220 
POPULAR SCIENCE REVIEW. 
interesting fact that, in such a hook-like leaf as that of Clematis 
mticella (Fig. 1), the hooked end of the leaf, which has the best 
chance of coming into contact with obstacles, is the most sensi- 
tive part. This has been made out by hanging small weights 
on different parts of the leaf, and it is found that the terminal 
leaflet bends in a few hours after a loop of string weighing less 
than a grain is hung on it, and which produced no effect in 
twenty-four hours on the other petioles. One may see proof 
of the sensitiveness of the leaf-stalks of the wild English 
Clematis, which sometimes catches withered leaves or delicate 
stalks of the quaking grass. The same thing is shown by 
a leaf after having been touched with a little water-colour, 
the delicate crust of dry paint being mistaken for something 
touching the plant. In such cases, or when the leaf has been 
merely rubbed with a twig, which is taken away before the 
leaf seizes it, the plant discovers that it has been deceived, and 
after bending for a time, it unbends and becomes straight 
again. 
The bending, which enables a leaf to seize a twig, is not 
the only change which the stimulus of a touch produces. The 
leaf- stalk swells and becomes thicker and more woody, and turns 
into a strong, permanent support to the plant. The thickening 
of the leaf-stalks is to be made out in Fig. 2, which repre- 
sents a shoot of clematis, hearing two leaves, each of which has 
seized a twig; in one of the leaf-stalks this thickening has 
commenced, and is fairly evident. The thickened and woody 
leaf- stalks remain in winter after the leafy part has dropped off, 
and in this condition they are strikingly like real tendrils. 
The genus Tropaeolum, whose cultivated species are often 
called Nasturtiums, also consists of leaf-climbing plants, which 
climb like Clematis by grasping neighbouring objects with 
their leaf- stalks. 
In some species of Tropaeolum we find climbing organs 
developed, which cannot logically be distinguished from 
tendrils ; they consist of little filaments, not green like a leaf, 
but coloured like the stem. Their tips are a little flattened and 
furrowed but never develope into leaves ; and these filaments are 
sensitive to a touch, and bend towards a touching object, which 
they clasp securely. Filaments of this kind are borne by the 
young plant, hut it subsequently produces filaments with slightly 
enlarged ends, then with rudimentary or dwarfed leaves, and 
finally with full-sized leaves; when these are developed they 
clasp with their leaf-stalks, and then the first-formed filaments 
wither and die off ; thus the plant, which in its youth was a 
tendril-climber, gradually developes into a true leaf-climber. 
During the transition, every gradation between a leaf and a 
tendril may be seen on the same plant. 
