A leaf is a thin flattened structure which grows from the nodes of a stem or its branches and has a bud in its axil. Leaves are generally green although some are red or brown. The leaf is made up of 3 main parts;
External structure of a leaf
The monocot leaf
Leaf base; this is the part which attaches the leaf to the stem.
Petiole; this is the part which connects the leaf base to the leaf blade. Leaves with a petiole are called petiolate and those without are called sessile.
The leaf stalkis a characteristic of dicots while a leaf sheath is found in monocots. The leaf stalk/sheath can be hairy or smooth.
Lamina; this is the expanded and flattened portion of the leaf consisting of veins and midrib.
Texture of lamina; the lamina may be hairy or smooth. It may be hard or soft
The arrangement of veins in the lamina of a leaf is called venation. Two broad types of venation are;
In network venation, the veins in the lamina branch while intersecting to form a network. It’s a characteristic of dicots.
In this venation, the veins run side by side without branching. This is a characteristic of monocotyledonous plants.
Leaves can be classified according to whether the leaf lamina is completely divided or not divided. Two broad types are:
- Simple leaves
- Compound leaves
A simple leaf has a single lamina which isn’t divided upinto leaflets e.g. Avocado, mango, orange, hibiscus, pawpaw, cassava, etc.
Cassava and pawpaw leaves are partly divided. The lobes are not considered to be leaflets because the divisions do not reach down the midrib. They are simple digitate i.e.
A swelling at the base of the leaf stalk is called pulvinus e.g. beans and cassava. Some leaves have it while others do not have it.
A compound leaf has a lamina which is completely divided into leaflets. They resemble leaves but are not leaves because the axillary buds are absent in the axis of leaflets e.g. beans, oxalis, cassia, etc.
Types of compound leaves
i)Compound pinnate leaves
These are compound leaves with leaflets arranged either in pairs opposite one another or alternately along the midrib called rachis of the leaf. If the terminal leaflet is present, the leaf is said to be imparipinnate and if the terminal leaflet is absent,the leaf is said to be paripinnate.
ii)Compound bipinnate leaves
These are compound leaves with 2 orders of leaflets. Leaflets are further divided up to form leaf-like structures called pinnules e.g. jacaranda.
iii)Compound digitate leaves
These are compound leaves with leaflets radiating out from the end of the petiole-like fingers of the hand.
iv)Compound trifoliate leaves
These are compound leaves with only 3 leaflets. They include soya beans, oxalis and straw berry.
NB:stipules (foliar appendages) are attached to the leaf base or petiole e.g. in beans, hibiscus and cassia.
This is the outer most layer of a leaf. It acts as a skin covering the whole leaf surface. It’s covered by a transparent water porous layer of cutin called cuticle.
This cuticle allows light penetration into the leaf and prevents excess water loss from theleaf surface.
The epidermal tissue is divided into 2 according to the location on a leaf i.e. upper and lower epidermis. The upper epidermis is a single layer of brick-shaped cells covered by a thick cutin in case of terrestrial plants or land plants. Inmost plants, it does not possess stomata and if present are few.
This is so as to control the amount of water loss during transpiration process.The major function of this epidermis is to prevent evaporation of water from the leaf cells and protection of the inner cells.The lower epidermis is usually made up of one layer of cells and contains numerous openings called stomata. These stomata are protected by 2 guard cells. In water plant e.g. water lily or hyacinths, stomata are few on this side of the leaf. Some chloroplasts are present in this layer of cells.
This is located between the upper and the lower epidermis. It’s differentiated into two layers. i.e.
It’s found just below the upper epidermis. It’s made up of cylindrical shaped cells. The cells are closely packed together without air spaces. The palisade cells contain many chloroplasts which are the major sites for photosynthesis.
Chloroplasts are small and made up of proteins. They contain chlorophyll which gives green plants their colour. The chlorophyll absorbs sun light energy that is used in the process of food manufacture (photosynthesis).
ii)Spongy mesophyll layer
It’s found under the palisade layer. It consists of cell called spongy cells which are irregularly arranged. These cells are not closely arranged, and therefore have large intercellular air spaces between them. The air spaces are connected with each. There is also the sub-stomatal air chamber where the gases collect before moving out of a leaf. Spongy cells contain fewer chloroplasts than the palisade cells hence they manufacture food
These are vascular bundles consisting of veins. Each vein has a phloem for transporting manufactured food and the xylem for conducting and distributing water and mineral salts. The veins also provide mechanical support to the leaf lamina.
Functions of leaves to plants
- The major function is to manufacture food for the plant during photosynthesis.
- Leaves have stomata which allow exchange of gases i.e. O2 and CO2.
- Leaves facilitate transpiration which sometimes helps the removal of excess water within the plant.
b)Modified or secondary functions
- They store food and water for the plant e.g. the thick fleshy leaves of onions.
- Some plant leaves are useful in vegetative reproduction e.g. bryophytes.