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The leaf is a lateral generally flattened structure borne on the stem. The leaves develop from the nodes. Their main function is photosynthesis and food making, axillary buds are found in its axil. All the leaves of a plant are known as phyllome. Axillary bud later develops into a branch. Leaves originated from shoot apical meristem and are arranged in acropetal order.
Leaf is divided into 3 main parts :
Leaf base (Hypopodium) –
- Leaves are attached to stem by leaf base.
- In some plants, leaf base becomes swollen and is called pulvinus which is responsible for sleep movement e.g., Cassia, mimosa, bean.
- In some plants, leaf base expands into sheath (Sheathing leaf base), e.g., grasses and banana (monocots).
- When the leaf base partially encloses the stem, it is called semi amplexicaul e.g., Prickly poppy, alotropis procera (Madar).
- It completely encloses the stem, it is called amplexicaul e.g., Sonchus, Polygonum.
Petiole (Mesopodium) –
- The part of leaf connecting the lamina with the branch of stem. Petiole help to hold the blade to light.
- In Eichhornia petiole swell and in citrus it is winged.
- The petiole is modified in tendrils in Nepenthes.
- In Australian acacia, petiole is modified in phyllode.
- Long thin flexible petiole allows leaf blade to flutter in air, thereby cooling the leaf and bringing fresh air to leaf.
Lamina (Epipodium) –
It is a broad and flattened part of leaf. Its main functions are photosynthesis and transpiration. Shape of lamina are :
- Acicular – Lamina is long and pointed, like a needle. eg. Pinus
- Lanceolate – In this type lamina is pointed or narrower at the ends while broader in the middle. eg. Bamboo, Nerium
- Linear – The lamina is long and narrow having parallel margins. eg. Grass
- Ovate – In this type lamina is egg-shaped having broad base with slight narrow top. eg. Ocimum, Banyan, China rose.
- Cordate – Its shape is like a heart. eg. Betel.
- Oblong – Long and broad lamina. eg. Banana
- Sagittate – The lamina is triangular in shape. eg. Sagittaria
- Spatulate – The lamina is broad spoon-shaped. eg. Calendula
- Orbicular or Rotund – In this types the lamina is spherical. eg. Lotus.
- Elliptical or Oval – In this type, the middle part of the lamina is broad while the ends are narrow and oval. eg. Guava.
- Oblique – In this types midrib divides, lamina into two unequal halves. eg. Bignonia, Neem.
Leaves of some plants have lateral appendages on either side of leaf base, known as stipules. If stipules are present in leaf it is called stipulated leaf, if it is absent then leaf is called exstipulated.
Stipules are of various types –
- Free lateral – They are independently present on both sides of leaf base. eg. Hibiscus rosasinensis (China rose)
- Interpetiolar – When two leaves are meet oppositely at the node then nearest stipules of each leaf join with each other. In this way, only two stipules of two leaves are found in place of four. eg. Ixora, Anthocephalus.
- Intrapetiolar – In this type both stipules of a single leaf join with each other to form a single stipule. eg. Gardenia
- Foliaceous – These type of stipules form a leaf-like structure. eg. Pea
Scaly – Stipules are dry, small and paper like. eg. Desmodium
- Spiny – Stipules modified into spine. eg. Zizyphus (Beri), Acacia.
- Ochreate – When both stipules of leaf combine together and form a tube-like structure, It is called ochreate. eg. Polygonum
- Adnate – Both stipules are attached with petiole. eg. Rose
- Tendrillar – Stipules are modified into tendrils like structure. eg. Smilax
- Bud scale – Protect the young Bud. e.g. Ficus
TYPES OF LEAVES
- Foliage leaf – They are usually green coloured and their main function is photosynthesis.
- Cotyledonary leaf – This leaf comes out during germination and helps in nutrition until the first leaf is not formed.
- Scaly leaf (Cataphylls) – Such leaves are usually dry membrane like and they can not perform photosynthesis
- Bract (Hypsophyll) – Bract is the leaves which are present in flower axis.
- Bracteole – These are leaf-like structure found on pedical.
- Floral leaf – Sepals, petals, stamen and carpel are found in flower which is included in this type of leaf.
- Perianth – In some flowers, Calyx and Corolla are not distinct and are termed as Perianth and unit of perianth is called tepal. eg. Lily
Duration of leaf :
- Persistent / Evergreen – Leaves of such plants are found in all season and do not (fall) shed combindly. eg. Pinus, Saraca indica, Datepalm.
- Deciduous – All leaves of such plants shed at the same time eg. Azadirachta.
- Caducous – Leaves fall soon just after appearance or after the opening of bud. eg. Rose
Leaf insertion :
- Cauline leaves – When the leaves are found on node of stem, then these are called cauline leaves. eg. Maize, Hollyhock.
- Ramal leaves – When leaves are found on branches, then these are called ramal leaves. eg. Delbergia, Zizypus.
- Radical leaves – During favourable season, leaves develop from the nodes of underground stem and seem that they are developing from roots. This type of leaves is known as radical leaves. eg. Radish, Turnip.
VENATION OF LAMINA
The arrangement of veins and veinlets in leaves (Lamina) is known as venation. It is of 2 types
- Reticulate: It is found in dicots. Exception – Calophyllum, Eryngium. It has parallel venation.
- Parallel: It is found in monocots. Exception – Smilax, Dioscorea, Alocasia, Colocasia. It has reticulate venation
Reticulate venation :
In its main vein divided into various branches (veinlets) and form a net-like structure. Reticulate venation is of two types.
Unicostate or pinnate – In this type of venation leaf has only one principal vein or midrib that give off many lateral veins which proceed toward margin and apex of lamina of the leaf and form a network. eg. Mango, guava, Peepal
Multicostate or palmate – In this type of venation many principal veins arising from the tip of petiole and proceed towards tip of lamina. This is again two types –
- Multicostate divergent – Many principal veins arising from the tip of petiole diverge from the another toward the margin of leaf blade eg. Cotton, Caster, Cucurbita, grape.
- Multicostate convergent – Many principal veins arising from the tip of petiole. At the base of leaf, they are closely arranged but diverage from one another in middle part and converge towards the apex of leaf. eg. Camphor, Zizyphus, Tejpat, China rose, plum.
Parallel venation :
In this type of venation, all veins run parallel to each other and they do not from the network. They are of two types.
Unicostate or pinnate – This type of pattern having only one principal vein, that gives off many lateral veins, which proceed toward the margin of leaf blade in a parallel manner but they do not have veinlets. eg. Banana, Ginger, Canna.
Multicostate or palmate – Having many principal veins arising from the tip of the petiole and proceeding upwards.
- Multicostate divergent – Many principal veins arising from the tip of petiole and diverge toward the margin of leaf. They do not divide into veinlets and do not form network. eg. Coconut, Date palm
- Multicostate convergent – Many principal veins arising from the tip of petiole run in a curved manner in lamina and converge towards the apex of leaf blades. eg. – Wheat, Sugarcane, Bamboo.
Furcate venation – The veins branch dichotomously but the reticulum is not formed by the finer branches. eg. Adiantum (fern).
TYPES OF LEAF
Simple and Compound Leaf :
A leaf which may be incised to any depth, but not down to the midrib or petiole, then this type of leaf called simple leaf. eg. Mango, China rose, Ficus, etc.
A leaf in which the leaf blade is incised up to the midrib or petiole, thus dividing it into several small parts, known as leaflets. This type of leaf is known as compound leaf. It is of two types –
Pinnately compound leaf – In this type of leaf, midrib is known as rachis. Leaflets are arranged on both sides of rachis. eg. Neem. It is of following types –
- Unipinnate – In this type of leaf, division occurs only once and leaflets are directly attached on both sides of rachis.
- If the number of leaflets is even, then leaf is known as paripinnate. eg. Cassia fistula, Sesbania
- If the number of leaflets is odd, it is known as imparipinnate. eg. Rose, Neem.
- Bipinnate – A twice pinnate compound leaf eg. Acacia, Delonix regia, Mimosa.
- Tripinnate – A thrice pinnate compound leaf eg. Moringa.
- Decompound – A compound leaf, which is more than thrice pinnate. eg. Carrot, Coriander.
Palmate compound leaf – In this type incision of leaf are directed from leaf margin to apex of petiole and all leaflets are attached on the upper end of petiole.
It is of following types –
- Unifoliate – When single leaflet is found. eg. Lemon
- Bifoliate – When two leaflets are present. eg. Bauhinia, Regnelidium, Bignonia.
- Trifoliate – When three leaflets are attached. eg. Oxalis, Aegle, Trifolium
- Tetrafoliate – When four leaflets are attached to the petiole. eg. Marsilea.
- Multifoliate – when more than four leaflets are found, then leaf is called multifoliate palmate compound leaf. eg. Silk cotton.
It is of following type –
Alternate or spiral – Single leaf arising at each node. eg. Cyprus rotandus, China rose, mustard & Sunflower.
Opposite – Leaves occurring in pairs at the node, they may be
- Decussate: Leaves that stands at right angle to next upper or lower pair eg. Calotropis, Mussaenda.
- Superposed: Successive pairs of leaves stand directly over a pair in the same plane eg. Psidium (guava), Ixora.
Whorled – More than two leaves at each node eg. Nerium, Alstonia.
It is the occurrence of more than one type of leaves on the same plant. It is of three types
- Developmental Heterophylly: Leaves of different forms and shape occur at different period or places on the same plant eg. Mustard, Sonchus, Eucalyptus.
- Environmental Heterophylly: It is aquatic adaptation which is commonly found in rooted emergent hydrophytes. In this, submerged leaves differ from the floating and aerial leaves. eg. Limnophila, Heterophylla, Ranunculus aquatiles, Sagittaria.
- Habitual Heterophylly: Due to habit mature leaves differ in their shape and incissions eg. Artocarpus (Jackfruit).
MODIFICATION OF LEAVES
- Leaf tendril – In it, whole leaf is modified into thin threadlike structure which is called leaf tendril eg. Lathyrus aphaca (wild pea).
- Leaflet tendril – When leaflet is modified into tendril-like structure than it is called leaflet tendril. eg. Pisum sativum (Garden pea), Lathyrus odoratus (sweet pea)
- Leaf spine – Leaves or any part of leaflet are modified into pointed spine. eg. Asparagus, Opuntia, Aloe, Argemone.
- Leaf scale – In it, leaves become thin, dry and form a membrane or paper-like structure and serve to protect axillary buds as in Ficus and Tamarix, Ruscus, Casurina.
- Leaf pitcher – Leaves of some plants are modified to pitcher shape. eg. Nepenthes, Dischidia.
- Leaf bladder – In some plant, leaves are modified into bladder-like structure eg. Utricularia. Leaf Hooks – In some plants terminal leaflets are modified into curved hooks for helping the plant in climbing. eg. Argemone, Opuntia, Aloe, Cat’s nail (Bignonia unguis – cati)
- Phyllode – In its, petiole becomes flat structure and function as normal leaf. eg. Australian acacia.
- Flashy leaves – In onion and garlic food storing flashy leaves are present