232 
the stimulus caused by the strain set up in the pulvinar tissues. 
Dalbergia linga is very evidently in an interesting transition 
stage, the pulvini being capable of undergoing modification in 
_two divergent directions, either into hooks or into normal 
tendrils. In both cases the leaflet portion would be permanently 
reduced or lost, but in the former case the power of thickening 
would become most marked, in the latter case the power of 
coiling and rapid curvature, whilst in correspondence with these 
changes would come the shortening and stiffening to form the 
hook, or the lengthening and thinning to form the tendril. 
Mitier ') has shewn, that the tendrils of the Cucurbitaceae 
increase in thickness after contact, to sometimes as much as 
3 times their original thickness, but that the thickening is 
generally chiefly or entirely on the surface in contact and is 
due simply to an enlargement of the cortical cells. He points 
out that the rapid contact curvature is quite a distinct and 
separate phenomenon from the slow winding and thickening 
which follows it; the former is a rapid action and may be 
repeated, whilst the latter is slow and permanent. It is only 
by means of the second of these phenomena that connections 
exist between tendrils in general and certain hooks; the first 
phenomenon, the rapidly produced contact curvature, is entirely 
peculiar to the fully specialized tendril, though all series of 
intermediate stages exist, where this power is less and less 
obviously developed. This power of producing a rapid curvature 
is necessary in thin and flexible tendrils of the Passiflora type, 
as otherwise they would hardly ever become attached, though 
exceptionally their high sensitivity may be a disadvantage, 
as for example, when the tendrils grasp loose twigs, straws 
or fibres, which instead of supporting the plant are a drag 
upon it. The stiff and rigid hooks of irritable hook climbers 
only clasp objects of a diameter varying within relatively nat 
row limits, and only become permanently attached after the 
1) Miitter, Untersuch tiber die Ranken der Cucurbitaceen. Coun, Beitrige- at 
IV, Hft. If, 1887. 
