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transmitted to the tissues, where the curvature is produced. 
Whether a contact stimulus, in the sense employed by Prerrzr , 
can act as a stimulus to cambial activity, has not been decis- 
ively answered by the observations recorded. That however, 
a mechanical stress or strain does act as such a stimulus, in 
the absence of all contact stimulation of the peripheral layers 
(injured hooks in contact), is certain. An injury may, however, 
act as a weak stimulus, and here the stimulation is undoubtedly 
transferred by vitalistic means across the intervening tissues 
to the cambial layers, where an increased activity is excited. 
This result is produced by slight injury to the most peripheral 
layer, as well as by ones in which a large portion of the cortex 
is removed, and since the resulting growth is not localized at 
the point injured, but spreads over a certain area on the same 
surface, it follows that the injury acts directly as a stimulus, 
and not indirectly by causing the diminution of pressure pro- 
duced by removal of the outer layers to operate as a stimulus 
to cambial activity. 
As the pressure becomes stronger the cambial activity, and the 
growth of the concave surfaces of hooks in contact, are gradu- 
ally inhibited and finally cease, growth being now restricted 
to the sides and back of the hook. It is in this manner, that 
the excentric growth of old attached hooks in produced. When 
first attached, growth is most active over the surfaces in contact. 
The growth response due to the cambial activity, set up by 
the action of traction, strains and stresses, is much more directly 
proportional to, and correlated with, the character and intensity 
of the stimulus at work, than is the contact curvature, which 
the stimulatory effect, exerted by discrete particles touching 
the sensitive epidermal layer, produces in tendrils. 
In hook-tendrils such as those of Sérychnos, and in tendril 
hooks such as those of Bauhinia, in which a slowly produced 
curvature is induced by contact stimulation, any differences 
of turgidity, which may be induced between the growing tissues 
of the concave and convex sides, are too slight, and too rapidly 
followed up by growth, to allow of any perceptible straight- 
