192 
to the smaller hook, the opposite one may still go on growing 
more rapidly, and become or remain, larger than the smaller hook, 
even though the growth of the latter has also increased some- 
what in rapidity. When the irritability of the hook is on the wane, 
moderate stimulation is unable to counteract a difference in 
the rate of growth already induced between the two hooks of a 
pair, but marked differences are always produced between such, if 
the contact stimulation is applied, whilst the hooks are still young. 
' Hooks several weeks old may still be sensitive to contact. 
If the lignification, as determined by Phloroglucin or Anilin 
sulfate, extends and is equally well marked right to the cam- 
bium, this is a sign, that the hook has ceased for some time to 
grow ; such hooks no longer respond to stimuli. If, when, as judged 
by successive external measurements, a hook has just ceased 
to grow, a stimulus is applied, a distinct response is given. In 
such hooks, on examination, the outer layers of the wood are 
less, and in the extreme outside layers, not at all, lignified. A 
certain similarity hence exists, in this respect, between hooks 
and tendrils, for in the latter, lignification and loss of irritability 
proceed side by side, until, in the adult fully lignified organ 
all irritability is absent. 
The hook increases in thickness entirely by the cambial 
activity, the cortex also growing somewhat, but undergoing 
none, or only a very slight, increase in thickness. Since contact 
with sufficient pressure, applied to any part of the hook, will 
produce a thickening over the whole hook, it follows, that the 
stimulus is transmitted to the different parts of the hook. 
There is no reason to doubt, that the transmission of the 
stimulus is, mainly at any rate, a vital phenomenon, though 
a pressure stimulus, exerting considerable mechanical strain, 
may be mechanically transferred to, and directly act upoD, 
the regions immediately affected by it. As will be shewn later, 
in more highly irritable hooks, in which contact, with but 
slight pressure, is sufficient to produce a marked effect, a distinct 
and localized swelling can be seen at the point of contact. 
The amount of growth, which follows contact stimulation, 
