Leaves provide trees with all their food because they turn
sunlight into food energy.
Chlorophyll makes this energy transformation possible.
Leaves also make the oxygen in the air that we breathe.
Chlorophyll is a pigment found in the cells of leaves which
is formed only in the presence of light and is the substance that
colors plants green. Chorophyll is contained in chloroplasts
and has the property of capturing light energy.
The process of
Photosynthesis (click on
Photosynthesis to view diagram):
- Sunlight shines through the top of the leaf and reaches the
next layer of cells. The light energy is trapped by the
chlorophyll in the
chloroplasts. In the chloroplasts, a process that uses
water changes the light energy into a kind of chemical energy. This
chemical energy is stored in the chloroplasts.
- The chloroplasts use the chemical energy to make food.
Air enters the leaf through the
stomata and moves into tiny spaces around the food-making
cells in the leaf. Carbon dioxide from the air passes through the
cell walls and membranes of the cells. Carbon dioxide enters
the chloroplasts where the previously stored chemical energy
converts the carbon dioxide into sugar.
- Tubes in the plant carry sugar from the leaf cells to other
parts of the plant, such as roots, stems, and fruits. Cells in
these parts of the tree store some of the sugar.
Three Main Parts to a Leaf
base which is the point at which the leaf is joined to the
petiole is the thin section joining the base to the lamina -
it is generally cylindrical or semicircular in form. The lamina
blade is the wide part of the leaf.
Leaves can be many different shapes. Primarily, leaves are
simple or compound.
All blades are attached to a single leafstem. Where the
leafstem attaches to the twig there is a bud. View a
Leaves may be arranged on the stem either in an alternate
arrangement - leaves that are staggered or not placed directly
across from each other on the twig; or in an opposite arrangement -
2 or 3 leaves that are directly across from each other on the same
twig. View a diagram
The margin may
be entire, singly-toothed, doubly-toothed, or lobed. View a diagram of simple
leaf margin structures.
Compound leaves may be
pinnate. View a diagram
The form of leaves is related with all their functions and their
environment. In addition to photosynthesis, the leaf also carries
out all the other exchanges with the atmosphere. It is through the
leaf that the plant "breathes" (absorbs oxygen and gives off carbon
dioxide plus energy) and transpires. Epidermic tissues in the
stomata. In most plants the stomata are located on the
underside of the leaves. Their function is regulated so that
plants living in dry climates have a substantially smaller number
of them than those in humid climates, where they are numerous and
prominent. Where humidity is low the stomata may actually be
recessed or partly protected by soft hairs which can prevent
All trees produce flowers of some kind. The main parts of
a generalized flower are diagramed below. Flower parts occur
in a standard arrangement of 4 whorls around the flower stem, or
pedicel or pedicuncle. The
outermost whorl is the
calyx whose job is to protect the developing flower.
The calyx is usually green and its separate parts (
sepals) are what we would recognize as the outside covering
of a bud, as in a rose. As the flower opens, the sepals are
pushed apart by the petals.
The whorl of petals is collectively called the corolla. Petals
are usually bright-colored to attract pollinators to visit
flowers. Inside the petals, the next whorl consists of the
male parts of the flowers, the stamens.
The stamen's job is to make pollen. Tiny, microscopic pollen
grains carry sperm from flower to flower. Stamens have two
anthers. Filaments are thread-like structures like the
filaments in a light bulb, that support the anthers out from the
flower base. The pollen grains are produced in the sac-like
anthers, which open in intricate ways to release pollen.
A generalized flower would also have female parts as the center
whorl. These are known as the
pistil or carpel. Like stamens, they are made of
style extends from the center of the flower and supports the
stigma, the sticky surface on which the pollen adheres
during pollination. At the base of the carpel in the center
of the flower is the
ovary, where eggs are borne.
This is a generalized description; in nature, there are many
variations on the general theme. Some plants have separate
sexes, so that an individual bears only flowers with male (stamens)
or femal (carpels) parts, not both. Some flowers have
colorful sepals, some have no petals, some have elaborate stamens
that look like petals. Different species also may have
different numbers of parts in each whorl. View a
Life Cycle of Flowering
Different trees produce different kinds of seeds. Some
produce seeds in a fleshy covering - a fruit or berry, while other
trees produce seeds tucked into the folds of cones or
catkins. Other trees produce seeds with wings as well as some
produce seed inside nuts or pods. View seed
Tree seeds vary greatly in size - some of the largest trees have
the smallest seeds. Tree fruits in various forms aid in the
dispersal of seeds. Fleshy fruits are eaten by animals, from whose
bodies the seeds may later be dropped. Winged fruits are spread by
wind. "Seed trees" left after lumbering will reforest the land.
Conifers (pine and fir trees) make their seeds in cones.
Pollen cones make pollen and the pollen spreads through the air
like dust. Seed cones are sticky and the pollen grains stick
to a seed cone. Tubes then grow from the pollen grains into
egg cells in the cone. A sperm enters an egg cell and
fertilizes the egg. Each fertilized egg cell grows into an
embryo and is housed in a seed. View Life
Twigs & Buds
Like a branch, a twig's job is to support and transport.
Twigs support the leaves which have the job of making food.
Because leaves need to collect the Sun's rays to make food, they
must be held up as high as possible by the twigs. Twigs also
transport water to the leaves and sugars from the leaves, using
tiny tubes. View a twig diagram or view a bud diagram