4 BOTANICAL GAZETTE [JANUARY 
Pfeffer (57) proved that Darwin was right, and that the root tip 
alone is sensitive to external stimuli, and the stimulus is trans- 
mitted to the part of the root in which the curve takes place, a 
distance which may be as much as ten millimeters. Darwin was 
also the first to show that injury to one side of the root tip may 
act as a stimulus, causing the root to curve away from the side 
injured. He regarded curvatures in growing organs as modifica- 
tions of circumnutation, and thought the immediate cause was a 
difference in turgescence of the cells on opposite sides of a curv- 
ing organ. In the work called out by Darwin’s Power of move- 
mentin plants a number of new theories were evolved as to the 
mechanism of root curvature, and at first the theories had a very 
close relation to the question of the function of the root tip. 
Detlefsen opposed Darwin’s view (13), but his purely mechanical 
explanation of the mechanism of curvature has been shown by 
Spalding (69, p. 442) to be untenable. Spalding adopts the 
term traumatropism, proposed by Pfeffer (56, p. 3 74) to designate 
a response of plant organs to the stimulus of injury or wounding, 
and this term has now been generally accepted. 
Wiesner was probably Darwin’s most vigorous opponent, and 
in several papers of considerable length (76-78) he discusses 
growth-curvatures in both stems and roots. He presents an 
explanation of these curvatures which, in a somewhat modified 
form, has been adopted by some of the most recent writers on 
the subject. He accounts for the traumatropic curvature of 
roots, which he calls the Darwinian curve, by an increased duc- 
tility of the cell membranes on the injured side. The turgor in 
these cells then stretches them more rapidly than those of the 
Opposite side, with the result that a curve is produced whose 
convexity is on the injured side (78, pp. 268-9). The mem- 
branes of the convex side are not only more ductile, but also less 
elastic than those of the concave side. 
favorable to the formation of a curve. 
dence offered by 
cur 
This condition is also 
The experimental evi- 
Wiesner in Support of his explanation of the 
ve was the fact that roots injured by decapitation grew faster 
than normal ones if they were grown in water, and even in those 
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