PLANT ORGANS AND ORGANISMS 



159 



The leaf of the Tulip Poplar or Liriodendron affords a good exam- 

 ple 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 

 on the %>per 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 3p of branch- 

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

 tissue called mesophyll, or leaf parenchyma, 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 Ferns. 



Parallel Venation is typical of the Monocotyledons, as Palms, 

 Lilies, Grasses, etc. 



Reticulate or Netted Veins characterize the Dicotyledons, as the 

 Poplar or Oak. The primary veins in these are generally pinnate while 

 the secondary ones and their branches are arranged in netted fashion. 



Pinni-veined or Feathered-veined leaves consist of a mid- vein with 

 lateral veinlets extending from mid-vein to margin at frequent 

 intervals and in a regular manner. Example: Calla. 



Palmately Veined leaves consist of a number of veins of nearly the 

 same size, radiating from petiole to margin. Example: Maple leaf. 



Veins are said to be anastomosing when they subdivide and join 

 each other, as the veins near the margin of Eucalyptus leaves. 



Leaf Insertion. The point of attachment of the leaf to the stem is 

 called the insertion. A leaf is: 



Radical, when inserted upon an underground stem. 



Cauline, when upon an aerial stem. 



Ramal, when attached directly to a branch. 



When the base of a" sessile leaf is extended completely around the 

 stem it is perfoliate, the stem appearing to pass through the blade. 

 Example: Uvularia perfoliata or Mealy Bellwort. 



