Leaf : Leaves are lateral outgrowth developed exogeneously from the nodes of the stem or branches. Buds develop in the axils of leaves. These are thin flat green parts which prepare food materials through the process of photosynthesis.
Functions of Leaf
(i) Leaves carry out photo-synthesis by which food is synthesized.
(ii) Diffusion in leaves controls oxygen and carbon dioxide.
(iii) The process of transpiration controls water and minerals in leaves.
Main Parts of a Leaf
1. Leaf base : It is the lower most part of leaf by which leaf is attached to nodes or the part of the leaf attached to the stem. Leaf base are of three types—
(a) Sheathing leaf base,
(b) Pulvinus leaf base,
(c) Succulent leaf base.
Stipules
These are small green appendanges at the point of junction of petiole and stem. These are present on either side of the leaf base.
Types of Stipules
(a) Free-lateral stipules : e.g. Hibiscus.
(b) Adnate stipules: e.g. Rosa.
(c) Interpetiolar Stipules : e.g. Ixora.
(d) Intrapetiolar stipules or Axillary stipules : e.g. Tabernaemontana, Gardenia.
(e) Ocharaceous stipules : e.g. Rumex.
(f) Bud scales : e.g. Ficus.
Modified stipules
(i) Tendrillar stipules : e.g. Smilax.
(ii) Spinous stipules : e.g. Acacia arabica.
(iii) Foliaceous or Foliose stipules : e.g. Lathyrus.
Petiole
Petiole is the stalk-like structure of a leaf. It raises the leaf and helps to secure more sun light.
Modifications of petiole
(a) Tendrillar petiole : e.g. Clematis.
(b) Winged petiole : e.g. Citrus.
(c) Spongy petiole : e.g. Acacia melanoxylon.
(d) Spinous petiole : e.g. Quisqualis.
Lamina
Lamina is the green flattened part of a leaf present at the tip of the petiole.
I. Shape of lamina
1. Acicular : e.g. Pinus.
2. Linear : e.g. Grasses.
3. Lanceolate : e.g. Nerium.
4. Rotund : e.g. Nelumbo.
5. Elliptical : e.g. Annona squamosa.
6. Ovate : e.g. Hibiscus.
7. Obovate : e.g. Terminalia.
8. Spathulate : e.g. Drosera.
9. Oblique : e.g. Azadirachta indica.
10. Oblong : e.g. Musa paradisiaca.
11. Reniform : e.g. Hydrocotyle vulgaris.
12. Cordate : e.g. Ficus religiosa and Thespesia.
13. Obcordate : e.g. Bauhinia.
14. Sagittate : e.g. Sagittaria.
15. Hastate : e.g. Ipomoea.
16. Lyrate : e.g. Brassica.
17. Centric : e.g. Allium.
18. Cuneate : e.g. Pistia.
19. Faclete : e.g. Eucalyptus.
20. Lorate or strap shaped : e.g. Vallisnaria.
21. Auriculate : e.g. Calotropis.
22. Fabelliform : e.g. Borassus flabellifer.
II. Margins of leaf
1. Entire : e.g. Annona.
2. Undulate or Repand or Wavy : e.g. Polyalthia.
3. Denate : e.g. Abutilon.
4. Serate : e.g. Hibiscus.
5. Crenate serriate : e.g. Bryophyllum.
6. Spinous : e.g. Argemone.
III. Leaf apex
1. Accuminate : e.g. Ficus religiosa.
2. Acute : e.g. Hibiscus.
3. Obtuse : e.g. Terminalia.
4. Truncate : e.g. Paris polyphylla.
5. Retuse : e.g. Pistia.
6. Emarginate : e.g. Bauhinia.
7. Cuspidate : e.g. Agave americana.
8. Mucronate : e.g. Tephrosia.
9. Tendrillar : e.g. Gloriosa superba.
10. Cirrhose : e.g. Musa paradisiaca.
IV. Surface of the Leaf
1. Glabrous : e.g. Mangifera
2. Glacous : e.g. Calotropis.
3. Scabrous : e.g. Ficus.
4. Pubescent : e.g. Lycoperiscum.
5. Pilose : e.g. Gravia pilosa.
6. Hispid : e.g. Cucurbita.
7. Glandular : e.g. Cleome.
8. Spiny : e.g. Argemone.
Texture of Leaf
1. Mesophytic : e.g. Annona.
2. Coraceous : e.g. Nerium.
3. Succulent : e.g. Bryophyllum.
4. Hygroscopic : e.g. Pistia.
5. Scaly : e.g. Casuarina.
6. Crestaceous : e.g. Borassus flabe alifer.
Venation
Venation is the mode of arrangement of veins in the lamina.
1. Reticulate Venation : Reticulate venation is found in dicots. They are of the following types—
(a) Pinnately reticulate venation: e.g. Hibiscus and Mangifera.
(b) Palmate reticulate venation:
(i) Convergent : e.g. Cinnamomum, Zizyphus.
(ii) Divergent : e.g. Ricinus, Cucurbita.
2. Parallel Venation : This type of venation is mainly found in monocots. In the lamina the veins formed from the midrib run parallel to each other towards the margins.
(i) Pinnately parallel venation : e.g. Musa Paradisiaca.
(ii) Palmately parallel venation : (i) Convergent : e.g. Oryza Sativa and grasses. (ii) Divergent : e.g. Borassus.
Kinds of Leaves
1. Dorsiventral : In dorsiventral leaves lamina is flattened and spreads horizontally. The dorsal and ventral sides differ in structure. Its upper surface is more exposed to sun, and that is why it is dark green in colour. Lower surface is less exposed to the sun, therefore it is light green in colour, e.g. Hibiscus.
2. Isobilateral : Such leaves are found in monocot plants. These leaves are directed vertically, and, therefore, uniformly exposed to the sun. This gives both the surfaces equal light for photosynthesis and produces uniform green colouration.
3. Centric : These leaves are round and cylindrical in shape. They are directed upwards, e.g. Allium cepa.
Different Leaves
(a) Simple Leaves : A simple leaf consists of a single lamina. The lamina is usually lobed, e.g. Psidium.
(b) Compound Leaves : In compound leaves the lamina is dissected upto the tip of the petiole to form many leaflets. Compound leaves are mainly of two types.
(i) Pinnately Compound Leaf : This leaf is defined as the one in which the midrib bears leaflets laterally arranged alternately. They are—(a) paripinate
(b) imparipinate—(i) unipinnately, (ii) bipinnately, (iii) tripinnately,
(iv) decompound.
(ii) Palmately Compound Leaf: In this all the leaflets are attached to the tip of the petiole like the fingers of the palm. Rachis are not there in this leaf. They are—
(a) Unifoliate compound leaf,
(b) Bifoliate compound leaf,
(c) Trifoliate compound leaf,
(d) Tetrafoliate compound leaf and (e) Multifoliate compound leaf.
Phyllotaxy
Phyllotaxy is defined as the type of arrangement of leaves on the stem and branches. It is of three types—
1. Alternate phyllotaxy : In this only one leaf arises at a node. Leaves are arranged with definite gaps around the stem.
2. Opposite phyllotaxy : In this two leaves are developed at each node lying opposite to each other.
(i) Opposite superposed : In this the opposite leaves of all the nodes lie one above the other, e.g. Quisqualis.
(ii) Opposite decussate : In this a pair of leaves lie at right angle to the next pair of leaves, e.g. Calotropis and Ixora.
3. Circular Phyllotaxy : In it more than two leaves are developed at each node, e.g. Nerium odorun.
In Croton bonplandianum both alternate and circular phyllotaxy are found.
Leaf mosaic : In it all the leaves adjust themselves in such way that they get maximum sunlight, e.g. Carica papaya.
Heterophylly : In this the plants have two or more types of leaves.
(a) Ecological heterophylly : The formation of different types of leaves takes place on the stem according to the ecological conditions, e.g. Limnophila heterophylla and Ranunculus.
(b) Developmental heterophylly: In some plants during growth stages, different types of leaves are formed, e.g. Dolichos lablab.
(c) Habitual heterophylly : In this heterophylly different types of leaves are formed at nodes, e.g. Artocarpus.
Leaf Modifications
Leaf modification is one in which the entire leaf or a part of it undergoes permanent change to perform some special functions along with the normal functions.
1. Cotyledons : These are the first leaves formed in the embryo stage. They store food and supply it to the embryo during germination. In Ricinus and Cucurbita the cotyledons become green and perform photosynthesis.
2. Tendrils : In weak stemmed plants the entire leaf modifies into tendrils.
3. Spines : In some plants leaves are modified into sharp pointed spines. The spines reduce the rate of transpiration in xerophytic plants and also protect the plants from herbivorous animals.
4. Scale leaves : In xerophytic plants and in underground stems the leaves are reduced to thin, small, colourless, dry, membranous leaves called ‘scales’ e.g. Allium sativum.
5. Phyllode : In phyllode the petiole is changed into a flattened, green, leaf-like structure to carryout photosynthesis, e.g. Acacia melanoxylon.
6. Storage leaves : The storage leaves store food and water and become thick and fleshy, e.g. Aloe vera.
7. Reproductive leaves : In some plants leaves reproduce epiphyllous buds which help in vegetative propagation, e.g. Bryophyllum and Begonia.
8. Absorbing leaves : These leaves help in absorption of water and minerals, e.g. Hydrilla and Utricularia.
9. Floral leaves : Flower is a modified shoot concerned with reproduction. So, the floral parts like sepals, petals, stamens and carpels are considered as floral leaves.
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