34 



MOBPHOLOGY OF THE ROOT. 



into which they penetrate. More remarkable cases abound in 

 those tropical regions where the sultry air. saturated with moist- 

 ure for a large part ol'lhc year, favors the utmost luxuriance of 

 vegetation. In the Palm-like Pandanus or Screw-Pine l (l-'i^. 



O V O 



(''.I), very strong root>, emitted in the open air from the trunk, 



and >ooii reaching 

 the soil, give the 

 appearance of a tree 

 partially raised out 

 of the ground. The 

 fanioii> Iian\ an-tive 

 of India (Fig. 71 ) is 

 a still more striking 

 illustration : for the 

 aerial roots strike 

 from the horizontal 

 branches of tin- tree. 

 often at a great 

 height, at first swing- 

 ing free in the air, 

 but linally reach- 

 ing and establishing 

 themselves in the 

 ground, where they 

 increase in diameter 

 and form accessory 

 trunks, surrounding 

 the original bole and supporting the wide-spread canopy of 

 branches and foliage. Very similar is the economy of the Man- 

 grove (Fig. 70), which forms impenetrable thickets on low and 

 muddy sea-shores in the tropics throughout most parts of the 

 world, extending even to the coast of Florida and Louisiana. 

 Here aerial roots spring not only from the main trunk, as in 

 tin- Pandanus, but also from the branchlets. as in the Hainan. 

 Kven the radicle of the embryo starts into growth, protrudes, 

 and attains considerable length while the fruit is still attached to 

 the branch. 



.V.i'. Aerial Rootlets for (-limiting are familiar in the Ivy of tin- 

 Old World (Iledera), Trumpet-Creeper (Teeoma radicans). and 

 our Poison Ivy (Rhus Toxicodendron) : by the adhesion of 



1 So named, not from :iny resemblance to a Pine-tree, but from a like- 

 ness of the folia i- to that of a Pine-Apple. 



FIG. 69. PaiKianus, or Screw-Pine; and in the background, 70, a Mangrove-tree 



(Khizopliora Manglr). 



