Dicot and Monocot Leaves
Angiosperm leaves have many shapes and sizes (see Photographic Atlas of
Plant Anatomy by Curtis, Lersten and Nowak, Leaf External Morphology -
http://botweb.uwsp.edu/anatomy/).
In general, dicot leaves exhibit net veination and have a differentiated
mesophyll with a palisade layer associated with the adaxial (generally upper)
surface and a spongy layer associated with the abaxial (generally lower)
surface as shown below in the Ligustrum leaf.
Oc
casionally the palisade layer is found both top and bottom with a spongy layer
in between. The leaf epidermis is typically one cell layer thick but as with most
plant characteristics there are exceptions. You can note the multiple epidermal
layers in the Oleander leaf shown below.
1
Stomates are variably located. In some leafs they are found in the abaxial
surface only, the adaxial surface only, both surfaces, or as in the Elodea leaf
and other submerged aquatic leafs, stomata are absent. Often they are located
at the surface of the leaf but in the Oleadnder leaf shown above, they are in
cavities or crypts.
There are additional features of the dicot leaf to consider. Given that dicot
leaves generally exhibit net veination, secondary and tertiary veins are seen in
all views in a cross section of the leaf, as noted in the Ligustrum leaf shown
previously. However, the main vein or midvein will always be seen in cross
section (see the Ligustrum midvein shown below).
The veins of leaves typically lack a vascular cambium but one can be seen
quite clearly in the bundle of the midvein. However, even when present cells of
the vascular cambium undergo few if any divisions, thus the leaf generally
contains only primary tissue. In addition, the bundles may or may not be
surrounded by an endodermis.
2
Monocot leaves in general differ from the leaves of dicots in several ways.
3
As noted in the Zea leaf shown above, the mesophyll is not differentiated but
consists solely of spongy mesophyll tissue. As the veins typically all parallel to
each other in the blade, all appear in a cross sectional view in a cross section of
the leaf. Some monocot leaves also exhibit a type of leaf anatomy shown above.
The vascular bundles are surrounded by two bundles sheaths, an inner one
called the mestome sheath that can be seen here as small thick walled cells.
The mestome sheath can be considered to be an endodermis and the cells that
comprise it are non-photosynthetic. Surrounding the mestome sheath is an
outer sheath of larger thin walled parenchyma cells. In members of the grass
family, such as Zea, the outer parenchyma sheath is encircled by mesophyll
cells. This pattern (Kranz anatomy) is associated with C4 photosynthesis, and
while not all C4 plants exhibit this pattern, all plants that exhibit this pattern
have C4 photosynthesis. A comparison of C3 and C4 plants is shown below.
Comparison of leaf anatomy of C3 and C4 plants. Reproduced through the
courtesy of Dr. Stephen Grace, University of Arkansas.
