Glossary:
1. air space
2. chlorophyll
3. compound leaf
4. compound leaf
5. entire
6. epidermis
7. guard cell
8. lamina
9. mesophyll
10. midrib
11 palisade mesophyll
12. petiole
13 photosynthesis
14 spongy mesophyll
15 stipule
16 stoma
17. vein (vascular bundle)
Leaf is an above-ground plant organ specialized for the process of photosynthesis.
Leaves are typically flat (laminar) and thin which evolved as a means to maximise
the surface area directly exposed to light. The internal organisation of leaves has
evolved to maximise exposure of the photosynthetic organelles, the chloroplasts, to
light and to increase the absorption of carbon dioxide, all of which assist
photosynthesis. These adaptations are at the expense of water loss and most leaves
have stomata which regulate carbon dioxide, oxygen and water vapour exchange
with the atmosphere.
The shape and structure of leaves varies considerably depending on climate,
primarily due to the availability of light and potential for water loss due to
temperature and humidity. Leaves are also the primary site, in most plants, where
transpiration and guttation take place. Leaves can also store food and water, and
are modified in some plants for these purposes. The concentration of
photosynthesis in leaves makes them rich in protein, minerals and sugars. Because
of their nutritional value leaves are prominent in the diet of many animals,
including humans as leaf vegetables.
Plant Parts - Leaves
Leaves are the food making factories of green plants. Leaves come in many
different shapes and sizes. Leaves can be simple. They are made of a single leaf
blade connected by a petiole to the stem. An oak leaf or a maple leaf are examples.
A compound leaf is a leaf made up of separate leaflets attached by a petiole to the
stem like an ash or a locust.
Leaves are made to catch light and have openings to allow water and air to come
and go. The outer surface of the leaf has a waxy coating called a cuticle which
protects the leaf. Veins carry water and nutrients within the leaf. Leaves are the site
of the food making process called photosynthesis. In this process, carbon dioxide
and water in the presence of chlorophyll (the green pigment) and light energy are
changed into glucose (a sugar). This energy rich sugar is the source of food used
by most plants. Photosynthesis is unique to green plants! Photosynthesis supplies
food for the plant and oxygen for other forms of life. A green plant helped make
the oxygen you are
breathing
today.
Leaves and Leaf Anatomy
Leaf Function: Leaves are the powerhouse of plants. In most plants, leaves are
the major site of food production for the plant. Structures within a leaf convert the
energy in sunlight into chemical energy that the plant can use as food. Chlorophyll is
the molecule in leaves that uses the energy in sunlight to turn water (H2O) and carbon
dioxide gas (CO2) into sugar and oxygen gas (O2). This process is called
photosynthesis.
Leaf Structure: A leaf is made of many layers that are sandwiched between two
layers of tough skin cells (epidermis). The epidermis also secretes a waxy substance
called the cuticle. These layers protect the leaf from insects, bacteria, and other pests.
Among the epidermal cells are pairs of sausage-shaped guard cells. Each pair of guard
cells forms a pore (called stoma; the plural is stomata). Gases enter and exit the leaf
through the stomata. Most food production takes place in elongated cells called
palisade mesophyll. Gas exchange occurs in the air spaces between the oddly-shaped
cells of the spongy mesophyll. Veins support the leaf and are filled with vessels that
transport food, water, and minerals to the plant.
Leaf Margins: Leaves come in many sizes and shapes; they are often used to
help identify plants. Some leaves are flat and wide; others are spiky and thin. Plant
spines (like cactus spines) are actually modified leaves.
Leaf Margins