6s6 ECOLOGY 



their thorns approaching the impenetrability of tropical tangles. It is 

 assumed, and probably correctly, that lianas have come from erect 

 ancestors, and that their evolution was subsequent to that of trees, 

 although potential lianas well may have existed before trees and even 

 may have climbed over rock cliffs. Probably the first lianas were 

 leaners, the twiners and tendril climbers developing later. It is the 



Fig. 967. — A tropical meso phytic forest, rich in lianas; at the left is a sinistrorse twiner; 

 note the twisting of the liana stems at the center; the dominating liana is Agelaea Wal- 

 lichii, but in the undergrowth are many rattan palms {Calamus), which at first are ordinary 

 palm rosettes, but later develop into lianas of extraordinary length ; Lamao Forest Reserve, 

 Philippine Islands. — From Whitford (Courtesy of the Philippine Bureau of Forestry). 



prevalent view that lianas have resulted from the " struggle for light " 

 in the forest. Of this there is no evidence whatever, except in so far as 

 stems in general tend to elongate where there is decreased light (p. 726).' 



' Recently evidence of the inception of a twining habit has been discovered in a race of 

 snapdragons (Antirrhinum Majus), the new form appearing to be a mutant. This 

 form has the characteristic anatomical features of twiners, such as a small pith region, 

 compact vascular tissues, and cortical differences on the convex and concave surfaces. 

 Furthermore, the twining variants, however caused, come true to seed. It is diiScult to 



