110 



MR. DARWIN" ON CLIMBING PLANTS. 



Tendril -bearers can, from their first growth, ascend along the outer 

 branches of any neighbouring bush, and thus always keep in the 

 full light ; twiners, on the contrary, are best fitted to ascend bare 

 Stems, and generally have to start in the shade. In dense tropical 

 forests, with crowded and bare stems, twining plants would pro- 

 bably succeed better than most kinds of tendril-bearers • but the 

 majority of twiners, at least in our temperate regions, from the 

 nature of their revolving movement, cannot ascend a thick trunk 

 whereas this can be effected by tendril-bearers, if the trunks carry 

 many branches or twigs ; and in some cases they can ascend by- 

 special means a trunk without branches, but with rugged bark. 



The object of all climbing plants is to reach the light and free 

 air with as little expenditure of organic matter as possible ; now, 

 with spirally ascending 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, ascending by its 

 tendrils, would, on the other hand, have been but little longer 

 than the height gained. That this saving of stem is really an 

 advantage to climbing plants I infer from observing that those 

 that still twine, but are aided by clasping petioles or tendrils, 

 generally make more open spires than those made by simple 

 twiners. Moreover, such plants very generally, as was observed 

 over and over again with the several leaf-climbers, after taking 

 one or two turns in one direction, 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 be possible ; and they can do it with 

 safety, as they secure themselves at intervals by their clasping 

 petioles. 



We have seen that tendrils consist of various organs in a modified 

 state, namely, leaves and flower-peduncles, and perhaps branches 

 and stipules. The position alone generally suffices to show when 

 a tendril has been formed from a leaf; and in Bignonia the lower 

 leaves are often perfect, whilst the upper ones terminate in a ten- 

 dril in place of a terminal leaflet ; in JEccremocarpus I have seen a 

 lateral branch of a tendril replaced by a perfect leaflet; and in 

 Vicia sativa, on the other hand, leaflets are sometimes replaced by 

 tendril-branches ; and many other such cases could be given. But 

 he who believes in the slow modification of species will not be 

 content simply to ascertain the homological nature of different 

 tendrils ; he will wish to learn, as far as possible, by what steps 



