868 
MECHANICS OF GROWTH. 
irritation is communicated along the tendril. The whole consequence of irritation 
does not however end with these phenomena ; for tendrils that are fixed to a sup- 
port also increase subsequently in thickness, sometimes very considerably, like the 
petioles of Solanum jasminoides ; they become woody, and have a longer term of life 
than those which have coiled spontaneously, or generally than those that have not 
attached themselves. 
There is *ill another point in which attached tendrils differ from those that 
have coiled spontaneously. In the latter all the coils of the spiral run in one direc- 
tion; those of a tendril attached to a support have, on the contrary, points (Fig. 487, 
w) at which the direction changes ; between any two of these points is a number 
of coils in the same direction, those beyond them being in the opposite direction ; in 
long tendrils with close coils there are often as many as five or six of these points. 
Darwin has already shown that this is no special property of tendrils, and still less 
a specific result of irritation, but is rather a physical necessity; for if a body which 
coils up is fixed at both ends so that no twisting can take place at either end, the 
coils must necessarily be produced in opposite directions in order that the torsions 
which are unavoidably produced may counterbalance one another. This behaviour 
of fixed tendrils can be imitated by cementing a narrow stretched strip of india- 
rubber firmly along another strip which is not stretched, and then releasing the 
former ; it contracts and forms the inside of a spiral, the outer side of which is 
formed by the strip that is not stretched. If the double strip is held at each end 
and first stretched out straight and then relaxed, coils will be produced, some to the 
right, others to the left, as in a tendril. If one end is now let go, the strip will twist 
itself anew into a spiral. 
Since all the movements of tendrils that have been described are the result of 
growth, they take place only when the external conditions of growth are favourable, 
and the more energetically the more favourable they are ; this is the case when food 
is abundant, temperature high, and the plant contains abundance of sap, the result 
of a copious supply of water combined with small loss by transpiration. Under 
these conditions tendrils can, as I have shown, carry on their nutation and sensitive 
movements even in the dark, and can twine and coil round supports. An instance 
is afforded by plants of Cucurbita Pepo, the upper parts of which are grown in a 
dark vessel, and which are nourished by green leaves exposed to light. 
As regards the mechanism of the curvatures caused by contact, as well as the 
coiling of free tendrils, it cannot be doubted that we have here to do with processes 
of growth and of its alteration by transverse pressure on the side which is growing 
less rapidly. The tendrils are only sensitive to contact or pressure so long as they 
are in a growing state. A curvature due to irritation may be effaced during growth, 
in the same manner as the curvature of growing shoots caused by concussion ; but 
if the irritation from the support lasts for a longer time and a coiling takes place, 
the difference in length between the convex and concave surfaces becomes per- 
manent. The cells of the convex are longer than those of the concave surface (as 
in roots which have curved downwards or nodes of Grasses which have curved 
upwards) ; in thick tendrils which coil round slender supports the difference in 
length is so great that it strikes the eye at once without measuring. De Vries's 
recent experiments on tendrils that have not yet coiled, which he marked with trans- 
