Plant Morphology: Roots

• Morphology refers to the study of the form and features of plants.

The Root

• The root is the underground part of the plant and is responsible for absorbing water and minerals, anchoring the plant, storing food, and synthesizing regulators.
• Roots develop from the radicle of the plant embryo.
• The first root to emerge is the primary root.
• In some plants, the primary root persists and forms the taproot system.
• In other plants, the primary root is short-lived and is replaced by a large number of roots, forming the fibrous root system.
• Adventitious roots arise from parts of the plant other than the radicle.
• The root cap is a protective layer of cells that covers the apex of the root.
• The region of meristematic activity is responsible for the growth of the root.
• The region of elongation is where the cells increase in size.
• The region of maturation is where the cells differentiate into specialized cells.
• Root hairs are slender, thin-walled extensions of the root epidermal cells and help absorb water and minerals.
• Roots can be modified to perform various functions such as storage, support, and respiration.
• Prop roots are aerial roots that provide support to the plant.
• Stilt roots arise from the lower nodes of the stem and provide support to the plant.
• Pneumatophores are specialized roots that help plants obtain oxygen from the atmosphere.

The Stem

• The stem is the aerial part of the plant and bears branches, leaves, flowers, and fruits.
• The stem develops from the plumule of the embryo.
• The stem conducts water and minerals from the roots to the leaves and other parts of the plant.
• The stem also transports food from the leaves to other parts of the plant.
• The stem may be modified to perform various functions such as storage, support, and protection.
• Tendrils are slender, wiry structures that help plants climb.
• Thorns are sharp, pointed structures that protect plants from herbivores.
• Some stems are modified to store food, such as potato tubers and ginger rhizomes.
• Underground stems of potato, ginger, turmeric, zaminkand, and Colocasia are modified to store food and perennate under unfavorable conditions.
• Stem tendrils are found in gourds (cucumber, pumpkin, watermelon) and grapevines which help the plant to climb.
• Axillary buds of stems may also get modified into woody, straight and pointed thorns and are found in Citrus and Bougainvillea.
• Phylloclade is a flattened or cylindrical stem and is modified to perform photosynthesis.

The Leaf

• The leaf is the lateral, flattened structure borne on the stem and is responsible for photosynthesis.
• It develops at the node and bears a bud in its axil.
• Leaves originate from shoot apical meristems and are arranged in an acropetal order.
• The petiole is the stalk of the leaf that attaches it to the stem.
• The lamina or leaf blade is the expanded, green part of the leaf.
• Veins provide rigidity to the leaf blade and act as channels for transport of water minerals and food materials.
• The arrangement of veins and veinlets in the lamina is called venation.
• Reticulate venation is when the veins are arranged in a network.
• Parallel venation is when the veins run parallel to each other.
• Simple leaf: A leaf with a single, undivided lamina.
• Compound leaf: A leaf where the lamina is divided into multiple leaflets.
• Pinnately compound leaf: Leaflets arranged on a common axis called the rachis.
• Palmately compound leaf: Leaflets attached at a common point.
• Phyllotaxy is the pattern of arrangement of leaves on the stem or branch.
• Alternate phyllotaxy: A single leaf arises at each node in an alternate manner.
• Opposite phyllotaxy: Two leaves arise at each node and lie opposite to each other.
• Whorled phyllotaxy: More than two leaves arise at a node and form a whorl.
• Leaves are modified to perform various functions such as storage, protection, and support.
• Some leaves are modified into tendrils for climbing.
• Spines (e.g., cactus) are modified leaves that provide protection.
• In some plants, leaves are modified to store food.
• Pitcher plant leaves are modified to trap insects.
• In Australian Acacia, the leaves are small and short-lived. The petioles in these plants expand, become green and synthesize food.

The Inflorescence

• An inflorescence is the arrangement of flowers on the floral axis.
• Racemose inflorescence: The main axis continues to grow, and flowers are borne laterally.
• Cymose inflorescence: The main axis terminates in a flower, and the flowers are borne in a basipetal order.
• It is defined as the arrangement of flowers on the floral axis.
• Main types are racemose and cymose based on whether the apex continues to grow or terminates in a flower.

The Flower

• The flower is the reproductive unit in the angiosperms, specialized for sexual reproduction.
• A typical flower has four kinds of whorls arranged successively on the thalamus or receptacle: calyx, corolla, androecium and gynoecium.
• Calyx: The outermost whorl of the flower, composed of sepals.
• Corolla: The second whorl of the flower, composed of petals.
• Androecium: The male reproductive part of the flower, composed of stamens.
• Gynoecium: The female reproductive part of the flower, composed of carpels.
• Bisexual flower: A flower containing both androecium and gynoecium.
• Unisexual flower: A flower containing either androecium or gynoecium, but not both.
• Actinomorphic flower: A flower that can be divided into two equal halves in any radial plane.
• Zygomorphic flower: A flower that can be divided into two equal halves in only one vertical plane.
• A complete flower has all four floral whorls.
• An incomplete flower is missing one or more floral whorls.
• Hypogynous flower: The gynoecium occupies the highest position, and the other floral parts are attached below it.
• Perigynous flower: The gynoecium is situated in the center, and other floral parts are located on the rim of the thalamus at the same level.
• Epigynous flower: The margin of the thalamus grows upward, enclosing the ovary completely, and other floral parts arise above the ovary.
• Aestivation refers to the mode of arrangement of sepals or petals in a floral bud with respect to the other members of the same whorl.
• Valvate: Sepals or petals in a whorl just touch one another at the margin, without overlapping.
• Twisted: One margin of the appendage overlaps that of the next one, and so on.
• Imbricate: The margins of the sepals or petals overlap one another, but not in any particular direction.
• Vexillary: The largest petal (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel).

The Androecium

• The androecium is the male reproductive part of the flower, composed of stamens.
• A stamen consists of a filament and an anther.
• The anther is the bilobed structure that produces pollen grains.
• The filament is the stalk that supports the anther.
• Staminode: Sterile stamen.
• Epipetalous: Stamens are attached to the petals (e.g., brinjal).
• Epiphyllous: Stamens are attached to the perianth (e.g., lily).
• The cohesion of stamens can vary; they may be free (polyandrous) or united in varying degrees.
• Monadelphous: Stamens united into one bundle (e.g., china rose).
• Diadelphous: Stamens united into two bundles (e.g., pea).
• Polyadelphous: Stamens united into more than two bundles (e.g., citrus).
• Variation in stamen length can occur within a flower.

The Gynoecium

• The gynoecium is the female reproductive part of the flower, composed of carpels.
• A carpel consists of an ovary, style, and stigma.
• The ovary is the enlarged basal part that contains the ovules.
• The style is the elongated stalk that connects the ovary to the stigma.
• The stigma is the receptive surface for pollen grains.
• Apocarpous: Carpels are free (e.g., lotus and rose).
• Syncarpous: Carpels are fused (e.g., mustard and tomato).
• The ovary contains one or more ovules attached to a flattened, cushion-like placenta.
• Placentation is the arrangement of ovules within the ovary.
• Marginal placentation: The placenta forms a ridge along the ventral suture of the ovary, and the ovules are borne on this ridge (e.g., pea).
• Axile placentation: The placenta is axial, and the ovules are attached to it in a multilocular ovary (e.g., china rose, tomato, and lemon).
• Parietal placentation: The ovules develop on the inner wall of the ovary or on peripheral part. Ovary is one-chambered but becomes two-chambered due to the formation of a false septum (e.g., mustard and Argemone).
• Free central placentation: Ovules are borne on a central axis without septa (e.g., Dianthus and Primrose).
• Basal placentation: The placenta develops at the base of the ovary, and a single ovule is attached to it (e.g., sunflower and marigold).

The Fruit

• The fruit is the mature or ripened ovary, developed after fertilization.
• Fruits formed without fertilization of the ovary are called parthenocarpic fruits.
• A typical fruit consists of the pericarp (fruit wall) 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.
• Dry fruits do not have a fleshy pericarp.
• Fleshy fruits: Drupes develop from mono- or multi-carpellary, superior ovaries and are one-seeded (e.g., mango and coconut).
  • Mango pericarp differentiates into an outer epicarp, a fleshy edible mesocarp, and an inner stony hard endocarp.
  • Coconut mesocarp is fibrous.
• Fruits are classified into simple, aggregate, and composite fruits.

The Seed

• The seed is the fertilized ovule and consists of an embryo and a seed coat.
• The seed coat develops from the integuments of the ovule.
• The embryo consists of an embryonal axis and one (in monocotyledons) or two (in dicotyledons) cotyledons.
• The hilum is a scar on the seed coat through which the seed was attached to the fruit.
• The micropyle is a small pore above the hilum that facilitates entry of oxygen and water into the seed during germination.
• Albumious seeds: Seeds having residual endosperm (e.g., wheat, maize, barley, castor, and sunflower).
• Non-albuminous seeds: Seeds without residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).
• The plumule is the future shoot.
• The radicle is the future root.
• The cotyledons are the seed leaves that store food.
• The endosperm is the nutritive tissue that provides nourishment to the developing embryo.

Floral Formula and Floral Diagrams

• The floral formula is a symbolic representation of the flower, showing the number, arrangement, and relationships of the floral parts.
• The floral diagram is a diagrammatic representation of the flower, showing the number, arrangement, and aestivation of the floral parts.
• Symbols used in floral formula:
  • Br: Bracteate
  • Ebr: Ebracteate
  • ⚥: Bisexual
  • ⊕: Actinomorphic
  • %: Zygomorphic
  • K: Calyx (number of sepals)
  • C: Corolla (number of petals)
  • A: Androecium (number of stamens)
  • G: Gynoecium (number of carpels)
  • _G: Superior ovary
  • ͞G: Inferior ovary
  • G with a line above: Half-inferior ovary
  • ( ): Fusion
  • Numbers indicate the number of parts in each whorl.