THE TYPICAL LEAF AND ITS PARTS 29 



the leaves altogether, as in the Lathyrus aphaca (Fig. 17), 



14. Adnate stipules of 

 clover. 



15. Leaves of smilax, show- 

 ing stipular tendrils. 



16. Leafy stipules of 

 Japan quince. 



a near relative of the sweet pea, where they function as 



foliage. But under whatever form 



they occur, their true nature may 



be recognized by their position on 



each side of the base of the petiole, 



and not in the axil, or angle formed 



by the leaf with the stem. 



17. Leaf of LatAyrus 

 aphaca, reduced to a pair 

 nd a tendril 



33. Petioles. The normal use of 

 the petiole is to secure a better 

 light exposure for the leaves, but 

 like other parts of the leaf, it is 

 subject to modifications. In some vines, such as the jas- 

 mine nightshade and tropaeolum of the gardens, it is 

 twisted into a tendril for climbing. Occasionally the leaf 

 blade disappears altogether and the leaf stalk takes its 

 place, as in some of the Australian acacias frequently seen 

 in greenhouses. Simulated leaves of this kind can gen- 

 erally be distinguished by their edgewise position, the 

 blades of true leaves being usually horizontal. Other 

 instances occur, such as the onion, jonquil, hyacinth, etc., 

 where the distinction, if any exists, is difficult to make out. 



