MOTION AND SENSITIVENESS IN CLIMBING PLANTS. 
63 
claviculata ; the branchlets of which become deeply sinuous 
or zigzag, which may be the first indication of the spiral con- 
traction which takes place on the lower surface, as indicated 
by the abruptly bending of the petiole, when it has not seized 
an object. This “ indication 39 would seem to corroborate the 
statement already made, that this plant is an example of a 
state of transition between a leaf-climber and a tendril-bearer. 
Tendrils of many plants, if they catch nothing, contract after 
several days or weeks into a close spire ; whereas when caught, 
they contract immediately, and in other instances (as Virginian 
Creeper) wither and drop off without contracting spirally ; 
thereby showing the intimate connection between the spiral 
contraction of a tendril and the previous act of clasping a 
support. 
The use of the spiral contraction is varied. If it has caught 
a twig higher than the shoot which is inclined, it drags it 
up. Again, when it has once secured a hold, and the inter- 
nodes of the shoot continue to lengthen, were it not for this 
contraction, the shoot would be slackened. Another most 
important service is that the tendrils are thus made more 
highly elastic. The strain (as in Virginian Creeper) is equally 
distributed to the several attached branches of the tendril, 
thereby vastly strengthening it. Little can be said upon the 
exciting cause of the spiral contraction. At present, therefore, 
it must be called a vital action without any further explana- 
tion being attempted. 
IY. Hook Climbers and Root Climbers. — In this group there 
is no spontaneous revolving movement ; the former of these, 
as Galium Aparine, Rubus Australis , and climbing roses, 
apparently depend solely upon the mechanical support gained 
by their hooks, as is the case with certain palms in the Hew 
and Old Worlds. In the latter group are a good many plants 
which are excellent climbers. 
“ One of the most remarkable is the Marcgravia umbellata, which in the 
tropical forests of South America, as I hear from Mr. Spruce, grows in a 
curiously flattened manner against the trunks of trees, here and there putting 
forth claspers (roots), which adhere to the trunk, and, if the latter he slender, 
completely embrace it. When this plant has climbed to the light, it sends 
out free and rounded branches, clad with sharp-pointed leaves, wonderfully 
different in appearance from those borne by the stem as long as it is 
adherent . . . .” 
The following are Mr. Darwin^s concluding remarks : — 
“Plants become nlimbers, it may be presumed, to reach the light, 
and to expose a large surface of leaves to its action and to that of the 
free air. This is effected by climbers with wonderfully little expendi- 
ture of "organized matter, in comparison with trees, which have to 
