Cuap. V. CONCLUDING REMARKS, 193 
bearers if the trunks are branched or bear twigs, and 
by some species if the bark is rugged. 
The advantage gained by climbing is to reach the 
light and free air with as little expenditure of organic 
matter as possible; now, with twining plants, the stem 
is much longer than is absolutely necessary; for 
instance, I measured the stem of a kidney-bean, which 
had ascended exactly two feet in height, and it was 
three feet in length: the stem of a pea, on the other 
hand, which had ascended to the same height by the 
aid of its tendrils, was but little longer than the height 
reached. That this saving of the stem is really an 
advantage to climbing plants, I infer from the species 
that still twine but are aided by clasping petioles or 
tendrils, generally making more open spires than 
those made by simple twiners. Moreover, the plants 
thus aided, after taking one or two turns in one direc- 
tion, generally ascend for a space straight, and then 
reverse the direction of their spire. By this means 
they ascend to a considerably greater height, with the 
same length of stem, than would otherwise have been 
possible; and they do this with safety, as they secure 
themselves at intervals by their clasping petioles or 
tendrils. 
We have seen that tendrils consist of various organs 
in a modified state, namely, leaves, flower-peduncles, 
branches, and perhaps stipules. With respect to 
leaves, the evidence of their modification is ample. 
In young plants of Bignonia the lower leaves often 
remain quite unchanged, whilst the upper ones have 
