Root:

• In the majority of dicotyledonous plants, the direct elongation of the radicle leads to the formation of a primary root that grows inside the soil.

• It bears lateral roots of several orders that are referred to as secondary, and tertiary, roots.

  • The primary roots and their branches constitute the tap root system.

    • In monocotyledonous plants, the primary root is short-lived and is replaced by a large number of roots that originate from the base of the stem and constitute the fibrous root system.

      • Example: Wheat.

• In some plants, roots arise from parts of the plant other than the radicle and are called adventitious roots.

  • Examples: Grass, Monstera, and Banyan Tree.

Functions of Root:

• Absorption of water and minerals from the soil.

• Providing a proper anchorage to the plant parts.

• Storing reserve food material.

• Synthesis of plant growth regulators.

Regions of the Root:

• The root is covered at the apex 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.

• Above the root cap is the region of meristematic activity.

  • The cells in this region are very small, thin-walled, and with dense protoplasm which divides rapidly.

• The cells proximal to this region undergo rapid elongation and enlargement and are responsible for the growth of the root in length which is called the region of elongation.

  • The cells of the elongation zone gradually differentiate and mature.

• The zone, proximal to the region of elongation, is called the region of maturation.

  • The epidermal cells form very fine and delicate, thread-like structures called root hairs from this region which 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

  • Tap roots of carrot, turnip, and adventitious roots of sweet potato, get swollen and store food.

  • The hanging structures that support a banyan tree are called prop roots

  • The stems of maize and sugarcane have supporting roots coming out of the lower nodes of the stem which are called stilt roots.

  • In some plants such as Rhizophora growing in swampy areas, many roots come out of the ground and grow vertically upwards which are called pneumatophores, and this help to get oxygen for respiration.

Stem:

• It is the ascending part of the axis bearing 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 is called nodes while internodes are the portions between two nodes.

• The stem bears buds, which may be terminal or axillary.

• The stem is generally green when young and later often become woody and dark brown.

Functions of Stem:

• Spreading out branches bearing leaves, flowers, and fruits.

• It conducts water, minerals, and photosynthates.

• Storage of food, support, protection, and vegetative propagation.

Modifications of Stem:

• They are modified to perform different functions:

  • Underground stems of potato, ginger, turmeric, zaminkand, Colocasia are modified to store food in them.

  • They also act as organs of perennation to tide over conditions unfavorable for growth.

  • Stem tendrils which develop from axillary buds, are slender and spirally coiled and help plants to climb such as gourds (cucumber, pumpkins, watermelon) and grapevines.

  • Axillary buds of stems may also get modified into woody, straight and pointed thorns. Example: Citrus, Bougainvillea

  • They protect plants from browsing animals.

  • Some plants of arid regions modify their stems into flattened (Opuntia), or fleshy cylindrical (Euphorbia) structures. They contain chlorophyll and carry out photosynthesis

  • Underground stems of some plants such as grass and strawberry, spread to new niches, and when older parts die new plants are formed.

  • In plants like mint and jasmine, a slender lateral branch arises from the base of the main axis and after growing aerially for some time arch downwards to touch the ground.

  • A lateral branch with short internodes and each node bearing a rosette of leaves and a tuft of roots are found in aquatic plants like Pistia and Eichhornia.

  • In banana, pineapple, and Chrysanthemum, the lateral branches originate from the basal and underground portion of the main stem, grow horizontally beneath the soil, and then come out obliquely upward giving rise to leafy shoots.

Leaf:

• 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 -

  • 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 leaf base may become swollen, which is called the pulvinus.

  • 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 the leaf surface.

  • The lamina or the 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.

  • The shape, margin, apex, surface, and extent of incision of lamina vary in different leaves.

Venation:

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

• When the veinlets form a network, the venation is termed as reticulate.

  • Leaves of dicotyledonous plants generally possess reticulate venation

• When the veins run parallel to each other within a lamina, the venation is termed parallel.

  • Parallel venation is the characteristic of most monocotyledons.

  Types of Venation:

• A leaf is said to be simple, when its lamina is entire or when incised, the incisions do not touch the midrib.

• When the incisions of the lamina reach up to the midrib breaking it into a number of leaflets, the leaf is called a compound.

• The compound leaves may be of two types:

  • In a pinnately compound leaf, a number of leaflets are present on a common axis, the rachis, which represents the midrib of the leaf as in neem.

  • In palmately compound leaves, the leaflets are attached at a common point, i.e., at the tip of the petiole, as in silk cotton.

• 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, opposite and whorled

  • In the alternate type of phyllotaxy, a single leaf arises at each node in an alternate manner, as in china rose, mustard, and sunflower plants.

  • In the opposite type, a pair of leaves arise at each node and lie opposite to each other as in Calotropis and guava plants.

  • If more than two leaves arise at a node and form a whorl, it is called whorled, as in Alstonia.

Modifications of Leaves:

• Leaves are often modified to perform functions other than photosynthesis

  • They are converted into tendrils for climbing as in peas or into spines for defense as in cacti

  • The fleshy leaves of onion and garlic store food.

  • In some plants such as Australian acacia, the leaves are small and short-lived.

  • The petioles in these plants expand, become green, and synthesize food.

  • Leaves of certain insectivorous plants such as pitcher plants and venus-fly traps are also modified leaves.

Inflorescence:

• A flower is a modified shoot wherein the shoot apical meristem changes to a floral meristem.

• Internodes do not elongate and the axis gets condensed.

• The apex produces different kinds of floral appendages laterally at successive nodes instead of leaves.

• When a shoot tip transforms into a flower, it is always solitary.

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

• Depending on whether the apex gets developed into a flower or continues to grow, two major types of inflorescences are defined – racemose and cymose

  • In racemose inflorescences, the central axis continues to grow, and the flowers are born laterally in acropetal succession.

  • In the cymose type of inflorescence the main axis terminates in a flower, hence is limited in growth. The flowers are borne in a basipetal order.

The Flower:

• The flower is the reproductive unit in the angiosperms and 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.

  • Calyx and corolla are accessory organs, while androecium and gynoecium are reproductive organs.

    • In some flowers like lily, the calyx and corolla are not distinct and are termed perianth.

• When a flower has both androecium and gynoecium, it is bisexual.

• A flower having either only stamens or only carpels is unisexual.

• In symmetry, the flower may be actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry).

  • When a flower can be divided into two equal radial halves in any radial plane passing through the center, it is said to be actinomorphic, as in mustard, datura, and chili.

  • When it can be divided into two similar halves only in one vertical plane, it is zygomorphic, as in pea, Gulmohar, bean, and Cassia.

  • A flower is asymmetric (irregular) if it cannot be divided into two similar halves by any vertical plane passing through the center, as in canna.

• A flower may be trimerous, tetramerous, or pentamerous when the floral appendages are in multiple of 3, 4, or 5, respectively.

• Flowers with bracts-reduced leaves found at the base of the pedicel - are called bracteate and those without bracts, ebracteate.

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

  • In the hypogynous flower, the gynoecium occupies the highest position while the other parts are situated below it and the ovary in such flowers is said to be superior, as, in mustard, china rose, and brinjal.

  • If gynoecium is situated in the center and other parts of the flower are located on the rim of the thalamus almost at the same level, it is called perigynous and here The ovary is said to be half inferior, as in plum, rose, peach.

  • In epigynous flowers, the margin of the thalamus grows upward enclosing the ovary completely and getting fused with it, the other parts of the flower arise above the ovary. Hence, the ovary is said to be inferior to in flowers of guava and cucumber, and the ray florets of sunflower.

Parts of a Flower:

Each flower normally has four floral whorls, viz., calyx, corolla, androecium, and gynoecium.

Calyx:

• The calyx is the outermost whorl of the flower and the members are called sepals.

• Generally, sepals are green, and leaf-like and protect the flower in the bud stage.

  • The calyx may be gamosepalous (sepals united) or polysepalous (sepals free).

Corolla:

• Corolla is composed of petals.

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

  • Like calyx, corolla may also be gamopetalous (petals united) or polypetalous (petals free).

  • The shape and color of corolla vary greatly in plants.

  • Corolla may be tubular, bell-shaped, funnel-shaped, or wheel-shaped.

Aestivation:

• The mode of arrangement of sepals or petals in floral buds with respect to the other members of the same whorl is known as aestivation.

• The main types of aestivation are valvate, twisted, imbricate, and vexillary.

  • When sepals or petals in a whorl just touch one another at the margin, without overlapping, as in Calotropis, it is said to be valvate.

  • If one margin of the appendage overlaps that of the next one and so on as in china rose, lady’s finger, and cotton, it is called twisted.

  • If the margins of sepals or petals overlap one another but not in any particular direction as in Cassia and Gulmohar, the aestivation is called imbricate.

  • In pea and bean flowers, there are five petals, the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel); this type of aestivation is known as vexillary or papilionaceous.

Androecium:

• The androecium is composed of stamens.

  • Each stamen which represents the male reproductive organ consists of a stalk or a filament and an anther.

    • Each anther is usually bilobed and each lobe has two chambers, the pollen-sacs.

      • The pollen grains are produced in pollen-sacs.

• A sterile stamen is called a staminode.

• Stamens of flowers may be united with other members such as petals or among themselves.

  • When stamens are attached to the petals, they are epipetalous as in brinjal, or epiphyllous when attached to the perianth as in the flowers of lily.

    • The stamens in a flower may either remain free (polyandrous) or may be united in varying degrees.

  • The stamens may be united into one bunch or one bundle (monadelphous) as in china rose, or two bundles (diadelphous) as in pea, or into more than two bundles (polyadelphous) as in citrus.

• There may be a variation in the length of filaments within a flower, as in Salvia and mustard.

Gynoecium:

• The 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.

  • The ovary is the enlarged basal part, on which lies the elongated tube, the style.

  • The style connects the ovary to the stigma.

  • The stigma is usually at the tip of the style and is the receptive surface for pollen grains.

    • Each ovary bears one or more ovules attached to a flattened, cushion-like placenta.

  • When more than one carpel is present, they may be free (as in lotus and rose) and are called apocarpous.

  • They are termed syncarpous when carpels are fused, as in mustard and tomato.

• After fertilization, the ovules develop into seeds and the ovary matures into a fruit.

Placentation:

• The arrangement of ovules within the ovary is known as placentation.

• The placentation is of different types namely, marginal, axile, parietal, basal, central, and free central.

  • In marginal placentation, the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows, as in pea.

  • When the placenta is axial and the ovules are attached to it in a multilocular ovary, the placentation is said to be axile, as in china rose, tomato, and lemon.

  • In parietal placentation, the ovules develop on the inner wall of the ovary or on the peripheral part.

    • The ovary is one-chambered but it becomes two-chambered due to the formation of the false septum, e.g., mustard and Argemone.

  • When the ovules are borne on the central axis and septa are absent, as in Dianthus and Primrose the placentation is called free central.

  • In basal placentation, the placenta develops at the base of the ovary and a single ovule is attached to it, as in a sunflower, or marigold.

The Fruit:

• The fruit is a characteristic feature of flowering plants.

  • It is a mature or ripened ovary, developed after fertilization.

• If the fruit is formed without fertilization 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 the pericarp is thick and fleshy, it is differentiated into the outer epicarp, the middle mesocarp, and the inner endocarp.

    • In mango and coconut, the fruit is known as a drupe.

      • They develop from monocarpellary superior ovaries and are one seeded.

      • In mango, the pericarp is well differentiated into an outer thin epicarp, a middle fleshy edible mesocarp, and an inner stony hard endocarp.

      • In coconut which is also a drupe, the mesocarp is fibrous.

The Seed:

• The ovules after fertilization, 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, and 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.

• In some seeds such as castor, the endosperm formed as a result of double fertilization is a food storing tissue and is called endospermic seeds.

• In plants such as beans, gram, and peas, the endosperm is not present in mature seeds and such seeds are called non-endosperms.

Structure of 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 the endosperm separates the embryo by a proteinous layer called the 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