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LEAVES
I. Angiosperm leaves
A. Megaphylls, derived from webbing and fusion of several
branches
B. However, since they are modified for gathering the sun's energy and
converting it to carbohydrates, their internal anatomy is a bit different than
stems
C. Leaves are merely extensions of the shoot and the procambial
system is continuous with that of the stem
D. A vascular bundle that extends from a stem bundle to the
base of a leaf, where it connects with the vascular system of the leaf, is
called a leaf trace and the gap in the ground tissue left behind is
called a leaf gap
II. Leaf morphology - the following will be drawn on the board in
class. See also the illustrated web pages at Texas
A&M University
A. Lamina (blade) - the expanded, photosynthetic portion
of the leaf
B. Petiole - the stalk of a leaf
1. Sessile - a leaf that lacks a petiole
C. Leaves may be simple or compound, i.e.
they may have one or more lamina
1. The individual laminas of a compound leaf are called leaflets
and they may be arranged either
a. palmately or
b. pinnately
2. A bud defines a leaf
a. Except for a brief period in the spring leaves have a bud in the
upper axil, where they attach to the stem
b. The key to determining whether or not a leaf is simple or compound
is to look for the this bud
c. Everything external to this bud is the leaf, regardless of how many
laminas there are
d. If more than one lamina then it is a compound leaf
D. Leave arrangement - leaves and leaflets may be arranged:
1. Opposite - 2 per node
2. Alternate - 1 per node
3. Whorled - more than 2 per node
E. Stipule - a small leaf-like structure where the leaf is
attached to the shoot
1. Some plants have them and some don't
2. They are especially conspicuous on rose bushes
F. Most monocots have sheathing leaf bases
G. Leaf characteristics are often used to aid in the identification of
plants. Diagnostic features include:
1. Leaf arrangement, e.g. opposite, alternate or whorled
2. Simple vs. compound
3. Lamina shape, base and apex (tip) shapes
4. Characteristics of the margins, e.g. toothed, lobed, entire, etc.
5. Different types of trichomes (hairs) may also be present on one or
both surfaces
III. Leaf "ecology"
A. Anatomy of leaves is dependent upon the particular habitat in which they
grow. There are three major types of plant habitats, defined by water
availability, and the plants that grow in them may be:
1. Xerophytes - adapted to very dry habitats
2. Hydrophytes - adapted to moist habitats, often floating or
submerged
3. Mesophytes - adapted to moderate habitats
IV. Leaf anatomy (internal structure)
A. Epidermis - Since leaves are the result of
primary growth their dermal system consists of an epidermis
1. Leaves have both an upper and lower epidermis consisting
of compactly arranged cells. The epidermis is often covered by a cuticle
(waxy or fatty layer) to reduce water loss. Hairs called trichomes, may also
be present
2. Most leaves have an epidermis only one cell thick.
However, some xerophytes may have up to three layers of cells.
3. Stomata (pores for gas exchange) may
occur on both leaf surfaces, but they are usually more numerous on the lower
surface
a. Floating leaves usually have stomata only on the upper
surfaces and submerged leaves usually lack stomata
b. The stomata of dicots are scattered whereas the stomata
of monocots are arranged in parallel rows
B. Mesophyll - the mesophyll (middle, leaf) is the ground tissue of
the leaf
1. It contains many intercellular air spaces for the diffusion of gasses
as well as chloroplasts for photosynthesis. It usually consists of two
distinct layers:
a. Palisade layer - the cells are arranged in a
compact, columnar fashion. Most of the photosynthetic activity takes place
in the palisade layer. Therefore its cells have many more chloroplasts
than the spongy layer
b. Spongy layer - the cells are irregularly
shaped and not as compact as the palisade layer, giving them a
"spongy" appearance. The "spongy" appearance is
particularly evident in floating aquatic leaves.
Cells of the spongy layer have fewer chloroplasts than the palisade layer
and photosynthetic activity is lower than in the palisade layer.
C. Vascular bundles - the veins of leaves are extensions of the stem
vascular bundles
1. Vascular bundles contain xylem (usually towards the upper surface) and
phloem (usually towards lower surface)
2. In dicots veins are usually arranged in a net-like fashion, whereas in
monocots they are arranged in parallel rows
3. Much like the human vascular system, leaf veins spread out in
progressively smaller units, usually from a large, central midvein
4. The vascular tissues of veins are usually enclosed by parenchyma cells
or compactly arranged bundle sheath cells
a. This ensures that the vascular system is not exposed to the air and
that all substances entering and leaving the vascular tissues must pass
through the sheath (somewhat like the endodermis of root)
5. The larger veins of dicots are usually bordered by collenchyma or
sclerenchyma for strength. Monocot veins are usually bordered by fibers
V. C4 leaves
A. Shortly you will learn that there are two major types of
photosynthesis called C3 and C4
B. The majority of plants are C3 and their
anatomy is similar to that described above
C. In C4 grasses the mesophyll cells and
bundle-sheath cells form two concentric layers around the vascular bundles
1. The bundle-sheath cells are large parenchyma cells with
many large chloroplasts
2. Photosynthesis takes place within these two concentric
layers.
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