THE LEAF 



31 



The leaf of the Tulip Poplar or Liriodendron affords a good example 

 of a Complete Leaf. 



Sometimes the lamina or blade is attached directly to the stem 

 by its base and is then said to be sessile. If the petiole is present, 

 petiolate. 



When leaf stipules are absent, the leaf is said to be exstipulate, 

 when present, stipulate. 



The petiole is seldom cylindrical in form, but usually channelled 



Fig. 25. — Stereogram of leaf structure. Part of a veinlet is shown on the right. 

 Intercellular spaces are shaded. {From Steiens.) 



on the upper side, flattened, or compressed. The stipules are always 

 in pairs and closely resemble the leaf in structure. 



The blade of the leaf consists of the framework, made up of branching 

 vessels of the petiole, which are woody tubes pervading the soft tissue 

 called mesophyll, or leaf parenchyme, and serve not only as supports 

 but as veins to conduct nutritive fluids. Veins are absent in simple 

 leaves such as many of the Mosses. 



Leaf Venation. — Furcate or Forked Venation is characteristic 

 of many Cryptogams. 



