Morphology and functions of leaf:

Leaves are the most important green appendages of shoot which are meant for manufacture of organic food by the process of photosynthesis.

It develops from the leaf primordium.

Leaf may be defined as:

" A dorsi-ventrally compressed, lateral appendage of stem, produced at the nodes and is specialized to perform photosynthesis".

Characteristics of Leaf :

- Leaf is a thin, expanded, green structure.

- The green color of the leaf is due to the presence of chlorophyll pigment.

- It is exogenous in origin. It is borne on the stem at the node.

- An axillary bud is often present in the axil of each leaf.

- It has limited growth.

- It does not possess apical bud or a regular growing point.

 

A typical foliage leaf

It has three main parts:

a) Leaf base or Hypopodium

b) Petiole or Mesopodium and

c) Lamina or Leaf blade or Epipodium

 

A) Leaf Base or Hypopodium:

The part of leaf attached to the stem or branch is known as leaf base.

It may assume different shapes in different plants.

Leaves of some plants possess a pair of lateral outgrowths at the base, on either sides of axillary bud.

These outgrowths are called stipules and such leaves are called stipulate leaves.

The leaves without stipules are called ex-stipulate leaves.

Stipules are usually green. The functions of stipules are to protect the bud and carry out photosynthesis.

 

B) Petiole or Mesopodium:

Petiole is the part of leaf connecting the lamina with the branch or stem.

Leaves that possess petiole are called petiolate and leaves without petiole are called non-petiolate or sessile leaves.

Petiole is usually cylindrical, but may be hollow (Papaya), tubular or flattened.

Function of petiole is to raise the lamina to expose it to more light and air and to help in conduction.

 

C) Lamina or Epipodium:

This is the largest, most important, green and flattened part of the leaf.

It plays a vital role in photosynthesis, gaseous exchange and transpiration.

The leaf is known as dorsi ventral when its ventral surface is structurally different from dorsal surface. e.g. dicotyledonous leaves.

The leaves having both similar surfaces are called isobilateral. Such leaves are found in monocot plants.

Exceptionally some leaves assume such forms that it becomes difficult to identify both the surfaces of leaves. Such leaves are called centric or cylindrical leaves. e.g. onion, garlic, etc.

 

Venation :

The arrangement of veins and veinlets in the lamina is known as venation.

The veins are in fact conducting strands of lamina.

They are concerned with the conduction of water, mineral salts and food and form the structural framework of the lamina.

 

The venation is mainly of two types.

I) Reticulate Venation :

When the veins and veinlets form a network ,it is called reticulate venation. Here the midrib is centrally placed and veins and veinlets remain distributed laterally.

It is found in dicotyledonous plants.

On the basis of number of mid-veins, reticulate venation is of two types :

1. Pinnate or Unicostate

It is with a single mid-vein e.g. Mango, Peepal etc.

2. Palmate or Multicostate

It is with two or more prominent veins. It may be convergent e. g. Zizyphus or divergent e.g. castor.

II) Parallel Venation:

In this type of venation, veins in lamina run almost parallel to one another. e.g., maize, grasses, etc.

It is of the following two types :

1. Unicostate

e.g., Banana, Canna.

2. Multicostate.

It may be convergent e.g., grass, rice, bamboo, or divergent e.g., fan palm.

 

Simple and Compound Leaf :

On the basis of incision of lamina, leaves may be of two types :

 

1. Simple leaf:

Simple leaves are those in which single leaf blade or lamina is present. e.g., Mango, Peepal, Papaya, etc.

2. Compound leaf:

Compound leaves are those in which the leaf blade or lamina is divided into number of segments known as leaflets or pinnae.

The leaflets never bear axillary buds in their axil.

The compound leaves are of two types:

a) Pinnately compound leaves:

In this type the leaflets are present laterally on a common axis called rachis, which represents the midrib of the leaf.(e.g. Gold mohur, Cassia)

There are of four kinds of pinnately compound leaves as

i) Unipinnate

ii) Bipinnate

iii) Tripinnate

iv) Decompound

 

b) Palmately compound leaves:

All the leaflets of the palmately compound leaves are attached at a common point, i.e. at the tip of petiole, like fingers of the palm.

There are five types of palmately compound leaves as

i) Unifoliate

ii) Bifoliate

iii) Trifoliate

iv) Quadrifoliate

v) Multifoliate