MORPHOLOGY OF FLOWERING PLANTS NOTES

•  The wide range in the structure of higher plants is fascinating, particularly the diversity found in angiosperms.

• Despite the large diversity in external structure or morphology, all angiosperms share certain characteristics: roots, stems, leaves, flowers, and fruits.

• Standard technical terms and definitions are essential for successful classification and understanding of higher plants and other living organisms.

• Variations in different plant parts are often adaptations to their environment, such as adaptations to various habitats, protection, climbing, or storage.

• When pulling out any weed, you will observe that they all have roots, stems, and leaves. Some may also bear flowers and fruits.

• The underground part of a flowering plant is known as the root system, while the portion above the ground is called the shoot system.

5.1 THE ROOT

• In dicotyledonous plants, the radicle directly elongates to form the primary root, which grows into the soil. This primary root gives rise to lateral roots of various orders, known as secondary, tertiary, etc. roots.

• The primary root and its branches collectively make up the tap root system, as seen in plants like mustard.

• In monocotyledonous plants, the primary root is short-lived and is replaced by numerous roots. These roots originate from the base of the stem and form the fibrous root system, as seen in plants like wheat.

• Certain plants, such as grass, Monstera, and the banyan tree, develop roots from parts other than the radicle, known as adventitious roots.

• The main functions of the root system include the absorption of water and minerals from the soil, providing anchorage to plant parts, storing reserve food material, and synthesizing plant growth regulators.

5.1.1 Regions of the Root

• The apex of the root is covered by a protective structure called the root cap (Figure 5.3), which safeguards the delicate tip of the root during its growth in the soil.

• A few millimeters above the root cap is the region of meristematic activity, where small, thin-walled cells with dense protoplasm undergo repeated divisions.

• The cells proximal to the meristematic region undergo rapid elongation and enlargement, contributing to the lengthening of the root. This region is known as the region of elongation.

• As the cells in the elongation zone mature, they gradually differentiate. The region proximal to the elongation zone is called the region of maturation.

• From the region of maturation, some of the epidermal cells develop fine and delicate structures known as root hairs.

• The root hairs play a crucial role in absorbing water and minerals from the soil.

5.2 THE STEM

• The stem is the ascending part of the plant axis that gives rise to branches, leaves, flowers, and fruits. It develops from the plumule of the embryo during seed germination.

• The stem is characterized by nodes and internodes. Nodes are the regions where leaves are attached, while internodes are the portions between two nodes.

• Buds are present on the stem, which can be terminal (located at the apex) or axillary (found in the leaf axils).

• In its young stage, the stem is generally green and later often becomes woody, turning dark brown.

• The main function of the stem is to spread out branches that bear leaves, flowers, and fruits. It also conducts water, minerals, and photosynthates (products of photosynthesis) throughout the plant.

• Some stems are involved in food storage, providing support to the plant, offering protection to delicate structures, and facilitating vegetative propagation (reproduction without seeds).

5.3 THE LEAF

• A leaf is a lateral and generally flattened structure that is borne on the stem. It develops at the node and has a bud in its axil, which later develops into a branch.

• Leaves originate from shoot apical meristems and are arranged in an acropetal (from base to apex) order.

• Leaves are crucial vegetative organs for photosynthesis.

• A typical leaf consists of three main parts: leaf base, petiole, and lamina (Figure 5.4a).

• The leaf is attached to the stem by the leaf base and may have stipules, which are small leaf-like structures located laterally.

• In monocotyledons, the leaf base may expand into a sheath that partially or wholly covers the stem. In some leguminous plants, the leaf base may become swollen, known as the pulvinus.

• The petiole helps hold the leaf blade to light. Long, thin, flexible petioles allow the leaf blades to flutter in the wind, which helps cool the leaf and bring fresh air to its surface.

• The lamina, also known as the leaf blade, is the green expanded part of the leaf containing veins and veinlets.

• The midrib is a prominent vein in the middle of the leaf blade. Veins provide rigidity to the leaf blade and serve as channels for transport of water, minerals, and food materials.

• The shape, margin (edge), apex (tip), surface, and extent of incision of the leaf blade can vary in different leaves.

5.3.1 Venation

• The arrangement of veins and veinlets in the leaf blade is referred to as venation.

• Reticulate venation is observed when the veinlets form a network-like pattern throughout the leaf blade

• Parallel venation is characterized by veins running parallel to each other within the leaf blade 

• Dicotyledonous plants typically exhibit reticulate venation in their leaves.

• Monocotyledonous plants, on the other hand, commonly have leaves with parallel venation.

5.3.2 Types of Leaves

A leaf is considered simple when its lamina is undivided and does not have incisions that reach the midrib.

In simple leaves with incisions, the incisions do not touch the midrib.

A leaf is classified as compound when the incisions of the lamina reach the midrib, dividing it into multiple leaflets.

Both simple and compound leaves have buds present in the axil of the petiole, but in compound leaves, there is no bud in the axil of the leaflets.

Compound leaves can be categorized into two types

Pinnately compound leaves have multiple leaflets attached to a common axis called the rachis, which represents the midrib of the leaf. An example is the neem tree.

Palmately compound leaves have leaflets attached at a common point, typically at the tip of the petiole. An example is the silk cotton tree.

5.3.3 Phyllotaxy

• Phyllotaxy refers to the pattern of leaf arrangement on a stem or branch.

• There are three main types of phyllotaxy: alternate, opposite, and whorled 

• In alternate phyllotaxy, a single leaf arises at each node in an alternate manner along the stem or branch. Examples include china rose, mustard, and sunflower plants.

• In opposite phyllotaxy, a pair of leaves arises at each node and they are positioned opposite to each other along the stem or branch. Examples include Calotropis and guava plants.

• Whorled phyllotaxy occurs when more than two leaves arise at a node and form a whorl-like arrangement. An example is Alstonia.

5.4 THE INFLORESCENCE

• A flower is a modified shoot in which the shoot apical meristem transforms into a floral meristem.

• Unlike regular shoots, the internodes of a flower do not elongate, and the axis becomes condensed.

• Instead of producing leaves, the floral meristem generates various floral appendages laterally at successive nodes.

• When a shoot tip undergoes the transformation into a flower, it is always solitary, meaning there is only one flower at the apex.

• The arrangement of flowers on the floral axis is known as an inflorescence.

• There are two major types of inflorescences based on the development of the apex: racemose and cymose.

• In racemose inflorescences, the main axis of the inflorescence continues to grow, and the flowers are borne laterally in an acropetal (from base to apex) succession 

• In cymose inflorescences, the main axis terminates in a flower, thus having limited growth. The flowers are borne in a basipetal (from apex to base) order 

5.5 THE FLOWER

• The flower is the reproductive unit of angiosperms, designed for sexual reproduction.

• A typical flower consists of four whorls arranged successively on the swollen end of the stalk or pedicel, known as the thalamus or receptacle.

• The four whorls are: calyx, corolla, androecium, and gynoecium.

• The calyx and corolla are accessory organs, while the androecium and gynoecium are the reproductive organs.

• In some flowers, such as lilies, the calyx and corolla are not distinct and are collectively referred to as the perianth.

• A flower is bisexual when it has both the androecium and gynoecium, while a flower with only stamens or only carpels is unisexual.

• Flowers can exhibit two types of symmetry: actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry).

• Actinomorphic flowers can be divided into two equal radial halves in any radial plane passing through the center, while zygomorphic flowers can be divided into two similar halves only in one particular vertical plane.

• Flowers that cannot be divided into similar halves are asymmetric or irregular.

• The number of floral appendages in a flower can be trimerous (in multiples of 3), tetramerous (in multiples of 4), or pentamerous (in multiples of 5).

• Flowers with bracts (reduced leaves) at the base of the pedicel are called bracteate, while those without bracts are ebracteate.

• Based on the position of the calyx, corolla, and androecium in relation to the ovary on the thalamus, flowers are described as hypogynous, perigynous, or epigynous.

• In hypogynous flowers, the gynoecium is positioned highest, while the other parts are below it. The ovary in such flowers is superior.

• In perigynous flowers, the gynoecium is in the center, and the other parts are located on the rim of the thalamus at the same level. The ovary is half-inferior.

• In epigynous flowers, the thalamus margin grows upward, enclosing and fusing with the ovary completely. The other parts of the flower arise above the ovary. The ovary is inferior in such flowers.

5.5.1 Parts of a Flower

 5.5.1.1 Calyx

• The calyx is the outermost whorl of the flower.

• The individual members of the calyx are called sepals.

• Sepals are generally green, leaf-like structures.

• The main function of the calyx is to protect the flower during the bud stage.

• The calyx can be either gamosepalous, where the sepals are fused together, or polysepalous, where the sepals are separate and free from each other.

 5.5.1.2 Corolla

• The corolla is the second whorl of the flower and is composed of petals.

• Petals are usually brightly colored to attract insects for pollination.

• Similar to the calyx, the corolla can be either gamopetalous, where the petals are fused together, or polypetalous, where the petals are separate and free from each other.

• The shape and color of the corolla vary greatly among different plant species. It can be tubular, bell-shaped, funnel-shaped, or wheel-shaped.

• Aestivation refers to the mode of arrangement of sepals or petals in the floral bud with respect to the other members of the same whorl.

• The main types of aestivation are valvate (sepals or petals just touching each other at the margin), twisted (one margin of the appendage overlaps the next one), imbricate (margins of sepals or petals overlapping one another but not in any particular direction), and vexillary or papilionaceous (specific to pea and bean flowers, where the largest petal overlaps the lateral petals, which in turn overlap the smallest anterior petals).

5.5.1.3 Androecium

• Androecium is the third whorl of the flower and is composed of stamens, which represent the male reproductive organs.

• Each stamen consists of a filament (stalk) and an anther. The anther is usually bilobed and contains two chambers called pollen-sacs, where the pollen grains are produced.

• A sterile stamen that lacks the ability to produce pollen is called a staminode.

• The stamens may be united with other floral parts such as petals or with each other.

• Epipetalous stamens are attached to the petals, while epiphyllous stamens are attached to the perianth (collective term for calyx and corolla).

• The stamens may remain free (polyandrous) or may be united to varying degrees. They can be united into one bundle (monoadelphous), two bundles (diadelphous), or more than two bundles (polyadelphous).

• Variation in the length of filaments within a flower can be observed in some plant species, such as Salvia and mustard.

5.5.1.4 Gynoecium

• Gynoecium is the fourth whorl of the flower and represents the female reproductive part.

• Gynoecium is composed of one or more carpels, which are the basic units of the female reproductive structure. Each carpel consists of three parts: stigma, style, and ovary.

• The ovary is the enlarged basal part of the carpel and contains one or more ovules. It is connected to the stigma by the style.

• The stigma is the receptive surface located at the tip of the style, where pollen grains land during pollination.

• The ovules are attached to the placenta, a cushion-like structure within the ovary. The ovules develop into seeds after fertilization.

• Gynoecium can have either a single carpel (simple gynoecium) or multiple carpels (compound gynoecium).

• Apocarpous gynoecium refers to the condition where carpels are free from each other, while syncarpous gynoecium refers to the condition where carpels are fused.

• After fertilization, the ovary undergoes maturation and develops into a fruit, which contains the seeds.

• Placentation refers to the arrangement of ovules within the ovary.

• Different types of placentation include marginal, axile, parietal, basal, central, and free central, based on the position of the placenta and the attachment of ovules within the ovary.

5.6 THE FRUIT

• Fruit is the mature or ripened ovary of a flower, developed after fertilization.

• Parthenocarpic fruit refers to a fruit that is formed without fertilization of the ovary.

• The fruit consists of a wall called the pericarp and seeds.

• The pericarp can be dry or fleshy, depending on the type of fruit.

• In fleshy fruits, the pericarp is thick and differentiated into three layers: the outer epicarp, the middle mesocarp, and the inner endocarp.

• Mango and coconut are examples of drupe fruits.

• A drupe fruit develops from a monocarpellary superior ovary and typically contains a single seed.

• In mango, the pericarp is well differentiated, with a thin epicarp, a fleshy edible mesocarp, and a stony hard endocarp.

• Coconut, also a drupe, has a fibrous mesocarp.

• These examples illustrate the diversity in fruit structure and composition.

5.7 THE SEED

• After fertilization, the ovules develop into seeds.

• A seed is composed of two main parts: the seed coat and the embryo.

• The seed coat, also known as the testa, is the protective outer covering of the seed.

• The embryo is the young, undeveloped plant contained within the seed.

• The embryo consists of several components, including the radicle, embryonal axis, and cotyledons.

• The radicle is the embryonic root of the plant.

• The embryonal axis is the portion of the embryo between the radicle and the cotyledons.

• In some plants like wheat and maize, the embryo has a single cotyledon.

• In other plants like gram and pea, the embryo has two cotyledons.

• The cotyledons serve as a source of nutrients for the developing seedling during germination.

• The embryo also contains other structures that will develop into the plant's shoot system, such as the plumule (embryonic shoot) and the hypocotyl (region below the cotyledons).

• The seed plays a vital role in the propagation and survival of plants. It contains all the necessary components for the plant's initial growth and development.

5.7.1 Structure of a Dicotyledonous Seed

• The outermost covering of a seed is the seed coat, which has two layers: the outer testa and the inner tegmen.

• The hilum is a scar on the seed coat where the seed was attached to the fruit or ovary.

• Above the hilum, there is a small pore called the micropyle, which allows water and gases to enter the seed.

• The embryo is located within the seed coat and consists of an embryonal axis and two cotyledons.

• The cotyledons are often fleshy and serve as a source of stored food materials for the developing seedling.

• The embryonal axis includes the radicle at one end, which develops into the primary root of the plant, and the plumule at the other end, which develops into the shoot system.

• Some seeds, such as castor seeds, have endosperm, a food-storing tissue formed as a result of double fertilization.

• In contrast, seeds of plants like bean, gram, and pea do not have endosperm in mature seeds and are referred to as non-endospermous seeds.

• The stored food reserves in the cotyledons or endosperm provide nourishment to the embryo during germination until it can establish photosynthesis and acquire nutrients from the environment.

5.7.2 Structure of Monocotyledonous Seed

• Monocotyledonous seeds are generally endospermic, meaning they have endosperm as a food storage tissue.

• However, some monocotyledonous seeds, like those of orchids, are non-endospermic, meaning they lack endosperm.

• In cereals such as maize (corn), the seed coat is membranous and fused with the fruit wall.

• The endosperm in cereal seeds is bulky and serves as the main storage tissue for food reserves.

• The endosperm is separated from the embryo by a proteinous layer called the aleurone layer.

• The embryo in cereal seeds is small and located in a groove at one end of the endosperm.

• The embryo consists of one large and shield-shaped cotyledon known as the scutellum, along with a short axis that contains a plumule (embryonic shoot) and a radicle (embryonic root).

• The plumule and radicle in cereal seeds are enclosed in protective sheaths called the coleoptile and coleorhiza, respectively.

5.8 SEMI-TECHNICAL DESCRIPTION OF A TYPICAL FLOWERING PLANT

• Habit: The plant belongs to the Brassicaceae family.

• Vegetative characters:

• Roots: Not specified.

• Stem: Not specified.

• Leaves: Not specified.

• Floral characters:

• Inflorescence: Not specified.

• Flower parts:

1. Br: Bracteate (presence of bracts).
2. K: Calyx (outermost whorl, sepals).
3. C: Corolla (petals).
4. P: Perianth (collective term for calyx and corolla if they are similar).
5. A: Androecium (stamens).
6. G: Gynoecium (carpels).
7. G: Superior ovary (ovary position above attachment of other floral parts).
8. G: Inferior ovary (ovary position below attachment of other floral parts).
9. ♀: Female (presence of gynoecium).
10. ♂: Male (presence of androecium).
11. ∗: Bisexual (presence of both male and female parts).
12. ⊕: Actinomorphic (radial symmetry).
13. ǀ: Zygomorphic (bilateral symmetry).

• Fusion: Indicated by enclosing the figure within brackets.

• Adhesion: Indicated by a line drawn above the symbols of floral parts.

• Floral diagram: Represents the arrangement and relation of flower parts, with calyx on the outermost and gynoecium in the center. The position of the mother axis is represented by a dot on the top.

• Floral formula: Represents the same information as the floral diagram, including cohesion and adhesion within and between whorls.

• The specific floral diagram and floral formula provided in Figure 5.16 represent the mustard plant (Family: Brassicaceae).

5.9 SOLANACEAE

• Family: Solanaceae (Potato family)

• Distribution: Widely distributed in tropical, subtropical, and temperate zones.

• Vegetative Characters:

• Plants: Mostly herbs, shrubs, and rarely small trees.

• Stem: Herbaceous, rarely woody. It can be aerial, erect, cylindrical, branched, solid or hollow. It may be hairy or glabrous. Some species have underground stems, such as the potato (Solanum tuberosum).

• Leaves: Alternate, simple, rarely pinnately compound. They are exstipulate and exhibit reticulate venation.

• Floral Characters:

• Inflorescence: Solitary or axillary, sometimes cymose as in Solanum species.

• Flower: Bisexual and actinomorphic (radially symmetric).

• Calyx: Consists of five united sepals, persistent, with valvate aestivation (overlapping in bud).

• Corolla: Composed of five united petals with valvate aestivation.

• Androecium: Consists of five stamens, epipetalous (attached to the petals).

• Gynoecium: Bicarpellary and syncarpous. The ovary is superior, bilocular, and has a swollen placenta with many axile ovules.

• Fruits: Can be a berry or a capsule.

• Seeds: Numerous and endospermous (containing endosperm).

• Floral Formula: ⊕ (Bisexual flower with actinomorphic symmetry)

• Economic Importance:

• Many plants in the Solanaceae family have significant economic importance:

• Food sources: Tomato, brinjal (eggplant), potato.

• Spices: Chilli (pepper).

• Medicinal plants: Belladonna, ashwagandha.

• Fumigatory: Tobacco.

• Ornamentals: Petunia.

IMPORTANT DIFFERENCES