447 
Curvature of Roots. 
That the living organ is at the same time a most delicately 
adjusted piece of mechanism, and that its response to what- 
ever acts upon it from without is executed on the strictest 
mechanical principles, every one understands, but the two facts 
are distinct, notwithstanding their frequent confusion, a con- 
fusion not confined to older writers. 
While, then, an absolute demonstration may at present be 
impossible, the experimental evidence, as a whole, points so 
uniformly in one direction as to fully justify the belief that 
the growing-point of the root is sensitive, that induction 
follows its irritation, and that traumatropic curvature is the 
result. Upon this interpretation not only are the results of 
these particular experiments consistent with each other, but 
they are in harmony with what is thus far known of the 
reactions of sensitive organs in general, and of the structural 
and physiological characteristics of the root in particular. 
Of special interest as an illustration may be cited recent 
studies of the propagation of heliotropic stimulus. It has 
been shown by Rothert 1 that in the case of certain leaf- 
structures, notably the cotyledons of different Grasses, the tip 
is most sensitive, and that heliotropic stimulus is transmitted 
from the tip to the lower part of the cotyledon, where, after an 
interval of time, the visible response by bending is observed. 
He emphasizes the fact that growth and sensitiveness to 
irritation are two entirely independent things, though both are 
factors upon which the curvature of the organ depends, a fact 
which also appears in the traumatropic curvature of roots. 
Other cases of Nachwirkung exhibited by different plant 
organs are so familiar as to require no reference. 
Passing to the phenomena of geotropism as seen in the root, 
and comparing those of traumatropism, there is abundant 
evidence that the principle is identical. Darwin held that in 
geotropism we have another case of transmitted effects. This 
view was rendered probable by experiments of Brunchorst 2 
1 Ueber die Fortpilanzung des heliotropischen Reizes, Ber. d. bot. Gesellsch. 
i§9 2 > P- 374- 
2 Ber. d. bot. Gesellsch. II, 1884 . p. 78 . 
