208 



LEAVES 



which are called leaflets. Leaves may be very deeply 

 divided, but still they are classed as simple unless the blade 

 is completely divided into leaflets. Compound leaves are 

 nearly always net-veined. A branching pinnate leaf is 

 said to be pinnately compound; a branching palmate leaf 



is said to be palmately 

 compound. Leaves of 

 locust or ash (see 

 page 187) are examples 

 of the former ; leaves of 

 clover are examples of 

 the latter. (See Fig- 

 ure 71.) 



C. Attachment and 

 Arrangement. — Leaves 

 are commonly attached 

 to stems by petioles. 

 The end of the petiole 

 which is attached to 

 the stem is usually 

 somewhat enlarged. 

 When this enlargement 

 appears to form a special organ it is called the pulvinus. 

 Leaf movements are generally due to change in the water 

 content of the cells of the pulvinus ; such movements are 

 characteristic of the clovers and locusts. In some groups 

 of plants, especially in the great rose and pea families, a 

 pair of structures like small leaf blades are borne at the 

 base of the petiole; these are called stipules. (See Fig- 

 ures 70 and 77.) 



Many leaves do not have petioles; their blades begin 



B 



Fig. 71. — Compound leaves: A, pinnately 

 compound leaf ol black locust ; B, palmately 

 compound leaves of red clover. The clover 

 leaves have sheathing stipules at the base of 

 the petioles. 



