Leaves and Leaf
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leaf arrangement. Redbud.
compound leaf. White Ash.
compound leaf. Ohio Buckeye.
Longitudinal section of Acer stem
x40 with leaf petiole coming off to the left. Arrow shows
the abscission zone. A vein enters the leaf passing
through the abscission zone.
leaf abscission x40. The pointer shows the
abscission zone in this stem ls. The petiole is on the
left with a vein passing through the abscission zone. A
group of flower buds is on the right.
leaf abscission x100. Note the cork cells on
both the petiole surface and extending across the
leaf xs. x400. Note the dense palisade
mesophyll in this leaf that was exposed to direct
sunlight. Spongy mesophyll is in the lower half of the
leaf xs. x400. The palisade mesophyll is less
dense and the spongy mesophyll is more extensive in this
shaded leaf compared to leaves on the same tree that were
exposed to direct sun.
Stoma and guard cells xs.
on Acer lower epidermis. x1000. Two
guard cells have a small opening (stoma) between them for
gas exchange. Note the large intercellular space, part of
the spongy mesophyll, immediately inside the stoma.
Monocot leaf bases
x20 frequently form sheaths around the stem that bears
them. Such leaves have no petioles and are broadly
attached to the stem at the base of the leaf.
leaf xs.. x40. This is a beach grass leaf that
is curled in the adaxial (upper surface) direction due to
temporary drought conditions. Unlike most leaves, most
stomata are on the upper surface where they are protected
from rapid water loss by the high relative humidity
within the curled leaf. White buliform cells can be seen
at the base of the V-shaped notches on the adaxial
(inner, upper) leaf surface. The leaf uncurles when these
buliform cells fill with water, enlarge, and become
turgid when drought conditions end.
x400. The large buliform cells are on the right at the
base of a V-shaped notch on the upper epidermis of a
curled beach grass (Ammophila) leaf.
Stomata in xs can also be seen associated with
intercellullar spaces within the leaf mesophyll. When the
buliform cells fill with water and enlarge this causes
the leaf to uncurl.
xs. x40. This is a monocot leaf. Note the
mesophyll which is not easily distinguishable as palisade
vs spongy. The cuticle is somewhat thicker on the upper
vein xs. x100. The large red ringed cells are
vessels, part of the xylem. The small cells below are
phloem. In leaves the xylem of veins is always toward the
adaxial (upper) surface and the phloem toward the abaxial
(lower) surface. In monocot leaves such as Clivia
xylem-phloem placement within a vein is the only way to
identify the upper and lower surface of a leaf seen in
Stoma, guard cells, and
cuticle xs. x400. The red material is cuticle
covering the lower epidermis of a Clivia
leaf. You can see a pair of guard cells, a stoma between
them, and cuticular hairs partically covering the stoma.
The covering of cuticular hairs helps maintain high
relative humidity near the stomatal opening and reduce
rapid water loss through the stoma.
Monocot leaf epidermis
x40. You can see pairs of guard cells and other cells of
this upper epidermis. Dark dots are nuclei. Rectangular
cells are over a leaf vein. Stomata are between veins.
upper leaf epidermis x100. Note the lack of
stomata on this leaf surface. The black dots are nuclei.
Unlike this species, many dicot plants have a few stomata
on their upper surface. Most of a dicot leaf's stomata
are usually found on the lower surface.
lower leaf epidermis x100. There are lots of
pairs of guard cells, each surrounding a stoma.
Stoma, surface view
x400. This shows a single pair of guard cells surrounding
a stoma on the lower epidermis of Tradescantia.
The large dark bodies are nuclei. The lighter circular
bodies within the guard cells are chloroplasts. In most
plants guard cells are the only cells within a leaf's
epidermis that contain chloroplasts. The production of
sugar by photosynthesis in the daytime within guard cells
and not in adjacent epidermal cells causes water to
diffuse into the guard cells. This makes the guard cells
swell, opening the stoma.
leaf xs. x40. A typical dicot leaf. Note the
large central vein as well as the palisade and spongy
Mid vein of a Ligustrum
leaf mesophyll and small veins x100. Note that
one of the small veins is in good cross section and
another is very oblique. This shows that the three
visible veins are not parallel to each other and thus
that the leaf does not have parallel venation. This means
that the leaf must have net venation and thus must be a
Net venation in
paradermal view x100. This Ligustrum
leaf is sectioned parallel to the leaf surface through
the spongy mesophyll. You can see the net-like
arrangement of the smallest veins which is the basis for
the name "net venation".
leaf xs x100. This corn leaf is typical of
monocot leaves with their characteristic parallel venation.
You can tell that the venation is parallel because the
veins are all shown in nice cross section. The only way
to identify the true upper surface of the leaf is to look
for the xylem in the veins, which is always positioned
toward the upper epidermis. There are stomata visible in
both the upper and lower epidermis. Some large buliform
cells are on the right side of the upper epidermis.
Shrinkage of these buliform cells would cause the leaf to
leaf vein x400. At the top and bottom of the
vein are groups of fibers. The secondary cell walls of
the xylem cells stain red, including the two very large
diameter vessels. Small sieve tubes and companion cells
in the phloem stain dark green.
buliform cells in the upper epidermis of a
leaf x400. The tiny cells in the two small veins are
xs x100. One of a group of two leaves near the
leaf base. The outer mesophyll is very dense and contains
a resin duct visible on the right. One of several visible
stomata can be seen on the right side of the bottom
to the Plant Biology index page.
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