240 
cambial or otherwise, being induced by contact. The root-tendrils 
of Vanilla, though widely different in origin and structure to 
those of Cucurbita, are not, as regards their physiological properties 
in relation to contact, very far removed from them. The tendrils 
of Cucurbita are however much more highly irritable, and this 
irritability is surpassed only in Passifora. Sicyos etc. In the 
root tendril of Vanilla the irritability to contact stimulation is 
retained so long as the original epidermal layer covering the 
root remains alive and normal. As soon as a cuticular exoder- 
mal layer is produced, the sensitivity to contact disappears , 
no formation of a contact curvature being now possible. 
In these last forms, contact causes a hardening but no thick- 
ening of the tendril and a special physiological property, reaches 
in them its highest differentiation, namely the power of respon- 
ding to contact by a rapid and at first non-permanent curva- 
ture, the production of which is in intimate connection with 
the maintenance of turgidity, and which may be removed or 
diminished, when all turgidity is removed. 
Between the stimuli of contact and of pressure no hard and 
fast line of demarcation can be drawn. Unilateral pressure , invol- 
ving internal mechanical strain, is possible without any con- 
tact stimulation being exerted by discrete particles, but all 
contact stimulation necessarily involves pressure of greater or 
less intensity. Irritable hooks are however more adapted to 
respond to the stresses and strains set up by pressure and trac- 
tion, tendrils to the contact stimulation exerted by the relati- | 
vely trifling localized pressure of the discrete particles pressing 
upon the sensitive epidermis of the tendril. 
The strains set up by pressure and traction in certain hooks, 
leaf petioles, or tendrils, act as direct stimuli to cambial activity, 
causing a thickening of the wood cylinder, corresponding, a8 
far as the inherent factors regulating growth will allow, in 
character, direction and amount, with the mechanical needs 
of the attached organ. The stimulus of actual contact in both 
tendrils, and in hooks (such as those of Strychnos), appears to 
be felt only by the epidermal layer in contact, and to be 
