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irritable hooks used in climbing, and thence to Uacaria and 
Artabotrys with irritable clasping hooks only. 
In Roucheria and Ancistrocladus in addition to the thickening , 
a slightly increased curvature may be produced by contact, 
the latter being especially marked in the former plant. In 
Strychnos the hook or hook-tendril is differentiated into a thin 
almost non-irritable peduncular portion, and a thicker irritable 
distal portion shewing thickening and pronounced coiling when 
in contact. The differentiation between concave and convex sur- 
faces is, as regards irritability, also pronounced, as is still more 
markedly the case in Bauhinia. In this last form the hook like 
tendril is stiff and permanently curved, it thickens when in 
contact and does not coil very much more rapidly or markedly 
than Strychnos does. Dalbergia linga is peculiar in so far as the 
pulvinar tendril is probably a stage in the conversion of a 
twining leaf stalk into a tendril. It is highly irritable , coils 
rapidly, and shews a marked thickening when in contact. 
The claw like tendrils of Bignonia unguis bear a very inter- 
esting though mainly external resemblance to hooks, appearing 
to be, not a stage in the transition of a hook into a tendril , 
but instead a physiologically degenerate though specially modified 
form of a more highly irritable clasping organ, such as the 
tendril of B. littoralis, presents to us. 
In Amphilobium mutisii we have a normal and fairly irritable 
tendril, which shews a marked thickening when in contact. In 
all the above cases, contact, and more especially pressure and 
mechanical strain, act as stimuli to cambial growth and cause 
an increase in the thickness of the wood cylinder. This irritable 
power of secondarily increasing in thickness by cambial growth, 
when in contact, appears to be a property of axial or radial atta- 
ching organs only, such as are derived from petioles, peduncles, 
branches, or stems; whilst it is absent from leaf-, leafiet- or sti- 
pular-tendrils, and also from many clasping organs derived from 
axial members, viz root-tendrils and many stem-tendrils. 
The tendrils of Cucurbita increase in thickness only by an 
increase in size of the cortical cells, without any proliferation , 
