Leaf is a green, flat, thin, expanded, dissimilar, lateral appendage of the stem.
It is borne at the node and bears a bud in its axil. It is exogenous in origin and develops from the leaf primordium of shoot apex. The green colour of leaf is due to the presence of chloroplast, which helps in synthesis of organic food. The green photosynthesis leaves of a plant are collectively called foliage leaves. They develop on the stem in acropetal succession.
Characteristics of leaf:
1. Leaf is lateral, dissimilar appendage of the stem.
2. Leaf is always borne at the node of stem.
3. Generally there is an axillary bud in the axil of a leaf.
4. It is exogenous in origin and develops from the swollen leaf primordium of the growing apex.
5. Growth of the leaf is limited.
6. The leaves do not possess any apical bud or a regular growing point.
7. The leaf has a leaf base, petiole, leaf lamina and sometimes, it may possess two lateral outgrowths at the base called stipules.
8. The leaf lamina is traversed by prominent vascular strands called veins.
Parts of Leaf:
A) Leaf base or Hypopodium:
1. The basal part of the leaf by which it is attached to the stem is called leaf base. Generally it is inconspicuous and not seen clearly.
2. In some cases the leaf base becomes swollen or acquires cushion-like form which is termed as pulvinus (e.g. Mango).
3. In many plants the leaf base is provided with a pair of small outgrowths called stipules. A leaf with stipules is termed as stipulate leaf (Rose, China rose, etc) and in the absence of stipules it is termed as exstipulate leaf (Mango). The stipules are commonly found in Dicotyledons.
4. In some grasses or monocots an additional outgrowth is present between the leaf base and lamina. It is called ligule. The leaf with ligule is termed as ligulate leaf (Jowar) and a leaf without ligule is known as eligulate leaf (Banana).
5. Sometimes small stipule like outgrowths are present at the base of the leaflets of a compound leaf called stipules (Clitoria).
B) Petiole or Leaf stalk or Mesopodium:
1. Petiole is a long or short, cylindrical or sub-cylindrical part of the leaf, which joins the leaf lamina to the leafbase.
2. Petiole pushes the leaf lamina away from the stem to get optimum sunlight.
3. A leaf with petiole is termed as petiolate leaf (Mango) and the one without petiole is said to be non-petiole or sessile leaf (Canna).
4. The petiole conducts water and solutes from the stem to the leaf blade. If also transports organic material from leaf lamina to stem. In addition, it projects the leaf blade properly to get optimum sunlight.
C) Leaf Lamina or Leaf Blade or Epipodium:
1. Leaf lamina is a flat, thin, green, expanded portion of the leaf and constitutes the major part of the leaf.
2. It is a site of photosynthesis, gaseous exchange and transpiration. A strong vein runs centrally through the leaf blade from its base to the apex, called midrib. It produces thinner lateral veins which in turn, are repeatedly branched and are called Veinlets.
3. The arrangement of veins and Veinlets in leaf lamina in some definite pattern is called venation. It is of two types, Reticulate and Parallel.
4. The venation provides mechanical support to the leaf blade and Also hepls in conduction of water and food elements. The lamina may be dorsi-ventral or isobbilateral. In case of a dorsi-ventral leaf the upper surface differs in structure from the lower surface while in isobbilateral leaf both the upper and lower surface are identical.
5. Some plants have cylindrical leaves in which there is no distinction between the two surfaces. They are called centric leaves.
Types of leaves:
There are two main types of leave Simple leaf and Compound leaf on the basis of degree or mode of incision of leaf lamina.
1. Simple leaf:
A leaf is said to be simple when its lamina is entire or undivided or may be incised to some depth but not up to the midrib or petiole, e g. Mango, Cotton etc.
The pattern of incision, if present, depends on the type of venation.
2. Compound leaf:
A leaf is said to be compound when the incision of the leaf blade goes up to the midrib or to the petiole so that the leaf is broken up or divided into a number of segments called leaflets or pinnae
1. The midrib in a compound leaf functions as an axis on which the leaflets develops. It is called rachis.
2. Compound leaves are classified as, pinnately compound leaf and palmately compound leaf.
3. In pinnately compound leaf (Pinna-feather) the leaf lamina is incised up to the midrib forming pinnae or leaflets. It's rachis bears leaflets laterally. A pinnately compound leaf nay be unipinnate (Rose) or bipinnate (Gulmohur) or tripinnate (Moringa).
4. In palmately compound leaf the leaf lamina is incised up to the petiole forming leaf lobes. It bears- Leaflets terminally.
a) Unifoliate - e.g. Citrus.
b) Bifoliate - e.g. Bauhinia.
c) Trifoliate - e.g Trigonella.
d) Quadrifoliate - e.g. Marsilia.
e) Pentafoliate - e g. Gynandropsis.
f) Multifoliate - e.g. Bombax.
An axillary bud is present in the axial of simple leaf or a compound leaf but it is never present in the axil of the leaflets of compound leaf.
Modifications of Leaf:
Leaves of many plants become modified to perform specialized functions as follows:
1. Leaf Tendril:
1. In some plants, the leaf is modified into a slender, wiry, often closely coiled spring-like structure called tendril.
2. It is a climbing organ which is very sensitive to touch or contact with any object. It coils around the object and helps the plant to climb.
3. They can be differentiated from stem-tendril by the absence of scaly leaves and are unbranded.
4. The leaf may be partially or wholly modified. Accordingly the leaf tendrils are of following types:
a) Whole leaf tendrils: In Lathyrus aphaca (wild pea) the entire leaf is modified into tendril whereas the stipules become leafy or foliaceous to perform the perform the functions of leaves.
b) Leaf apex tendril: In Gloriosa superba (glory lily) the leaf apex is modified into coiled tendril.
c) Leaflet tendrils: In peace (pisum sativum) the upper leaflets of compound leaf are modified into tendrils.
d) Petiolar Tendrils: Sometimes the petiole becomes long and gets modified into a tendril e.g. Clematis.
e) Stipular Tendrils : In Smilax stipules are modified into tendrils.
2. Leaf Spine:
1. In some plants the leaves or their parts are modified into small, hard, sharp pointed structures called spines.
2. A bud present in the axil of a spine indicates that the spine is a modified leaf.
3. The spines are defensive organs. They protect the plant from grazing animals.
4. In addition they also help the plant to reduce the rate of transpiration.
5. In Opunita the leaves are modified into spines.
6. In Argemone (pickly poppy) the margin of leaf lamina is modified into a spine.
7. In Zizyphus and Acacia the stipules are modified into spines.
3. Leaf hooks:
1. In Cat's nail (Bignonia unguis-cati), an elegant climber, the terminal leaflets become modified into three sharp, stuff and curved hooks which resemble the nails of a cat.
2. These hooks cling to the bark of a tree and act as organs of support for climbing.
4. Phyllode:
1. A flat, green, leaf-like modification of petiole or rachis is known as phyllode.
2. The leaflets either fall off very early (conducous) or become highly reduced. In such plants the phyllode performs photosynthesis.
3. It is a xerophytic modification in order to reduce the rate of transpiration. For e.g. in Australian Acacia the normal leaf is bipinnately compound. The leaflets soon fall off and the petiole gets flattened to form the phyllode.
Functions of leaf:
A. Normal functions or primary functions:
a. Photosynthesis-
The principal function of the leaf is to manufacture organic food through photosynthesis.
b. Gaseous exchange-
The stomata are primarily meant for gaseous exchange required for photosynthesis and respiration.
c. Transpiration-
Loss of water in the form of vapour through aerial parts of the plant is called transpiration. Most of the transpiration occurs through stomata present on leaf surface. This process helps to maintain temperature of the plant. It is also necessary for ascent of sap.
d. Protection of bud:
The terminal and axillary buds are protected by leaves from desiccation and mechanical injury.
e. Conduction:
The vascular bundles of veins and petioles of leaves serve the function of translocation of organic food through phloem and water and minerals through xylem.
B. Special functions or Secondary functions:
Besides normal functions, the leaf undergoes partial or total modification as an adaptation to various mechanical and physiological need of the plant. They include protection, climbing, vegetative propagation.