The Basic Biology of Plant Leaves
Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are
usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors,
caused by other plant pigments that mask the green chlorophyll.
The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species
maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests
have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have
a smaller surface area to minimize water loss.
Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are
attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant
stem are called sessile leaves. Small green appendages usually found at the base of the petiole are known as
stipules. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce
veins of vascular tissue. The edge of the leaf is called the margin.
Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern.
Monocots and dicots differ in their patterns of venation. Monocots have parallel venation; the veins run in straight
lines across the length of the leaf without converging at a point. In dicots, however, the veins of the leaf have a
net-like appearance, forming a pattern known as reticulate venation. One extant plant, the Ginkgo biloba, has
dichotomous venation where the veins fork.
(a) Tulip (Tulipa), a monocot, has leaves with parallel venation. The netlike venation in this (b) linden (Tilia
cordata) leaf distinguishes it as a dicot. The (c) Ginkgo biloba tree has dichotomous venation.
The arrangement of leaves on a stem is known as phyllotaxy. The number and placement of a plant’s leaves will
vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are
classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said
to be either alternate—meaning the leaves alternate on each side of the stem in a flat plane—or spiral, meaning the
leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point,
with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a
node, the leaf arrangement is classified as whorled.
Leaves may be simple or compound. In a simple leaf, the blade is either completely undivided—as in the banana
leaf—or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a compound leaf, the leaf
blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk, but is
attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating outwards
from one point Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant Schefflera
sp. (common name “umbrella plant”). Pinnately compound leaves take their name from their feather-like
appearance; the leaflets are arranged along the midrib, as in rose leaves, or the leaves of hickory, pecan, ash, or
Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant
(Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may
be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (Aesculus
hippocastanum), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from
the midrib, as on a scrub hickory (Carya floridana). The (d) honey locust has double compound leaves, in which
leaflets branch from the veins.
The outermost layer of the leaf is the epidermis. It is present on both sides of the leaf
and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the
regulation of gas exchange. It contains stomata: openings through which the exchange of gases takes place.
Two guard cells surround each stoma, regulating its opening and closing.
The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the
epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer
known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate
of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface.
Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or
bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface
Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or
“middle leaf.” The mesophyll of most leaves typically contains two arrangements of parenchyma
cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (also called the palisade
mesophyll) has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the
palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy
parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous
exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular
spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many
chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.
The central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an
upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy layer, comprised of loosely
packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all
aerial surfaces of land plants to minimize water loss. These leaf layers are clearly visible in the scanning electron
micrograph. The numerous small bumps in the palisade parenchyma cells are chloroplasts.
Chloroplasts are also present in the spongy parenchyma, but are not as obvious. The bumps protruding from the
lower surface of the leave are glandular trichomes.
Like the stem, the leaf contains vascular bundles composed of xylem and phloem. The xylem
consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem
transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter
how large or small, always contains both xylem and phloem tissues.
Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced
in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area:
two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to
spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves
with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels
||Carotene and Lycopene
||Yellow, Orange, Red
||Red, Blue, Purple, Magenta
Why do leaves change color in the fall? When leaves appear green, it is because they contain an abundance of
chlorophyll. There is so much chlorophyll in an active leaf that the green masks other pigment colors. Light
regulates chlorophyll production, so as autumn days grow shorter, less chlorophyll is produced. The
decomposition rate of chlorophyll remains constant, so the green color starts to fade from leaves.
At the same time, surging sugar concentrations cause increased production of anthocyanin pigments. Leaves
containing primarily anthocyanins will appear red. Carotenoids are another class of pigments found in some
leaves. Carotenoid production is not dependent on light, so levels aren't diminished by shortened days.
Carotenoids can be orange, yellow, or red, but most of these pigments found in leaves are yellow. Leaves with
good amounts of both anthocyanins and carotenoids will appear orange.
Leaves with carotenoids but little or no anthocyanin will appear yellow. In the absence of these pigments, other
plant chemicals also can affect leaf color. An example includes tannins, which are responsible for the brownish
color of some oak leaves.
Temperature affects the rate of chemical reactions, including those in leaves, so it plays a part in leaf color.
However, it's mainly light levels that are responsible for fall foliage colors.
Sunny autumn days are needed for the brightest color displays, since anthocyanins require light. Overcast days
will lead to more yellows and browns.
The main pigment classes responsible for leaf color are porphyrins, carotenoids, and flavonoids. The color that
we perceive depends on the amount and types of the pigments that are present. Chemical interactions within the
plant, particularly in response to acidity (pH) also affect the leaf color.
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