BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



229 



But to come back to temperature: as in 

 forests, the eflFects produced by low-growing 

 vegetation depend on the height of the 

 vegetation, its density, and on the amount 

 of interference with the penetration of the 

 sun's rays. Thus a stand of a broad-leafed 

 plant like the snapdragon (Antirrhinum) 



has a different effect from that produced by 

 stands of grasses. 



Plants with flat, horizontal leaves permit 

 the sun's rays to penetrate only with diffi- 

 culty, although the air may fall or rise 

 readil)' with changes in density. The upper 

 surface of the vegetation practically coin- 



E. 35 



WEDNESDAY 



7T 1 — — I 1 1 1 1 1 1 1 1 I I ; 



Fig. 58. Temperature record in degrees C. at three levels of the Barro Colorado Island tropi- 

 cal rain forest. D, 9 inches above the ground; E, 55 feet; and F, 86 feet. Graphs B and C 

 represent the corresponding cycles of evaporation (in artificial units), respectively, from 4 

 inches above the ground and at 75 feet. Graph A shovi^s the barometric pressure in inches. All 

 are for the same week in March. ( Redrawn from Allee. ) 



