CLIMBING PLANTS. 415 
ten years old, was still elastic and supported a weight of 
exactly two pounds. This tendril had five disk-bearing 
branches of equal thickness and of app: wently equal strength, 
so that this one tendril, after having been exposed during 
ten years to the weather, would have resisted a strain of ten 
pounds! 
Spiral Contractions. — Tendrils of many kinds of plants 
if they catch nothing, contract after an interval of several 
Woodbine. 
days or weeks into a close spire. A few contract into a 
helix. 
The spiral contraction which ensues after a tendril has 
‘aught a support is of high service to all tendril- bearing 
plants; hence its almost universal occurrence with plants of 
widely different orders. When caught the spiral contrac- 
tion drags up the shoot. Thus there is no waste of growth, 
and the stretched stem ascends by the shortest course. A 
far more important service rendered by the spiral contraction 
is that the tendrils are thus made highly elastic. The strain, 
as in Ampelopsis, is thus equally distributed to the several 
attached branches of a branched tendril. Tt is this elasticity 
which saves both branched and simple tendrils from being 
torn away during stormy weather. In one case observed 
