Morphology of Flowering Plants

Introduction to Morphology

Morphology refers to the study of the form and structure of organisms. In flowering plants (angiosperms), it deals with the vast diversity in external structures of various plant parts. Understanding morphology is essential for plant identification, taxonomy, and agriculture.

The Root

Root System Types

In majority of dicotyledonous plants, the direct elongation of the radicle leads to the formation of primary root inside the soil. It bears lateral roots of several orders that are referred to as secondary, tertiary, etc. roots. The primary roots and its branches constitute the tap root system, as seen in the mustard plant.

In monocotyledonous plants, the primary root is short lived and is replaced by a large number of roots. These roots originate from the base of the stem and constitute the fibrous root system, as seen in the wheat plant.

In some plants, like grass, Monstera and the banyan tree, roots arise from parts of the plant other than the radicle and are called adventitious roots.

Regions of the Root

• Root cap: The apex of the root is covered by a thimble-like structure called the root cap. It protects the tender apex of the root as it makes its way through the soil.
• Region of meristematic activity: A few millimeters above the root cap lies the cells of this region. These cells are very small, thin-walled and with dense protoplasms. They divide repeatedly.
• Region of elongation: The cells proximal to the region of meristematic activity undergo rapid elongation and enlargement and are responsible for the growth of the root in length.
• Region of maturation: The elongated cells gradually differentiate and mature. From this region, some of the epidermal cells form very fine and delicate, thread-like structures called root hairs. These root hairs absorb water and minerals from the soil.

Modifications of Root

Roots in some plants change their shape and structure and become modified to perform functions other than absorption and conduction of water and minerals. They are modified for support, storage of food and respiration.

The Stem

Stem Characteristics

The stem is the ascending part of the axis that bears branches, leaves, flowers and fruits. It develops from the plumule of the embryo of a germinating seed. The stem bears nodes and internodes. The region of the stem where leaves are born are called nodes while internodes are the portions between two nodes. The stem bears buds, which may be terminal or axillary. Stems are green when young and later often become woody and dark brown.

Modifications of Stem

The stem is not always exactly what it appears to be. It can be modified to perform various functions:

• Underground stems: Modified to store food and tide over conditions unfavourable for growth. Examples: Potato, ginger, turmeric, zaminkand, colocasia.
• Stem tendrils: Slender, spirally coiled structures developed from axillary buds that help plants climb. Examples: Gourds (cucumber, pumpkins, watermelon) and grapevines.
• Thorns: Woody, straight and pointed structures modified from axillary buds that protect plants from browsing animals. Examples: Citrus, Bougainvillea.
• Phylloclade: Flattened or fleshy cylindrical stems containing chlorophyll that carry out photosynthesis in arid regions. Examples: Opuntia (flattened), Euphorbia (cylindrical).
• Runners, stolons, suckers, offsets: Creeping stems for vegetative propagation. Examples: Grass and strawberry (runners), Mint and jasmine (stolons), Banana, pineapple and Chrysanthemum (suckers), Pistia and Eichhornia (offsets).

The Leaf

Leaf Structure

The leaf is a lateral, generally flattened structure borne on the stem. It develops at the node and bears a bud in its axil. The axillary bud later develops into a branch. Leaves originate from shoot apical meristems and are arranged in an acropetal order. They are the most important vegetative organs for photosynthesis.

A typical leaf consists of three main parts: leaf base, petiole and lamina.

• Leaf base: The leaf is attached to the stem by the leaf base and may bear two lateral small leaf-like structures called stipules. In monocotyledons, the leaf base expands into a sheath covering the stem partially or wholly. In some leguminous plants, the leafbase may become swollen, which is called the pulvinus.
• Petiole: The petiole help hold the blade to light. Long thin flexible petioles allow leaf blades to flutter in wind, thereby cooling the leaf and bringing fresh air to leaf surface.
• Lamina: The lamina or leaf blade is the green expanded part of the leaf with veins and veinlets. There is usually, a middle prominent vein, which is known as the midrib. Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals and food materials.

Venation

The arrangement of veins and the veinlets in the lamina of leaf blade is termed as venation.

• Reticulate venation: When the veinlets form a network, the venation is termed reticulate. Generally observed in dicotyledonous plants.
• Parallel venation: When the veins run parallel to each other within a lamina, the venation is termed parallel. Generally observed in monocotyledonous plants.

Types of Leaves

• Simple leaf: A leaf is said to be simple, when its lamina is entire or when incised, the incisions do not touch the midrib.
• Compound leaf: When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called compound compound.
• Pinnately compound leaf: A number of leaflets are present on a common axis, the rachis, which represents the midrib as in neem.
• Palmately compound leafs: The leaflets are attached at a common point, i.e., at the tip of petiole, as in silk cotton.

Phyllotaxy

Phyllotaxy is the pattern of arrangement of leaves on the stem or branch. It is usually of three types:

• Alternate: A single leaf arises at each node in alternate manner, e.g., china rose, mustard and sun flower.
• Opposite: A pair of leaves arise at each node and lie opposite to each other, e.g., Calotropis and guava.
• Whorled: If more than two leaves arise at a node and form a whorl, it is called whorled, e.g., Alstonia.

Modifications of Leaves

Leaves are often modified to perform functions other than photosynthesis. They are converted into tendrils for climbing as in peas, into spines for defense as in cacti, fleshy leaves to store food as in onion and garlic, and pitcher/trap mechanisms in insectivorous plants like pitcher plants and Venus fly trap.

The Inflorescence

A flower is a modified shoot wherein the shoot apical meristem changes to floral meristem. Internodes do not elongate and the axis gets condensed. The axis gets successively arranged on a floral axis termed the inflorescence. Depending on whether the apex gets converted into a flower or continues to grow, major types of inflorescences are defined:

• Racemose: The main axis continues to grow, the flowers are borne laterally in an acropetal succession (older flowers at the base and younger flowers at the tip).
• Cymose: The main axis terminates in a flower, hence is limited in growth. The flowers are borne in a basipetal order (older flowers at the apex and younger flowers at the base).

The Flower

The flower is the reproductive unit in the angiosperms. It is meant for sexual reproduction. A typical flower has four different kinds of whorls arranged successively on the swollen end of the stalk or pedicel, called the thalamus or receptacle. These are calyx, corolla, androecium and gynoecium.

Parts of a Flower

• Calyx: The outermost whorl of the flower and the members are called sepals. Generally, sepals are green, leaf-like and protect the flower in the bud stage.
• Corolla: Corolla is composed of petals. Petals are usually brightly coloured to attract insects for pollination.
• Androecium: Androecium is composed of stamens. Each stamen represents the male reproductive organ consisting of a stalk or a filament and an anther.
• Gynoecium: Gynoecium is the female reproductive part of the flower and is made up of one or more carpels. A carpel consists of three parts namely stigma, style and ovary.

Symmetry of Flowers

• Actinomorphic (Radial symmetry): When a flower can be divided into two equal radial halves in any radial plane passing through the centre, e.g., mustard, datura, chilli.
• Zygomorphic (Bilateral symmetry): When it can be divided into two similar halves only in one particular vertical plane, e.g., pea, gulmohur, bean, cassia.
• Asymmetric (Irregular): When it cannot be divided into two similar halves by any vertical plane passing through the centre, e.g., canna.

Position of Floral Parts on Thalamus

Hypogynous: The gynoecium occupies the highest position while the other parts are situated below it. The ovary is said to be superior, e.g., mustard, china rose and brinjal.
Perigynous: If the gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level, the ovary is half inferior, e.g., plum, rose, peach.
Epigynous: The margin of thalamus grows upward enclosing the ovary completely and getting fused with it, the other parts arise above the ovary. The ovary is said to be inferior, e.g., guava and cucumber, and the ray florets of sunflower.

The Fruit

The fruit is a characteristic feature of the flowering plants. It is a mature or ripened ovary, developed after fertilisation. If a fruit is formed without fertilisation of the ovary, it is called a parthenocarpic fruit.

Generally, the fruit consists of a wall or pericarp and seeds. The pericarp may be dry or fleshy. When pericarp is thick and fleshy, it is differentiated into the outer epicarp, the middle mesocarp and the inner endocarp.

The Seed

The ovules after fertilisation, develop into seeds. A seed is made up of a seed coat and an embryo. The embryo is made up of a radicle, an embryonal axis and one (as in wheat, maize) or two cotyledons (as in gram and pea).

Structure of a Dicotyledonous Seed

The outermost covering of a seed is the seed coat. The seed coat has two layers, the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle. Within the seed coat is the embryo, consisting of an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food materials. At the two ends of the embryonal axis are present the radicle and the plumule.

Structure of a Monocotyledonous Seed

Generally, monocotyledonous seeds are endospermic but some as in orchids are non-endospermic. In the seeds of cereals such as maize, the seed coat is membranous and generally fused with the fruit wall. The endosperm is bulky and stores food. The outer covering of endosperm separates the embryo by a proteinaceous layer called aleurone layer. The embryo is small and situated in a groove at one end of the endosperm. It consists of one large and shield shaped cotyledon known as scutellum and a short axis with a plumule and a radicle. The plumule and radicle are enclosed in sheaths which are called coleoptile and coleorhiza respectively.

Description of Family Solanaceae

Solanaceae is a large family, commonly called as the 'potato family'. It is widely distributed in tropics, subtropics and even temperate zones.

Vegetative Characters

• Plants: Herbaceous plants, shrubs and rarely small trees.
• Stem: Herbaceous rarely woody, aerial, erect, cylindrical, branched, solid or hollow, hairy or glabrous, underground stem in potato (Solanum tuberosum).
• Leaves: Alternate, simple, rarely pinnately compound, exstipulate; venation reticulate.

Floral Characters

• Inflorescence: Solitary, axillary or cymose as in Solanum.
• Flower: Bisectual, actinomorphic.
• Calyx: Sepals five, united, persistent, valvate aestivation.
• Corolla: Petals five, united; valvate aestivation.
• Androecium: Stamens five, epipetalous.
• Gynoecium: Bicarpellary obligate, syncarpous; ovary superior, bilocular, placenta swollen with many ovules.
• Fruits: Berry or capsule.
• Seeds: Many, endospermous.

Economic Importance

Many plants belonging to this family are source of food (tomato, brinjal, potato), spice (chilli), medicine (belladonna, ashwagandha), fumigatory (tobacco), and ornamentals (petunia).