48 



LEAVES 



broken off but " grow off " by the formation of definite cleav- 

 age planes as in the shedding of leaves for winter. This self- 

 pruning process is very common among willows, poplars, cotton- 

 woods and many other trees. (Fig. 33.) 



Parts of Leaves. — A simple, typical leaf (Fig. 34) has a 

 blade or lamella supported by a framework composed of libs 



Fig. 34. — Simple net-veined leaf. 



Fig. 35. — Simple parallel-veined leaf. 



and veins. The principal rib is known as the midrib and is a 

 continuation of the leaf stem or petiole. Leaves which do not 

 have petioles are sessile. These ribs and veins are continuations 

 of the woody bundles found in the stems and serve not only to 

 support the leaf but as channels through which the water and raw 

 food materials pass to the leaf, and through which more com- 

 plex food substances' pass to other parts of the plant. Minute 

 structures, known as stipules, frequently are found at the bases 

 of the petioles. They are extremely variable in form and 

 character. Sometimes they are leaf-like and may fall soon after 

 the unfolding of the leaves as in the willows ; sometimes they are 



