Orientation 



209 



gravity draws the organism through the medium, as is seen in a 

 "floating" butterfly or in slowly sinking plankton. 



Superimposed upon the primary orientation, many secondary re- 

 sponses to orienting influences take place. The tips of green plants 

 grow toward the light and hence exhibit positive phototropism. The 

 leaves are oriented generally at right angles to the incident radiation, 

 thus receiving maximum illumination, and they sometimes form a 

 symmetrical pattern (Fig. 6.11) and sometimes a mosaic, as is dis- 

 played by ivy leaves on a wall. 



Fig. 6.11. Alternating position and horizontal orientation of leaves on shoot of 



Norway maple, resulting in maximum exposiure to sunlight. (Shipley, 1925, after 



Kerner, Copyright, Cambridge Univ. Press.) 



In such species as the sunflower the top portion of the plant or its 

 leaves are turned by turgor changes during the course of the day, 

 keeping always in the direction of the sun. In regions where light 

 and heat from the sun are excessive, the leaves of some plants, such 

 as the compass plant, SilpJiiiun laciniatum, are oriented so as to 

 present their edges toward the sun. These examples will suffice to 



