Curvature of Roots . 441 
thus treated thirteen were bent away from the cut surface 
after intervals of from twelve to twenty-five hours.’ In still 
other cases nitrate of silver was used as an irritant, with 
similar results. He noticed the short mechanical bend in the 
opposite direction, at the place of injury, so that it is plain 
that he recognized two distinct forms of curvature, or bending, 
and attributed one to mechanical causes and the other to 
irritability 1 . 
Darwin’s work is open to criticism in a number of 
particulars which have been pointed out by later writers, 
especially Detlefsen, Sachs, and Wiesner. His chief error, 
however, lay in the view that the extremely slight pressure 
of various light objects fastened to the slanting side of the tip 
is the real occasion of the curvature that follows. This un- 
founded conclusion constantly appears in the discussion of his 
experiments, and leads to assumptions which the experiments 
themselves were far from proving. 
But notwithstanding these and possibly other errors incident 
to the first experimental investigation of the question, Darwin 
fully established the fact of the deflection of the root-tip, by 
processes already described, and gave a theoretical expla- 
nation of the fact. His interpretation included two distinct 
thoughts : first, that the apical portion of the root is sensitive ; 
and secondly, that extremely slight pressure, or, as he expressed 
it, simple contact, is an irritant sufficient to induce deflection. 
The latter has been clearly disproved by my own experiments, 
as well as by earlier ones of Wiesner and others. 
The ‘Power of Movement in Plants’ appeared in 1880. 
Of the various papers called forth by it, partly no doubt 
because of the striking form in which the conclusions were 
expressed, the first that calls for special notice is that of 
Detlefsen, ‘ Ueber die von Ch. Darwin behauptete Gehirn- 
function der Wurzelspitze V 
Detlefsen employed the various means that Darwin had 
used to induce deflection of the radicle, and introduced some 
1 Power of Movement in Plants, p. 151. 
2 Arbeiten des Bot. Inst, in Wurzburg, Bd. II, 1882. 
H h 
