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 upper side, flattened, or compressed. The stipules are always 
in pairs and closely resemble the leaf in structure. 
The blade ot the leaf consists of the framework, made up 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. 
