544 RELATION TO ENVIRONMENT. 



the leaf surface. Then the leaves of most tropical plants are 

 thick and firm, as in the rubber plant, banana, orchids, bromelias, 

 tillandsias, palms, etc., which afford them protection against the 

 beating action of heavy and prolonged rains. 8th, against exces- 

 sive heat and insulation, leaves of trees are protected by a vertical 

 position of the highest ones, while those lower in the canopy are 

 horizontal. 



1027. Competition in evergreen tropical forests. Thus cli- 

 mate is largely eliminated as a factor in the plant's struggle for 

 existence. Plants are left to compete among themselves for space 

 on the ground, space in the air, and for light. This competition 

 has reached its highest pressure in the evergreen tropical forest. 

 In all forest societies trees have outdistanced their rivals in the 

 struggle for space and light. This is because of their height, 

 by means of which they are enabled to reach above and beyond 

 other plants. They overcome their rivals by casting a shade 

 over them in which they cannot grow, or at least only preserve a 

 miserable existence. Only plants tolerant of shade can grow 

 beneath the forest canopy, and these rarely become successful 

 rivals of the trees. The most dangerous competitors of trees 

 under such circumstances are parasites and epiphytes and 

 lianas. 



1028. Epiphytes in the evergreen tropical forest. In all for- 

 est societies epiphytes are present, but they have reached their 

 climax in size, numbers, and effective rivalry with forest trees, 

 in the evergreen tropical forest. The darkness of the interior of 

 the forest largely prevents the development of even tolerant 

 shade plants on the forest floor. Undershrubs and herbs have, 

 as a consequence, migrated from the forest floor to the forest 

 canopy, where they are crowded in surprising masses in tier after 

 tier upon the trunks and branches, even to the upper extremity of 

 the same ; while on the uppermost leaves mosses, algae, and lichens 

 abound, as well as on the leaves and branches lower down. 

 Numerous orchids, ferns, bromelias, tillandsias, and shrubs load 

 the trees down in some cases with such a weight that the branches 

 or tree are broken down. Where there are open places in the 



