60 



PLANT RELATIONS. 



greatly that the working power of the leaves is reduced. 

 At no very great depth of water a limit is reached, beyond 

 which the light is no longer able to be of service to the 



leaves in their work. 



Fig. 50. A vine or liana climbing 

 the trunk of a tree. The leaves 

 are all adjusted to face the light 

 and to avoid shading one an- 

 other as far aB possible. 



Hence it is that water plants are 

 restricted to the surface of the 

 water, or to shoal places ; and in 

 such places vegetation is very 

 abundant. Water is so serious 

 an impediment to light that very 

 many plants bring their working 

 leaves to the surface and float 

 them, as seen in water lilies, thus 

 obtaining light of undiminished 

 intensity. 



46. The climbing type. — Climb- 

 ing stems are developed especially 

 in the tropics, where the vegeta- 

 tion is so dense and overshadow- 

 ing that many stems have learned 

 to climb upon the bodies of other 

 plants, and so spread their leaves 

 in better light (see Figs. 50, 55, 

 98, 212). Great woody vines 

 fairly interlace the vegetation of 

 tropical forests, and are known 

 as "lianas," or "lianes." The 

 same habit is noticeable, also, in 

 our temperate vegetation, but it 

 is by no means so extensively dis- 

 played as in the tropics. There 

 are a good many forms of climb- 

 ing stems. Eemembering that 

 the habit refers to one stem de- 

 pending upon another for 

 mechanical support, we may in- 

 clude many hedge plants in the 



