234 Field, Forest, and Wayside Flowers 



of tropical forests, and they are most successful 

 where masses of tangled vegetation uphold and 

 " boost" them. 



In the fourth class Darwin places the "root- 



FiG. 65. English ivy (Hedera Helix). 

 Climbing by means of aerial rootlets. (From the Vegetable World.) 



climbers." These produce roots both in earth and 

 air. The aerial roots, or "rootlets," are short, 

 woody threads, which half cover the main stem 

 and branches, and cling tightly to bare walls, naked 



