Download FUNCTIONS OF LEAVES

FUNCTIONS OF LEAVES
What are leaves
Leaves are an essential plant organ because they play a vital part in the nutrition of plants. Leaves vary
considerably in shape and form, but they are all have the same function.
A leaf is a lateral outgrowth from a stem, arises at a node and possesses a bud in its axil. The leaf has
two main parts; the blade (lamina) and the stalk (petiole). When there is no petiole the leaf is said to be
sessile.
The lamina is supported by a system of veins, and when a vein or midrib with numerous branches forms
a network, the venation is said to be reticulate. This pattern is typical of dicotyledons.
When several veins of approximately equal size run parallel to each other the venation is said to be
parallel. This arrangement is typical of monocotyledons.
Leaves may be simple or compound (the latter has more than one blade and the component leaflets are
called pinnae). Pinnae may be further divided into pinnules.
Leaf arrangement around the stem, the leaf shape, leaf base, leaf apex and leaf margin are all
morphological characters used in identification.
Functions of leaves
Photosynthesis
The production of food (sugars) using sunlight, carbon dioxide and water.
Transpiration
The process by which plants lose water in the form of water vapour.
Leaves have wide variations in morphology. In many instances there are adaptations to the particular
climatic conditions of the plant's environment, so that an adequate surface for photosynthesis is
combined with features to minimize excessive water storage. The thick fleshy bases of leaves that make
up most of the Daffodil bulb accumulate large quantities of food. In sweet peas, tendrils are the
equivalent of leaflets.
External leaf structure
Lamina
the blade of the leaf.
Margin
the edge of the leaf.
Veins
conducting tissue of the leaf. Contains xylem which conducts water and minerals from
the roots and phloem which moves sugars and other products from the leaves to the rest
of the plant.
Midrib
the central conducting vein in dicots.
Petiole
the stalk which holds the leaf.
Leaf base
point at which the lamina attaches to petiole.
Stipule
a small leaf-like appendage at the base of the petiole.
Node
the slightly enlarge portion of the stem where leaves and buds arise, and where
branches originate.
Internode
the region of the stem between two successive nodes.
Axil
the upper angle between the petiole of a leaf and the stem from which it grows.
Axillary bud.
an embryonic shoot which lies at the junction of the stem and petiole
Apex.
is the tip or end farthest from the base or point of attachment
How do we tell if we are looking at a leaf or leaflet?
The easiest way to determine this is to look at the position of the buds. Buds occur in the axils of leaves,
not in the axils of leaflets. Some wattles have phyllodes rather than leaves. The petiole is enlarged and
usually flattened and performs the functions of a leaf.
Internal leaf structure
In most leaves there are three types of tissue. These are as follows:
Epidermis
On both the upper and lower surface of the leaf. Usually consist of a single layer of tightly fitting cells. It
is covered with a waxy coating, the cuticle, that helps to prevent water loss and hence drying out of the
leaf.
Stomata (singular stoma or stomate) are pores that occur in the epidermis, usually more abundantly in
the lower epidermis. Carbon dioxide and oxygen pass in and out through these pores. Water that is
evaporated through transpiration also passes out through the stomata. Two guard cells surround each
stomate. The guard cells swell and shrink to regulate the opening and closing or the stomate. Hairs may
also grow out from the epidermis.
Mesophyll tissue
The mesophyll tissue is composed of parenchyma cells and forms the two layers between the upper and
lower epidermis. The palisade mesophyll cells are tall and cylindrical and contain many chloroplasts (with
the pigment chlorophyll) for photosynthesis. The spongy mesophyll cells are below the palisade
mesophyll, are irregular in shape, and contain fewer chloroplasts. They have large air spaces between
them. Carbon dioxide and oxygen move through this layer. This means that most of the food-producing
cells have free access to these gases.
Veins or vascular bundles
These are the conducting tissues. Xylem occupies the upper part of the veins, phloem the lower part.
The veins transport minerals and water (xylem), and products of photosynthesis (phloem).