negative traumatropic curve. To test the relative sensi- 
tiveness of the root apex, and of the elongating zone toward a 
known injurious substance, a block of gelatin was made up with 
a 0.01 per cent. aqueous solution of crystals of copper acetate, 
while the gelatin used on the opposite side of the roots was 
made with distilled water only. Sixteen seedlings of Lupinus 
albus were used. After six hours, the temperature being 21° to 
22°, the gelatin blocks were separated, and the roots were seen 
to be bending away from the side holding the copper salt. The 
gelatin was replaced and allowed to remain eighteen hours 
longer, when the experiment was ended. Thirteen of the six- 
teen roots were strongly negative toward the copper acetate, 
while the other three roots remained straight. All roots were 
living and had grown somewhat. 
It is thus demonstrated that when an injurious substance is 
presented at the same time to both the elongating zone and the 
apex of a root, the negative traumatropism of the root will over- 
come the tendency to form a positive mechanical curve. It is 
thus demonstrated that the curves toward sodium phosphate in 
the former experiments were not traumatic. 
There is still the possibility that the curves toward the sodium 
phosphate were due to a disturbance of the turgor of the root. 
The salt on the one side may have caused a shortening of the 
cells on that side by withdrawing water; or the low osmotic 
pressure of the water and the gelatin on the opposite side may 
have caused an inflow of water into the cells, and hence a length- 
ening on that side of the root. 
To test the possibilities named in the ciate paragraph, a 
gelatin block was made up with 3.5 per cent. watery solution of 
ioSpaLDING: Traumatropism of roots. Annals of Botany 8: 423. 1894. 
