CH. V, 8] SPECIAL FUNCTIONS OF ROOTS 



253 



In a few kinds of plants the roots are AERIAL, that is, fitted 

 to live in the air, temporarily or permanently; and they 

 exhibit corresponding modifications in structure. The 

 simplest case is illustrated by Corn, where roots grow out on 

 the stem ,a little above ground and run diagonally to the 

 soil, there acting as props to the stem, as well as organs of 

 absorption. This arrangement is much farther developed 

 in some tropical plants, notably the Screw Pine, or Pandanus 

 (Fig. 177), where these roots come to form almost the whole 

 support of the plant, the main stem remaining small. Similar 

 roots, but more irregularly placed, 

 cause the Mangroves of tropical 

 shores to form their dense thickets. 

 There are tropical plants, belonging 

 mostly to the Fig family, of which 

 the seeds germinate high up in the 

 crotches of trees to which they 

 happen to be carried; thence the 

 growing plants send down aerial 

 roots which, on reaching the ground, 

 thicken to trunks so robust that 

 often they strangle the supporting 

 tree, leaving the strangler a several- 

 trunked tree in its place. Analogous 

 effects occasionally are seen in our 

 own woods where trees have started on top of moss-covered 

 bowlders, excepting that here the bowlder does not vanish. 

 By similar aerial roots, put down vertically from horizontal 

 branches and later developed to trunks, a single Banyan 

 tree is enabled to spread to a many-trunked grove covering 

 several acres (Fig. 178). There is no difficulty, of course, 

 in understanding how roots can form trunks, since every- 

 where behind their young tips all roots are practically stems 

 in their structure (page 220). 



The extreme of the aerial habit is attained in some roots 

 which never reach the ground. Thus, in the epiphytic 



FIG. 



177. - - Pandanus, 



