THE PLANT AS A HOST 43 



for absorbing moisture from the air, the skin 

 being of a peculiarly spongy nature. There is 

 little doubt that in many cases the debris, com- 

 posed of rotting leaves, which collects in the forks 

 of the branches, forms a good rooting medium. 

 A large group of plants in tropical America 

 is formed by the many species of Tillandsia. 

 One of the most curious of these, known as 

 " Spaniard's Beard" (T. usneoides), offers an 

 example of a conspicuous and widely distributed 

 epiphyte. This species has no real roots, but 

 drapes itself round the branches of its host by 

 means of its long thread-like stems of a silver- 

 grey colour. A good proof that the Tillandsias 

 are not in the least dependent upon soil is 

 to be seen in the case of one of the species 

 at Kew. This example, which comes from 

 Trinidad, has been suspended from the roof of 

 a greenhouse for a long time. The roots of the 

 specimen are simply in the warm, damp atmo- 

 sphere of the hot-house, yet the plant thrives per- 

 fectly well. The great Aroids of the genus Philo- 

 dendron, and the strange Monsteras, perch high 

 in the branches of trees, whence they send down 

 long aerial roots. Although a great deal of 

 moisture is absorbed from the air by these pro- 

 cesses, they not infrequently find a final resting- 



