- Books Name
- ACME SMART COACHING Biology Book
- ACME SMART PUBLICATION
- CBSE Class 11
THE LEAF (PHYLLOPODIUM)
Leaves are lateral, flat, green and expanded parts of plant which arise from the nodes on the stem or branches.
Usually leaf has a bud in its axil.
The chief function of leaf is photosynthesis and transpiration.
Parts of a Leaf
A leaf consists of following three parts:
(i) 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).
A tongue like structure is also present between leaf base and axis in grasses, called ligule.
When the leaf base partially encloses the stem, it is called semi amplexicaul e.g., Prickly poppy, Calotropis procera (Madar); if it completely encloses the stem, it is called amplexicaul e.g., Sonchus, Polygonum.
(ii) Petiole (Mesopodium)
The stalk of leaf is called petiole.
Petiole in Eichhornia becomes spongy and bulbous.
In orange (Citrus), petiole becomes winged.
Petiole is modified to tendrils in Nepenthes.
In Australian Acacia, petiole is modified into leaf like flat structure called phyllode.
(iii) Lamina (Epipodium)
The broad, green, flat part of the leaf is called lamina ('leaf blade).
All the leaves of a plant are collectively called as phyllome.
Leaves are of the following types -
(1) Cotyledonary leaves: Embryonic or seed leaves, distinct in plants having epigeal germination.
(2) Foliage leaves: Common green, photosynthetic leaves.
(3) Scale leaves (Cataphylls) : Reduced scaly leaves.
(4) Bract leaves (Hypsophylls) : They bear flowers in their axil.
(5) Fertile leaves (Sporophylls) : They bear sporangia on their ventral surface.
(6) First leaf (Prophyll) : First few leaves different from the rest e.g., Citrus.
Prefoliation : Arrangement of leaves in bud condition. It is of two types:
(1) Ptyxis: The manner in which each individual leaf is folded or rolled in bud condition.
(2) Vernation: Arrangement of leaves with respect to each other in bud condition.
(1) Radical: Leaves borne on reduced stem, appear to arise directly from the roots e.g., radish, turnip.
(2) Cauline: Leaves found directly on the nodes of main stem e.g., maize, hollyhock.
(3) Ramal: Leaves present on the nodes of the stem branches e.g., Dalbergia, Zizyphus.
Types of Leaf
A. Simple leaf
Leaf which may be entire or incised, and the incisions do not touch the midrib e.g., mango, banyan.
B. Compound leaf
Leaf blade is incised upto midrib or petiole thus, divides it into two or more leaflets.
They are of two types :
(1) Palmately compound leaves. It has no rachis and all the leaflets are joined to a common point i.e., at the tip of petiole. They may be :
(i) Unifoliate e.g., Citrus (lemon and orange).
(ii) Bifoliate e.g., Bignonia.
(iii) Trifoliate e.g., Dolichos, Trifolium, Aegle, Butea
(iv) Quadrifoliate e.g., Marsilea, Paris
(v) Multifoliate e.g., Bombax (silk cotton tree)
(2) Pinnately compound leaves. Rachis bears a number of lateral leaflets. These may be :
(i) Unipinnate. Midrib of the leaf directly bears the leaflet and is now called rachis. The unipinnate compound leaf is called paripinnate when terminal leaflet is absent (leaflets are in even number) e.g., Cassia, Tamarindus or imparipinnate when terminal leaflet is present (leaflets are in odd number) e.g., Rosa, Tephrosia, Azadirachta.
(ii) Bipinnate. Midrib produces secondary axis or branches which bear leaflets e.g., Acacia, Mimosa, Delonix.
(iii) Tripinnate. Secondary axis produce tertiary axis which bear leaflets e.g., Moringa, Melia.
(iv) Decompound. Rachis is divided repeatedly without any definite pattern so that the lamina is dissected into narrow segments e.g., Carrot, Parthenium, Coriandrum.
Venation in Leaves
Arrangement of veins and the veinlets in the lamina is called venation. It is of three types :
1. Reticulate venation.
The branches of veins form a network, e.g., dicots. However there are some dicots which show parallel venation e.g., Calophylum, Eryngium and Corymbium. It may be
(i) Pinnate or unicostate : Having only one midrib which gives rise to lateral veins bearing vein lets forming reticulation e.g., Peepal , China rose.
(ii) Palmate or multicostate : Many veins arise from the tip of the petiole and reach the apex or margins of the lamina. Their lateral veins form reticulation. It is of 2 types
(a) Convergent : Veins is converge towards the apex of the lamina e.g., Zizyphus and Smilax (a monocot) .
(b) Divergent: Veins diverge towards the margins e.g., Castor (Ricinus), Luffa, Vitis (grape vine), etc.
(2) Parallel venation. The veins remain parallel to each other and veinlets are inconspicuous e.g., monocots. Some monocots which show reticulate ventation are Smilax, Dioscorea, Alocasia, Colocasia.
(i) Pinnate or unicostate parallel venation - Only one principal vein (midrib) is present and lateral veins run parallel without reticulation, e.g. , Banana, Canna
(ii) Palmate or multicostate parallel venation - Many principal veins arise from the base of the lamina. They may be:
(a) Convergent e.g., Bamboo, Grasses
(b) Divergent e.g., Fan palm
(3) Furcate venation - The veins branch dichotomously but the reticulum is not formed by the finer branches e.g., Adiantum (fern), Circeaster (angiosperm).
Small, lateral, leaf like appendage which arises in pair from the petiole axis of leaf is called stipule.
Stipule gives protection to the young axillary buds.
Leaves having these are called stipulate, while not having these are called exstipulate.
They may be of following types :
(i) Free lateral. On either side of leaf e.g., China rose, cotton.
(ii) Adnate. United with petiole e.g., Rose.
(iii) Ochreate. Form tube like covering e.g., Polygonum .
(iv) Scaly -Small membranous stipules e.g., Desmodium
(v) Axillary or intrapetiolar -Situated within the petiole towards axis e.g., Gardenia
(vi) Interpetiolar -Situated between the petioles of opposite leaves. e.g., Anthocephalus, Ixora.
(vii) Bud scales -Protect the young bud e.g., Ficus.
Modification of stipules
(1) Tendrillar stipule. In Smilax, stipule changes into tendril and helps in climbing.
(2) Foliaceous stipule. In Pisum and Lathyrus, stipules become leaf like.
(3) Spinous stipules. In Acacia and Zizyphus, stipule is modified into spines.
Types of stipules : A. Free lateral stipules, B. Adnate stipules, C. Interpetiolar stipules,
D. Intrapetiolar stipules, E. Ochreate stipules, F. Foliaceous stipules, G. Bud scales,
H. Tendrillar stipules, I. Spiny stipules
It is the mode of arrangement of leaves on the stem or its branches. It is of following types :
(1) Alternate. Single leaf arising at each node, e.g., Mustard.
(2) Opposite. Leaves occurring in pairs at the node. They may be :
(a) Decussate. Leaves that stand at right angle to upper or lower pair of leaves at nodes, e.g., Calotropis, Sacred basil, Zinnia.
(b) Superimposed. Successive pair of leaves stand directly over a pair in the same plane, e.g., Psidium (Guava), Syzygium (Jamun), Quisqualis.
(3) Whorled. More than two leaves at each node, e.g., Nerium, Alstonia.
Modification of Leaves
(1) Storage leaves - e.g., Members of family Crassulaceae have thick succulent leaves which store water (hydrophillic colloids). Such storage leaves prevent the leaf against desiccation e.g., Bryophyllum.
(2) Leaf tendrils : These coil around the support and help the plant to climb.
(3) Spines: e.g. , Opuntia, Berberis . This is a xerophytic adaptation, as spines reduce transpiration loss, besides it helps to protect the plant from grazing animals.
(4) Phyllode: e.g., Acacia auriculiformis (Australian acacia). Here, the leaflets fall off early and petiole becomes flattened to function as leaf. This is also a xerophytic adaptation.
(5) Insect catching leaves - e.g., Nepenthes, Drosera, Utricularia, etc.
(6) Scale leaves -Small dry membranous, brownish leaves, e.g., Casuarina, Ruscus.
(7) Coloured leaves - Leaves near inflorescence are brightly coloured to attract the insects, e.g., Euphorbia pulcherrima (Poinsettia).