Stem
It is a part of the shoot system, which develops from the plumule of the embryo.
Stem is the ascending part of the main axis of the plant.
It is generally above ground, green when young and does a little of photosynthesis. Older stem is often woody and dark brown.
It is negatively geotropic i.e. grows away from soil.
It is positively phototropic i.e. grows or bends towards the light.
It is differentiated into nodes and internodes. The branches and leaves arise from the nodes. The space between two successive nodes is called internode.
It often bears multicellular hairs.
Stem bears different kinds of buds.
1. Terminal or apical bud: present at the growing apex and is covered by tiny leaves. It helps the plant to grow upwards.
2. Axillary or lateral bud: present at the axil of the leaf and helps the plant to grow in different directions by giving rise to new shoots sideways.
3. Floral bud: could be terminal or axillary. It grows into flower.
Functions of the Stem
The main function of stem is to spread out branches bearing leaves, flowers and fruits.
It conducts water and mineral nutrients from the roots to the rest of the plant.
Also transports food manufactured by the leaves to roots and other storage organs.
Stem becomes modified for perennation, vegetative propagation, protection, support and photosynthesis.
Modifications of Stem
Underground stems
Stems of some plants remain underground and serve functions like perennation, vegetative propagation and storage of food.
They possess nodes and internodes.
They have scaly leaves.
They have axillary and terminal buds.
Sub-aerial stems
These are the weak branches of stems which lie horizontally on the ground and a part may be buried beneath the soil.
These are basically for the purpose of vegetative propagation.
Aerial branches and adventitious roots arise at every node.
They are commonly called creepers.
Aerial stems
Aerial stems are those stems which remain fully above ground.
These stems are modified to perform specific functions like support, protection, photosynthesis, vegetative propagation, etc.
They bear nodes and internodes.
Click Here for more details on modifications of stem.
Leaf
A leaf is a flattened, expanded and normally green structure borne on a stem and a specialized organ of photosynthesis.
It develops from a node as a lateral outgrowth of the stem.
It generally bears a bud in its axil called axillary bud. The axillary bud later develops into a branch.
The first leaf originates from the shoot apical meristem as leaf primordia and slowly enlarge/expand as they grow.
Parts of a Leaf
A typical leaf has three parts
(i) Leaf base (Hypopodium)
It is the lowermost part of a leaf by which it is attached to the node of the stem.
It protects a bud in its axil.
In monocot plants, the leaf base expands into a sheath-like structure called sheathing leaf base, that partially or fully encircles the stem.
In dicotyledons, the leaf base bears two lateral outgrowths called stipules.
(ii) Petiole (Mesopodium)
It is the stalk of the leaf by which it is attached to the stem. It helps a leaf to get more sunlight.
Petiolate, when the leaf has a petiole as in dicots.
Sessile, when petiole is absent as in most of the monocots.
(iii) Lamina/ Leaf blade (Epipodium)
It is a green, flat, thin, expanded portion of the leaf.
It has number of thin veins and veinlets traversing through its surface.
Venation
The arrangement of veins and veinlets on the lamina is called venation.
These veins supports and also conducts water, minerals and food through the leaf.
Types of venation
Types of Leaves
Based on the incision of the lamina, leaves are classified into two types:
(i) Simple leaves:
A leaf is said to be simple leaf when its leaf blade in undivided.
The leaf should have a single lamina or if incised, the incision do not touch the midrib.
Ex: Hibiscus
(ii) Compound leaves:
A leaf is said to be compound leaf when the leaf blade is divided into small segments called leaflets.
The incisions go up to the midrib, hence the leaflets are not connected by lamina and are free from one another.
Types of compound leaves
(a) Pinnate: The leaflets are attached to the midrib or rachis. The leaflets are arranged alternate or opposite of the midrib.
Ex: Drumstick, Rose
(b) Palmate: The leaflets radiate from the end of the petiole. The leaflets are joined at a common point.
Ex: Alstonia, papaya
Phyllotaxy
The mode of arrangement of leaves on stem/branch is called phyllotaxy.
The leaves are arranged so that they get appropriate amount of sunlight for photosynthesis.
Types of phyllotaxy
(i) Alternate or spiral
There is only a single leaf at each node and leaves are arranged in the same plane or spirally around the stem/branch.
Ex: China rose, Mango
(ii) Opposite
There are two leaves present at each node opposite to each other.
Opposite phyllotaxy is of two types.
(a) Opposite decussate: The successive pairs of leaves are at right angles to each other.
Ex: Calotropis, jasmine
(b) Opposite superposed: The successive pairs of leaves stand one over the other.
Ex: guava
(iii) Whorled
There are more than two leaves at each node. They may be present in a whorl or a circle.
Ex: Oleander (Nerium)
Modifications of Leaves
Leaves are modified for various special functions. Few examples are given below:
(i) Leaf tendrils: These are thin, wiry, slender and coiled structures that help the plant in climbing to a nearby object.
Ex: Pea, Gloriosa, Smilax
(ii) Leaf spines: The leaves may be wholly or partially modified into sharp and pointed structures, to reduce loss of water through transpiration and also protect the plant from browsing animals.
Ex: Cactus, Prickly pear (Opunita)
(iii) Scale leaves: Leaves are modified for storage of food materials and to reduce transpiration.
Ex: Asparagus
(iv) Phyllode: The petiole gets modified into green flattened, leaf like structure for carrying out photosynthesis as leaflets are small and short-lived.
Ex: Australian acacia
(v) Pitcher shaped leaves: The petiole gets modified into long, coiled tendril like structure and lamina is modified into pitcher, for capturing and digesting insects and small animals.
Ex: Nepenthes
.
It is a part of the shoot system, which develops from the plumule of the embryo.
Stem is the ascending part of the main axis of the plant.
It is generally above ground, green when young and does a little of photosynthesis. Older stem is often woody and dark brown.
It is negatively geotropic i.e. grows away from soil.
It is positively phototropic i.e. grows or bends towards the light.
It is differentiated into nodes and internodes. The branches and leaves arise from the nodes. The space between two successive nodes is called internode.
It often bears multicellular hairs.
Stem bears different kinds of buds.
1. Terminal or apical bud: present at the growing apex and is covered by tiny leaves. It helps the plant to grow upwards.
2. Axillary or lateral bud: present at the axil of the leaf and helps the plant to grow in different directions by giving rise to new shoots sideways.
3. Floral bud: could be terminal or axillary. It grows into flower.
Functions of the Stem
The main function of stem is to spread out branches bearing leaves, flowers and fruits.
It conducts water and mineral nutrients from the roots to the rest of the plant.
Also transports food manufactured by the leaves to roots and other storage organs.
Stem becomes modified for perennation, vegetative propagation, protection, support and photosynthesis.
Modifications of Stem
Stems of some plants remain underground and serve functions like perennation, vegetative propagation and storage of food.
They possess nodes and internodes.
They have scaly leaves.
They have axillary and terminal buds.
Sub-aerial stems
These are the weak branches of stems which lie horizontally on the ground and a part may be buried beneath the soil.
These are basically for the purpose of vegetative propagation.
Aerial branches and adventitious roots arise at every node.
They are commonly called creepers.
Aerial stems
Aerial stems are those stems which remain fully above ground.
These stems are modified to perform specific functions like support, protection, photosynthesis, vegetative propagation, etc.
They bear nodes and internodes.
Click Here for more details on modifications of stem.
Leaf
A leaf is a flattened, expanded and normally green structure borne on a stem and a specialized organ of photosynthesis.
It develops from a node as a lateral outgrowth of the stem.
It generally bears a bud in its axil called axillary bud. The axillary bud later develops into a branch.
The first leaf originates from the shoot apical meristem as leaf primordia and slowly enlarge/expand as they grow.
Parts of a Leaf
A typical leaf has three parts
It is the lowermost part of a leaf by which it is attached to the node of the stem.
It protects a bud in its axil.
In monocot plants, the leaf base expands into a sheath-like structure called sheathing leaf base, that partially or fully encircles the stem.
In dicotyledons, the leaf base bears two lateral outgrowths called stipules.
(ii) Petiole (Mesopodium)
It is the stalk of the leaf by which it is attached to the stem. It helps a leaf to get more sunlight.
Petiolate, when the leaf has a petiole as in dicots.
Sessile, when petiole is absent as in most of the monocots.
(iii) Lamina/ Leaf blade (Epipodium)
It is a green, flat, thin, expanded portion of the leaf.
It has number of thin veins and veinlets traversing through its surface.
Venation
The arrangement of veins and veinlets on the lamina is called venation.
These veins supports and also conducts water, minerals and food through the leaf.
Types of venation
Types of Leaves
Based on the incision of the lamina, leaves are classified into two types:
(i) Simple leaves:
A leaf is said to be simple leaf when its leaf blade in undivided.
The leaf should have a single lamina or if incised, the incision do not touch the midrib.
Ex: Hibiscus
A leaf is said to be compound leaf when the leaf blade is divided into small segments called leaflets.
The incisions go up to the midrib, hence the leaflets are not connected by lamina and are free from one another.
Types of compound leaves
(a) Pinnate: The leaflets are attached to the midrib or rachis. The leaflets are arranged alternate or opposite of the midrib.
Ex: Drumstick, Rose
(b) Palmate: The leaflets radiate from the end of the petiole. The leaflets are joined at a common point.
Ex: Alstonia, papaya
Phyllotaxy
The mode of arrangement of leaves on stem/branch is called phyllotaxy.
The leaves are arranged so that they get appropriate amount of sunlight for photosynthesis.
Types of phyllotaxy
(i) Alternate or spiral
There is only a single leaf at each node and leaves are arranged in the same plane or spirally around the stem/branch.
Ex: China rose, Mango
(ii) Opposite
There are two leaves present at each node opposite to each other.
Opposite phyllotaxy is of two types.
(a) Opposite decussate: The successive pairs of leaves are at right angles to each other.
Ex: Calotropis, jasmine
(b) Opposite superposed: The successive pairs of leaves stand one over the other.
Ex: guava
(iii) Whorled
There are more than two leaves at each node. They may be present in a whorl or a circle.
Ex: Oleander (Nerium)
Modifications of Leaves
Leaves are modified for various special functions. Few examples are given below:
(i) Leaf tendrils: These are thin, wiry, slender and coiled structures that help the plant in climbing to a nearby object.
Ex: Pea, Gloriosa, Smilax
(ii) Leaf spines: The leaves may be wholly or partially modified into sharp and pointed structures, to reduce loss of water through transpiration and also protect the plant from browsing animals.
Ex: Cactus, Prickly pear (Opunita)
(iii) Scale leaves: Leaves are modified for storage of food materials and to reduce transpiration.
Ex: Asparagus
(iv) Phyllode: The petiole gets modified into green flattened, leaf like structure for carrying out photosynthesis as leaflets are small and short-lived.
Ex: Australian acacia
(v) Pitcher shaped leaves: The petiole gets modified into long, coiled tendril like structure and lamina is modified into pitcher, for capturing and digesting insects and small animals.
Ex: Nepenthes
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