86 PRACTICAL COURSE IN BOTANY 
by coiling, the body of the plant 
is drawn upward proportionally. 
It will be observed that the helix 
is interrupted at one or more 
points, above and below which 
the coils turn in opposite direc- 
tions. This is because the ten- 
dril is attached at both ends and 
cannot adjust itself to the oppo- 
site strains of torsion. Twist 
with your fingers a piece of tape 
so attached, and you will see 
that on one side of your hand it 
turns from right to left and on 
the other from left to right. 
98. The cause of twining. — 
Fic. 98.—Leaf of common pea, . 
showing upper leaflets reduced to Botanists are not fully agreed 
cence, on this point. The explanation 
most generally accepted at present is that the twining of 
stems is due to the combined action of lateral and negative 
geotropism (51). The first 
causes one side tO grow 
morerapidly than the other, 
thus forming a succession of coils, while the 
second, by stimulating the upward growth 
of the axis, stretches it into a spiral, and in 
this way draws it more tightly round the 
support. For this reason twining stems do 
best on an upright support. 
In tendrils, the twining is thought to be 
due not to gravity, but to contact with a 
solid body, which, by inducing unequal de- 
velopment on opposite sides of the tendril, 
Fia. 99. — Stems ‘i ‘ 
of a passion flower Causes it to turn about an available object. 
transformed into The coiling of the free part of the twining 
tendrils. (After Bo ok x 
Gray.) organ is in response to the stimulus trans- 
