400 MacDougal . — The Mechanism of 
11. The strain exerted by a tendril during contact curvature 
is less than *5 grams. Rapidity, not force, is essential to the 
effectiveness of such curves. 
12. The time of formation of coils in the free portion of 
a tendril extends over 6 to 40 hours. 
13. The latent period of a contact-reaction varies from 
5 seconds to an hour in different species. 
14. Marked differences exist between the structure of the 
protoplasm of the convex and concave sides. The protoplasm 
of the concave side is more richly granular, and occupies 
a greater proportion of the cell-cavity than on the convex 
side. The density of the protoplasm of the convex side 
increases from the base toward the tip, and apparently cor- 
responds with the degree of irritability to contact exhibited. 
15. The parenchymatous cells of the concave side are 
markedly different in outline, structure, and form from those 
of the convex side. When plasmolysed they undergo 
a decrease in size amounting to 20 to 40 per cent, of their 
original volume, and changes in contour from oblong-ovoid 
to irregularly globoid or ovoid. During curvature they 
undergo similar alterations. After curvature they are found 
in such condition, and to their action must be ascribed the 
contact-curvatures of the tendrils under examination. This 
action consists in an increase in the permeability of their 
protoplasts, a consequent extrusion of water into the inter- 
cellular spaces, and a release of the stretching tension exerted 
upon their walls of the vascular, collenchymatous, and 
epidermal tissues. The elastic contraction of these cell-walls 
causes the resultant curvature. This action ensues in free 
portions of attached tendrils upon maturity, and early in 
the process of the formation of loose coils in an unattached 
tendril. 
