PLANTS AS MECHANICAL SUPPORTS. 2$ I 



Their food materials are derived entirely from the air and the 

 water which falls upon them, while the mineral salts are ob- 

 tained from the dust which has been carried by the air and 

 accumulated upon the surface of the supporting plant, or 

 among the mass of dead and decaying leaves and other de- 

 bris about the base of the epiphyte. Organic matter from 

 the decay of the older parts may also be reabsorbed. 



An adaptation to this mode of life is marked in the repro- 

 ductive bodies. Of all epiphytes the seeds or spores are either 

 light and carried by the wind ; or the seeds are sticky and 

 carried by birds and other animals ; or they are eaten by 

 birds and voided upon the trees where they are adapted to 

 germinate. 



358. Purpose. — In most cases, the use of other plants as 

 supports has been adopted to secure for the smaller and 

 weaker plants proper exposure to light for making food. 

 For example, so dense are the tropical forests that only by 

 climbing to the tree-tops or perching on the branches can the 

 lowlier plants secure an adequate amount of light. Even in 

 the temperate zone the advantage in climbing for light is 

 obvious. 



359. Summary. — Plants rooted in the soil adapt them- 

 selves to use others as mechanical supports by the develop- 

 ment of tendrils or aerial roots for climbing ; recurved leaves, 

 shoots, or prickles for clambering ; and long, swinging sensi- 

 tive shoots for twining. Others use their neighbors as the sole 

 support, being perched upon them but deriving no food from 

 them. (Those which do absorb food are parasites. See 

 *S 184). In most cases the purpose of such adaptations is to 

 secure light. 



