[Plant Parts and
Functions: stems | leaves |
buds | roots
| flowers |
fruit | seeds]
BOTANY: PLANT PARTS AND
Ch. 1, pp. 9 - 14
The blade of a leaf is the expanded, thin
structure on either side of the midrib. The blade is usually the
largest and most conspicuous part of a leaf. The petiole
is the stalk which supports the leaf blade. It varies in length
and may be lacking entirely in some cases where the leaf blade is
described as sessile or stalkless.
The principal function of leaves is to absorb sunlight
for the manufacturing of plant sugars in a process called
photosynthesis. Leaves develop as a flattened surface in order to
present a large area for efficient absorption of light energy. The
leaf is supported away from the stem by a stem-like appendage
called a petiole. The base of the petiole is attached to the stem
at the node. The small angle formed between the petiole and the
stem is called the leaf axil. An active or dormant bud or cluster
of buds is usually located in the axil.
The leaf blade is composed of several layers. On the
top and bottom is a layer of thickened, tough cells called the
epidermis. The primary function of the epidermis is protection of
leaf tissue. The way in which the cells in the epidermis are
arranged determines the texture of the leaf surface. Some leaves
have hairs that are an extension of certain cells of the
epidermis. The African violet has so many hairs that the leaf
feels like velvet.
Part of the epidermis is the cuticle, which is composed
of a waxy substance called cutin that protects the leaf from
dehydration and prevents penetration of some diseases. The amount
of cutin is a direct response to sunlight, increasing with
increasing light intensity. For this reason, plants grown in the
shade should be moved into full sunlight gradually, over a period
of a few weeks, to allow the cutin layer to increase and to
protect the leaves from the shock of rapid water loss or sun
scald. The waxy cutin also repels water and can shed pesticides if
spreader-sticker agents or soaps are not used. This is the reason
many pesticide manufacturers include some sort of spray additive
to adhere to or penetrate the cuticle.
Some epidermal cells are capable of opening and
closing. These cells guard the interior of the leaf and regulate
the passage of water, oxygen, and carbon dioxide through the leaf.
These regulatory cells are called guard cells. They protect
openings in the leaf surface called stoma. The opening and closing
of the cells are determined by the weather. Conditions that would
cause large water losses from plants (high temperature, low
humidity) stimulate guard cells to close. Mild weather conditions
leave guard cells in an open condition. Guard cells will close in
the absence of light. A large percentage of stomata occur in the
The middle layer of the leaf is the mesophyll and is
located between the upper and lower epidermis. This is the layer
in which photosynthesis occurs. The mesophyll is divided into a
dense upper layer, called the palisade layer, and a spongy lower
layer that contains a great deal of air space, called the spongy
mesophyll. The cells in these two layers contain chloroplasts
which are the actual sites of the photosynthetic process.
Types of Leaves
A number of rather distinct types of leaves occur on plants.
Leaves commonly referred to as foliage are the most common and
conspicuous, and as previously stated, serve as the
manufacturing centers where the photosynthetic activity of the
plant occurs. Scale leaves or cataphylls are found on
rhizomes and are also the small, leathery, protective leaves
which enclose and protect buds. Seed leaves, or
cotyledons, are modified leaves which are found on the embryonic
plant and commonly serve as storage organs. Spines and
tendrils, as found on barberry and pea, are specialized
modified leaves which protect the plant or assist in supporting
the stems. Storage leaves, as are found in bulbous
plants and succulents, serve as food storage organs. Other
specialized leaves include bracts, which are often
brightly colored. The showy structures on dogwoods and
poinsettias are bracts, not petals.
Conifers, (pines, firs, spruce, laurel, etc.) have "needles"
as leaves. They normally have waxy cuticles with sunken stomata
to help deter desiccation. Also, most have resin canals on
either side of the vascular system. The resin is thought to help
deter and guard against insect damage.
Venation of Leaves
The vascular bundles from the stem extend through the petiole
and spread out into the blade. The term venation refers to the
patterns in which the veins are distributed in the blade. Two
principal types of venation are parallel-veined and net-veined.
Parallel-veined leaves are those in which there
are numerous veins which run essentially parallel to each other
and are connected laterally by minute, straight veinlets. Possibly
the most common type of parallel-veining is that found in plants
of the grass family where the veins run from the base to the apex
of the leaf. Another type of parallel-venation is found in plants
such as banana, calla, and pickerelweed, where the parallel veins
run laterally from the midrib. Parallel-veined leaves occur on
plants which are part of the monocotyledon group.
Net-veined leaves, also called
reticulate-veined, have veins which branch from the main midrib(s)
and then subdivide into finer veinlets which then unite in a
complicated network. This system of enmeshed veins gives the leaf
more resistance to tearing than most parallel-veined leaves.
Net-venation may be either pinnate or palmate. In pinnate
venation, the veins extend laterally from the midrib to the edge,
as in apple, cherry and peach. Palmate venation occurs in grape
and maple leaves, where the principal veins extend outward, like
the ribs of a fan, from the petiole near the base of the leaf
blade. Net-veined leaves occur on plants which are part of the
Leaves as a Means of Identifying Plants
Leaves are useful in identifying species and varieties of
horticultural plants. The shape of the leaf blade and the type of
margin are of major importance as identifying characteristics.
Simple leaves are those in which the leaf blade is a
single continuous unit. A compound leaf is composed of several
separate leaflets arising from the same petiole. A deeply lobed
leaf may appear similar to a compound leaf, but if the
leaflets are connected by narrow bands of blade tissue it may be
classified as a simple leaf. If the leaflets have separate stalks
and if these stalks are jointed at the point of union with the
main leafstalk, the leaf is considered to be compound. Some leaves
may be doubly compound, having divisions of the leaflets.
Shape of the Leaf Blade
The following are some common shapes which are found in leaves
||Narrow, several times longer
than wide; approximately the same width.
||2 or 3 times longer than wide; tapering to an
acute or rounded apex and base.
||Egg-shaped, basal portion wide; tapering
toward the apex.
||Longer than wide; tapering
toward the apex and base.
||Heart-shaped, broadly ovate;
tapering to an acute apex, with the base turning in and
forming a notch where the petiole is attached
Shape of the Leaf Apex and Base
The following are common shapes found in leaves.
||Tapering to a long, narrow point.
||Ending in an acute angle, with a sharp, but not
||Tapering to a rounded edge.
||Arrowhead-shaped, with two pointed lower lobes.
||Having a relatively square end.
Studying leaf margins is especially useful in the
identification of certain varieties of fruit plants.
||A smooth edge with no teeth or notches.
||Having a pronounced sinuous or wavy margin.
||Having rounded teeth.
||Having teeth ending in an acute angle, pointing outward.
||Having small, sharp teeth pointing toward the apex.
||Margin cut into sharp, deep, irregular teeth or
||Incisions extend less than halfway to the midrib.
||Incisions extend more than halfway to the midrib.
Leaf Arrangement along a Stem
The various ways leaves are arranged along a stem are also used
to help identify plants. Rosulate arrangement is one in
which the basal leaves form a rosette around the stem with
extremely short nodes. Opposite leaves are positioned
across the stem from each other, two leaves at each node. Alternate
or spiral leaves are arranged in alternate steps along the stem
with only one leaf at each node. Whorled leaves are
arranged in circles along the stem.
Leaves as Food
The leaf blade is the principal edible part of several
horticultural crops including chive, collard, dandelion, endive,
kale, leaf lettuce, mustard, parsley, spinach, and Swiss chard.
The edible part of leek, onion, and Florence fennel is a cluster
of fleshy leaf bases. The petiole of the leaf is the edible
product in celery and rhubarb. In plants like Brussels sprouts,
cabbage, and head lettuce, the leaves form a large, naked bud and
are the edible product.