LEAF

The leaves develop from the nodes. The main function is to carry out photosynthesis or food formation. axillary buds are found in the axil of leaves. The axillary bud may develop into a branch. Leaves originate from shoot apical meristems and are arranged in an acropetal order.

Leaf Is a lateral outgrowth of stem developed exogenously at the node

Leal (Phyllopodium) is divided into 3 main parts :

(1) Leaf base (Hypopodium) - The part of leaf which is attached to stem is known as leaf base. Sheathing leaf base is found in monocots. In monocots, the leaf base expands into a sheath covering the stem partially or wholly. Pulvinus leaf bases are found in some legume plants Swollen leaf base is known as pulvinus leaf base.

(2) Petiole (Mesopodium) - The part of leaf connecting the lamina with the branch or stem is known as petiole. Petiole or stalk containing leaves are known as petiolate leaves and when petiole or stalk is absent then leaves are called sessile. In Eichhornia petiole swells up and in Citrus it is winged. The petiole helps hold the blade to light Long thin flexible petioles allow leal blades o flutter in wind, thereby couling the leaf and bringing fresh air to leaf surface.

(3) Lamina (Leaf blade-Epipodium) - It is a broad and green flattened part of leaf . Its main functions are photosynthesis and transpiration.

Stipules :

Leaves of some plants have two lateral appendages on either side of leaf base, known as stipules., Leaf with stipule is known as stipulate leaf. eg Fabaceae Leaf without stipule is called ex-stipulate leaf eg. Solanaceae, Liliaceae

Stipules are of various types -

1. Free lateral-They are independently present on both sides of leaf base Eg. Hibiscus rosa sinensis (China rose)

2. Foliaceous - These type of stipules are leaf like. Eg. - Pea

3. Spiny - Stipules modified into spine like structures. Eg. Ziziphus (ber) 

4. Ochreate - When both stipules of a leaf combine together and form a tube like structure, then it is called ochreate. Eg. Polygonum 

5. Tendrillar - Stipules are modified into tendrils like structure. Eg. Smilax

Types of Leaves -

Bracts - These are leaf like structure which may present at base of pedicel of flower.

Bracteoles - These are leaf like structures found on pedicel.

VENATION OF LAMINA

The arrangement of veins and veinlets in leaf lamina is known as venation. It is of 2 - types

(1) Reticulate It is found in dicots. Exception - Calophyllum (It has parallel venation) 

(2) Parallel. It is found in monocots. Exception - Smilax (It has reticulate venation)

1. Reticulate venation - In this type of venation lateral veins are divided into various branches (veinlets) and vein lets form a net like structure.

Reticulate venation is of 2 types:

(a) Unicostate or pinnate - This type of venation is having only one principal vein or midrib that gives off many lateral veins which proceed towards margin and apex of lamina of the leaf and form a network

(b) Multicostate or palmate - In this type of venation many principal veins arising from the tip of petiole and proceed upward, this is again of two types.

2. Parallel venation - In this type of venation, all veins run parallel to each other and they donot form network.

They are of 2 types: 

(a) Unicostate or pinnate This type of pattern is 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 donot have veinlets.

(b) Multicostate or palmate: This type of pattern is having many principal veins arising from the tip of the petiole and proceeding upwards.

MODIFICATION OF LEAVES

When leaves are modified into different structures then it is called modification of leaves.

• Leaf tendril - In some plants whole leaf is modified into a wire like structure which is called leaf tendril Tent helps is climbing. Eg. Lathyrus aphaca (wild pea) Peas 

• Leaf spine - Leaves are modified into pointed spines. Eg Opuntia Cacti. Argemone

• Leaf pitcher - Leaves of some plants are modified into pitcher shaped structure. Eg. Nepenthes (pitcher plant) (Only lamina is modified into pitcher). Water is stored in the pitcher of Dischidia (complete leaf is modified into pitcher). In Nepenthes insectivorous pitcher while in Dischidia - non insectivorous pitcher is formed.

• Leaf bladder - In some plants, leaves are modified into bladder like structure Eg. Utricularia (bladder wort)

• Phyllode - In some plants petiole becomes flat leaf like green. Synthesises food and functions as normal leaf. Eg. Australian acacia, Parkinsonia. 
•  Leaflet tendril - When leaflet is modified into tendril like structure then it is called leaflet tendril. Eg.: Pisum sativum (garden pea), Lathyrus odoratus (sweet pea).

Note: Dionaea venus flytrap, is insectivorous plant and also has modified leaves

• Leaflet hook - eg. Cat's nail (Bignonia unguis cati)

SIMPLE AND COMPOUND LEAF

Simple Leaf : A leaf is said to be simple, when its lamina is entire or may be incised to any depth. but not up to the midrib or petiole. Eg. :-Peepal, mango, radish,

Compound leaf A leaf in which the leaf blade (lamina) is incised up to the midrib and petiole, thus dividing it into several small parts, i.e. leaflets.

It has two types :

(A)Pinnately compound leaf : In this type of leaf, leaf blade (lamina) is incised upto the mid rib and mid rib is known as rachis. A number of leaflets are present on a common axis, the rachis.

Eg. Neem

(B) Palmately compound leaf: In this type incisions of leaf are directed from leaf margins to apex of the petiole and all leaflets are attached at a common point i.e. at the tip of the petiole. Rachis is absent in palmately compound leaf. Eg. Silk Cotton (Bombax)

• A bud is present in the axil of petiole in both simple and compound leaves, but not in the axil of leaflets of the compound leaf.

PHYLLOTAXY

Phyllotaxy is the pattern of arrangement of leaves on the stem or branch.

This is usually of three types.

Alternate : In this type a single leaf arises at each node at alternate manner. Eg. Mustard, china rose, sunflower.

Opposite : In this type of phyllotaxy a pair of leaves arise at each node and lie opposite to each other. eg. Guava, Calotropis, Ocimum (tulsi)

Whorled: If more than two leaves arise at each node and form a whorl. then it is called whorled phyllotaxy.

Eg. Alstonia (devil tree), Nerium

Function and structure

The principal function of leaves is to absorb sunlight need ed to manufacture plant sugars through a process called photosynthesis. Leaf surfaces are flattened to present a large area for efficient light absorption. The blade is the expanded thin structure on either side of the midrib and usually is the largest, most conspicuous part of a leaf.

A leaf is held away from its stem by a stem-like appendage called a petiole, and the base of the petiole is attached to the stem at a node. Petioles vary in length or may be lacking entirely, in which case the leaf blade is described as sessile or stalk less. A celery stalk is a leaf petiole.

The node where a petiole meets a stem is called a leaf axil. The axil contains single buds or bud clusters, referred to as axillary buds. They may be either active or dormant. Under the right conditions, they will develop into stems or leaves.

A leaf blade is composed of several layers. On the top and bottom is a layer of thick, tough cells called the epidermis. Its primary function is to protect the other layers of leaf tissue. The arrangement of epidermal cells determines the leaf's surface texture. Some leaves, such as those of African violets, have hairs (pubescence), which are extensions of epidermal cells that make the leaves feel like velvet.

The cuticle is part of the epidermis. It produces a waxy layer called cutin, which protects the leaf from dehydration and disease. The amount of cutin on a leaf increases with increasing light intensity. For this reason, when moving plants from shade into full sunlight, do so gradually over a period of a few weeks. This gradual exposure to sunlight is referred to as hardening off. It allows the cutin layer to build up and protect the leaves from rapid water loss or sun scald. Transplants produced indoors should be hardened off before planting outdoors.

The waxy cutin also repels water. For this reason, many pesticides contain a spray additive (sticker, spreader, etc.) to help the product adhere to, or penetrate, the cutin layer.

Special epidermal cells called guard cells open and close in response to environmental stimuli such as changes in weather and light. They regulate the passage of water, oxygen and carbon dioxide into and out of the leaf through tiny openings called stomata. In most species, the majority of the stomata are located on the undersides of leaves.

Conditions that would cause plants to lose a lot of water (high temperature, low humidity) stimulate guard cells to close. In mild weather, they remain open. Guard cells also close in the absence of light.

Located between the upper and lower epidermis is the mesophyll. It is divided into a dense upper layer (palisade mesophyll) and a lower layer that contains lots of air space (spongy mesophyll). Located within the mesophyll cells are chloroplasts, where photosynthesis takes place.

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