Environment and Adaptation 207 



leaves of tea-roses may have somewhat the same 

 function. The vertical position of the leaves of 

 Eucalyptus and of the Calif ornian Manzanita, and 

 the vertically placed phyllodes of the acacias al- 

 ready referred to, are also supposed to be pro- 

 tective devices against too powerful illumination. 

 It is not easy to distinguish between modifications 

 associated directly with excess of light, and those 

 concerned merely with checking transpiration, which 

 is increased by both light and high temperature. 



Climbing Plants. As light is the all-essential 

 factor for the growth of green plants, it is not 

 strange that the struggle for light in the teeming 

 vegetation of the tropics has resulted in many 

 adaptations. This explains the various types of 

 climbing plants, which, although sparingly repre- 

 sented in the temperate regions, must be seen in the 

 damp tropical jungles to appreciate their full possi- 

 bilities. These climbing plants, either by twining 

 their stems about others, or by lifting themselves up 

 by tendrils of various kinds, may climb to the tops 

 of the tallest trees, or stretch from one tree to an- 

 other, often completely smothering the lower 

 growths over which they spread themselves. Thus 

 lifted above the lower vegetation, they expand their 

 leaves and flowers in the full sunshine far aloft. 



Epiphytes. Another type of adjustment to light 

 is seen in the Epiphytes, or air plants, which also 

 are best developed in the tropical forests. In tem- 

 perate climates most of the epiphytes are humble 



