396 MacDougaL — The Mechanism of 
Furthermore the cells of the concave side are provided with 
tapering ends, and the end walls are united over a small 
area only. As a consequence of this arrangement, the inter- 
cellular spaces are large. The cells of the convex side are 
united more completely across the ends, and the intercellular 
spaces are relatively smaller. The protoplasm of the elements 
of this tissue in the concave side is much more densely 
granular, and occupies a much greater proportion of the cell- 
cavity than in the convex side. The parenchymatous cells 
are arranged in four or five layers on both the concave and 
convex side of the tendril, but owing to the greater diameter 
of the cells of the convex side, the tissue of this side attains 
a marked preponderance in thickness. The two outer layers 
are furnished with chloroplasts varying from 6 to io in 
number in each cell. The parenchymatous tissues of both 
the cortex and pith exhibit marked infoldings of the walls. 
In tendrils exposed to strong sunlight, a reddish cell sap is 
found in numerous cells both in the epidermis and cortex of 
the convex and concave sides. Previous to the maturity and 
coiling of the tendril, the prosenchymatous and tracheary 
tissue are not developed to such extent as to be of great 
mechanical value. The arrangement of the vascular elements 
conforms to the outline of the tendril and its mechanical 
needs. The chief mechanical element during the irritable 
stage of the tendril is the collenchyma-cylinder, which as 
w r ell as the epidermis and vascular tissue must be in a state of 
extreme elastic extension, due to a small extent to their own 
turgidity and to the excessive turgidity of the parenchymatous 
cells. Under such circumstances curvature might result from 
an increase in the extensibility of the walls of the convex 
side, or by a loss of turgidity or by an active contraction due 
to the loss of elasticity of the walls of the cells of the concave 
side. The contour of the parenchymatous cells of the two 
sides after plasymolysis as already described before curvature, 
after curvature, and in a mature coil which has fastened around 
a support, offers some conclusive evidence in favour of the 
latter view. 
