LEAF ARRANGEMENT 



49 



S3. All the leaves are exposed 

 to light. 



"leaf-arrangement"). Make out the 

 phyllotaxy on any plant. Try it 

 on a long potato tuber. 



113. In some plants, several 

 leaves occur at one level, being 

 arranged in a circle around the 

 stem. Such leaves are said to be 

 verticillate or whorled. Leaves 

 arranged in this way are usually 

 narrow. 



114. Although a definite 

 arrangement of leaves is the rule 

 in most plants, it is subject to mod- 

 ification. On shoots that receive the light only from one 

 side or that grow in difficult positions, the arrangement may 

 not be definite. Examine shoots that grow on the under 

 side of dense tree-tops or in other partially lighted positions. 



115. The direction or 

 "hang" of the leaf is 

 usually fixed, but there 

 are some leaves that 

 change their direction be- 

 tween daylight and dark- 

 ness. Thus, leaves of 

 clover (Fig. 85), bean, 

 locust, and many related 

 plants, "sleep" at night; 

 also oxalis. It is not a 

 sleep in the sense in which 

 animals sleep, however, 

 but its function is not 

 well understood. 



116. Leaves usually ex- 

 pose one particular surface 

 to the light. This is be- 



