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- 
Fic. 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 erial roots, or ‘‘ rootlets,” are short, 
woody threads, which half cover the main stem 
and branches, and cling tightly to bare walls, naked 
