CLIMBING STEMS 115 
quently it is in the opposite direction, as of a clock- 
hand moving backwards. The result of this movement 
is that if the shoot encounters, say, an upright stem, it 
will lap round it in a spiral manner, and unless the said 
stem be quite smooth and unbranched, the twining 
shoot will be eventually supported by it. How effec- 
tive the twining habit is as regards economy of build- 
ing material may be seen from comparing the weight 
of the stem of a Hop with that of some tall herbaceous 
plant of the same altitude, and bearing an equal 
weight of leaves and flowers. The tendril-climbers are 
still more efficient, for they avoid the increased length 
of stem which arises from a twining habit. They 
grow straight up towards the light. Both the top of 
the growing shoot and the spreading tendrils which 
arise from it are continuously revolving in search of a 
support. When a tendril encounters one (such as a 
twig), the contact produces a stimulus which results 
in the tendril taking several close turns round the 
support. Nor does the action stop there, for usually 
the lower unattached portion of the tendril contracts 
into a spiral, drawing the stem closer to the support, 
and woody growth ensues, by which the tendril be- 
comes exceedingly tough, often stronger than the 
stem itself. 
One other point concerning climbers may be noted. 
Did they exhibit in a marked degree that bending 
towards the light which is characteristic of most 
plants, they would often defeat their own object, as 
they would grow away from possible supports. But 
they grow boldly up into an overhanging canopy, 
apparently confident of their power to ascend into the 
light and air which exist above. In the root-climbers, 
