ECOLOGICAL 169 



To the twiners and scramblers of the tropical forest the general 

 name liana is often applied, and they form a prominent ecological 

 feature. Macgregor Skene writes: "In the great forests of the tropics 

 the trees find competitors and often conquerors in the mighty 

 creepers — the lianas — which twine their tough, slender stems round 

 supporting trees and round ieach other, crushing the life out of the 

 less resistant by constricting the conducting tissues, as does the ivy 

 with us. Hanging in festoons, climbing in inconceivable confusion, 

 they render the forest impassable to all but determined axemen. 

 And they reach out beyond the supporting trees, carrying their 

 leaves to the light above the roof of the forest, held up by pillars 

 of dead stems which they themselves have strangled." Of this 

 "woodland warfare" in which the lianas play such deadly part 

 there is an extraordinarily vivid picture in R. L. Stevenson's 

 Woodman. 



Leaf-climbers. — In Clematis and Tropaeolum the stalk of the 

 leaf is sensitive to contact and bends round a support; in Gloriosa 

 it is the tip that effects attachment. These types are approaching 

 tendril-bearers, but there is no marked specialisation of the climbing 

 structure. In the young leaves of the Climbing Fumitory {Corydalis 

 claviculata) the whole surface of the young leaf is sensitive to contact, 

 and bends towards the side on which pressure is exerted. Thus the 

 support is clasped. 



The sensitiveness of the leaf-stalks is sometimes exquisite; thus 

 Darwin observed one which responded to the slight but continued 

 pressure of a loop of soft thread, weighing only -j\. of a grain. The 

 region below the contact tends to become stronger, with more woody 

 tissue, approximating sometimes to a stem. It is interesting to 

 notice that in some species of Clematis and Tropaeolum the petioles 

 show little mobility or sensitiveness, but whether they have lost 

 these qualities or never had them, who can tell? 



Tendril-bearers. — A tendril is a speciahsed climbing structure, 

 very sensitive and mobile, and the tendril-bearers, such as vines and 

 passion-flowers, represent the highest grade of climbing plants. 

 Although it is a morphological, not an ecological question, we may 

 notice here the variety of structures that may be transformed into 

 tendrils. A tendril may develop from an entire leaf (e.g. in one of 

 the vetches, Lathyrus aphaca), from a leaflet (e.g. in the Sweet Pea), 

 from a stipule (e.g. in Smilax), from a shoot (e.g. in Bryony), from 

 an inflorescence (e.g. in Vine and Passion-flower), and even from an 

 aerial root (e.g. in Vanilla). This illustrates one of the methods of 

 Organic Evolution— the making of a new structure out of something 

 much older. 



The sensitiveness of tendrils to contact surpasses that of animals. 

 Thus Darwin notes in regard to the tendrils of the common pea: 

 "Whilst young and about an inch in length, with the leaflets on the 



