374 MacDougal . — The Mechanism of 
curvature effect the movement either by elongation of the 
tissues of the side of the organ becoming convex, or the 
contraction of a tissue on the concave side. As belonging to 
the first class, stems, petioles, and other organs which respond 
to geotropic and heliotropic stimuli, &c. may be mentioned ; 
and to the second class, the pulvini of Mimosa and other 
plants, the tentacles of Dr o sera, the leaves of Dionaea , and 
other structures. It may as well be stated in the beginning, 
that my conclusions justify the assumptions of Charles Darwin, 
that the clasping-movements of such highly specialized, 
initially dorsiventral tendrils as those of Passijiora are pro- 
duced by a contraction of the elements of a tissue on the 
concave (lower) side of the tendril. He says (I, p. 181) : £ At 
first I attributed this movement to the growth of the outside ; 
black marks were therefore made, and the interspaces measured, 
but I could not thus detect any increase in length. Hence 
it seems probable in this case and in others, that the curvature 
of the tendril from a touch depends on the contraction of 
the cells along the concave side.’ A similar conclusion had 
previously been reached by Knight : ‘ The external pressure 
of any body on one side of a tendril will probably drive the 
fluid from one side of the tendril which will consequently 
contract, to the opposite side which will expand, and the 
tendril will thence be compelled to bend around a slender bar 
of wood or iron 5 (VI). From the above it is apparent that 
while the conclusion that tendrils effected contact curvatures 
by means of the contraction of the irritated side of the organ 
is to be accredited to Knight, yet it is clear that to Darwin 
must be ascribed the first comprehension of the real nature of 
such contraction. De Vries, by a series of comparative 
measurements of the growth of the convex and concave sides 
of tendrils (II), and later by a series of plasmolytic tests, was 
led to the belief that the curvature and coiling of these organs 
was due to an increased osmotic activity of the convex side 
resulting in an accelerated growth-extension of the tissues of 
this side. Sachs (XX) adheres in the main to De Vries’ com 
elusions, yet he points out that a contraction of the lower side 
