209 
curved portion, that the thickening and lignification of the 
sclerenchyma first commences, then spreading rapidly apically, 
and slowly basally. As the sclerenchyma develops and lignifies, 
irritability declines, the hook resembling somewhat, in this 
respect, the tendril of Cwucurbita'), in which, as soon as the 
sclerenchyma commences to lignify, irritability disappears. 
The thickening of the sclerenchyma begins first on the down- 
wardly directed concave surface, and then spreads around the 
sides to the back, until finally a complete oval ring of scleren- 
chymatous wood tissue is formed. In the normally attached, 
fully developed, and coiled, hook-tendril, the main growth takes 
place in the xylem, but the same difference between the con- 
cave and convex sides persists for a time, the wood on the 
convex surface, at first, growing more in length and less in 
thickness, but finally, surpassing in all directions the growth on 
the concave surface. Contact exercises, at first, aspecial stimulus 
upon the concave surface, but later, as the pressure increases, 
this causes the growth on the surface in contact to be more or 
less markedly inhibited. The other tissues grow but little, ifat all. 
The epidermis stretches and cracks, in which case, the exposed 
outer cortical layers form corky tissue; the collenchymatous 
cortex grows but little, and becomes, by being stretched, slightly 
thinner, than it was originally; whilst the growth of the phloem- 
layer is very slight, as compared with that of the xylem. 
The response to contact stimulation is, under favourable 
conditions, rapid: in 12 hours, a slightly increased curvature , 
and in | day, a distinct increase in thickness, above the normal, 
being perceptible. Thus, with hook-tendrils attached to fairly 
rough fixed supports, 0.4 cm. in diameter, the following results 
were given. The numbers in brackets being the averages of 
similar untouched hook-tendrils: — 
1st measurements. After 1 day. After 3 days. — rie 
(e Toa tod oS Mob), $26.A98 dios tons a1 Lat 120.418]; @ 146. ia 6.200) 
ays. 
(6) 85.84 [90.90]; 100.105 [95.96]; 112.130 [96.97]; 128 . 152 (100. 102}. 
eee 
1) Munier. Conn’s Beitriige. Bd. IV. Hft. 2, 1887. 
