FALLING OF THE LEAF 



97 



evergreen. Most leaves fall by breaking off at the lower 

 end of the petiole with a distinct joint or articulation. 

 There are many leaves, however, which wither and hang 



138. Elliptic leaf 

 of purple beech. 



Ovate serrate leaf 



cf hibiscus. 



140. Orbicular lobed leaves. 



on the plant nntil torn off by the wind : of snch are the 

 leaves of grasses, sedges, lilies, orchids, and other plants 

 known as monocotyledons (Chap. XXIII). Most leaves 

 of this character are parallel -veined. Consult 439. 



205. Leaves also die and fall from lack of light. Ob- 

 serve the yellow and weak leaves in a dense tree -top or in 

 any thicket. Why do the lower leaves 

 die on house-plants? Note the carpet 

 of needles under the pines. All ever- 

 greens shed their leaves after a time. 

 Counting back from the tip of a pine 

 or spruce shoot, determine how many 

 years the leaves persist (Fig. 145). 

 In some spruces a few leaves may be 

 found on branches ten or more years 

 old. Leaves usually persist longest 



in the lightest positions (Fig. 77). ^^^- Tmucate leaf of tulip-tree 



206. Although the forms and positions of leaves often 

 have direct relation to the places and conditions in which 



