Comprehensive Plant Anatomy Notes
Meristem Classification Based on Cell Division Planes
Mass Meristem:
Cells divide in all planes to produce a mass of cells.
Examples include early stages of embryo development, endosperm development, pith and cortex development, and sporangia development.
Plate Meristem:
Cells divide mainly anticlinally in two planes, leading to a plate-like increase in surface area.
A single-layered plate meristem gives rise to the epidermis.
A two to several-layered plate meristem gives rise to the leaf blade (lamina).
Rib Meristem:
Cells divide periclinally relative to the plant body axis to form vertical columns or rows of cells.
Forms young roots, as well as the pith and cortex of young stems.
Shoot Apex Theories
Apical Cell Theory:
Put forward by Hofmeister and supported by Nageli.
States that the activity of a single apical cell governs the entire growth process in cryptogams (algae, bryophytes, and pteridophytes).
Histogen Theory:
Proposed by Hanstein (1870) and supported by Strasburger (1868).
States that the main plant body arises from a meristematic mass consisting of three distinct histogen zones:
Dermatogen: The outermost single layer of cells. Cells divide primarily by radial walls to form the epidermis. In certain plants (e.g., Ficus, Nerium, Pepromea), dermatogen produces a multilayered epidermis.
Periblem: The intermediate zone situated between the dermatogen and plerome. Composed of isodiametric cells that divide periclinally to form the primary cortex.
Plerome: The massive central core of the apices, composed of cells elongated in the longitudinal direction. Cells divide in all planes to yield the central cylinder (stele), which includes the pericycle, primary vascular tissues, medullary rays, and pith.
Haberlandt's Terminology: Haberlandt proposed the terms protoderm, ground meristem, and procambium to replace dermatogen, periblem, and plerome, respectively.
Tunica-Corpus Theory:
Proposed by Schmidt (1924) and supported by Foster (1949).
Asserts that the apical meristem consists of two tissue zones:
Tunica: The outer zone consisting of one or more cell layers surrounding a central core. The outermost layer of tunica gives rise to the epidermis.
Corpus: The central core mass of larger cells enclosed externally by the tunica. Corpus cells divide in various planes to give rise to the pith, vascular bundles, and part of the cortex.
Root Apex Structure and Theories
Histogen Theory of Root Apex:
Proposed by Hanstein (1870) for root apices.
Differs from the shoot apex by the presence of a fourth histogen layer at the tip called the calyptrogen, which gives rise to the root-cap.
Quiescent Centre:
Coined by Clowes.
In the root apex, periblem cells are arranged in a crescent-shaped, disc-like, or hemispherical region called the quiescent centre.
Cells of the quiescent centre remain inactive under normal conditions but become activated whenever the initials of the apical meristem are damaged.
Körper-Kappe (Body) Theory:
Proposed by Schüepp.
States that cells divide in two planes (transverse followed by longitudinal) forming straight or T-shaped configurations.
Structural Characteristics of Root Apex:
Covered and protected at the apex by a root-cap.
Completely lacks nodes, internodes, leaf primordia, and branch primordia.
Lateral roots arise endogenously from the pericycle layer.
Collenchyma Types:
Classified into angular, lacunar (lacunate), and lamellar collenchyma based on cell arrangement and localized irregular cell wall thickenings.
Anatomy of Monocot Stem
Ground Tissue and Stele:
Stele Type: The stele is an atactostele, characterized by numerous vascular bundles randomly scattered throughout the ground tissue. It represents the most advanced stele type.
Ground Tissue Regions:
Outer peripheral region consists of smaller, compactly arranged cells containing chloroplasts involved in photosynthesis.
Inner region consists of larger cells lacking chloroplasts, specialized for food storage.
Endodermis, pericycle, medulla, and medullary rays are not organized.
Vascular Bundle Properties:
Distribution: Peripheral vascular bundles are smaller, more numerous, and arranged compactly in 1–2 outer rings. Central/inner vascular bundles are larger, fewer, and spaced farther apart.
Bundle Sheath: Each oval vascular bundle is enclosed by a sclerenchymatous fibrous sheath, defining them as fibro-vascular bundles.
Type: Conjoint, collateral, and closed (cambium is completely absent between xylem and phloem).
Xylem and Phloem Organization:
Xylem: Contains four tracheary elements arranged in the shape of a 'Y':
Two metaxylem elements form the two upper arms of the 'Y'.
Two protoxylem elements form the stalk of the 'Y'.
Includes xylem parenchyma and xylem fibers.
Protoxylem Lacuna: During rapid internodal stretching during stem elongation, one or both protoxylem elements break down to form a water-filled lysigenous cavity known as the protoxylem lacuna.
Phloem: Positioned between the two metaxylem arms towards the periphery. Contains sieve tubes, companion cells, sieve plates, and phloem fibers. Phloem parenchyma is completely absent.
Anatomy of Dicot (Dorsiventral / Bifacial) Leaf
Epidermis:
Consists of a single layer of compactly arranged, barrel-shaped cells without intercellular spaces present on both upper (adaxial/ventral) and lower (abaxial/dorsal) surfaces.
Covered externally by a layer of cutin called the cuticle. The cuticle is thicker on the adaxial epidermis than on the abaxial epidermis.
Epidermal cells possess a vacuolated protoplast with a distinct nucleus and lack chloroplasts, except in sciophytes (shade-tolerant plants) and submerged aquatic plants.
Stomata occur predominantly on the lower abaxial surface (hypostomatic condition).
Trichomes are present on both upper and lower epidermal surfaces.
Mesophyll (Ground Tissue):
Photosynthetic assimilatory tissue situated between the upper and lower epidermis, consisting of thin-walled chloroplast-containing parenchyma cells.
Differentiated into two distinct regions:
Palisade Parenchyma: Located adjacent to the adaxial epidermis. Composed of 1–3 rows of vertically elongated, columnar cells. Cells are compactly arranged with narrow or indistinct intercellular spaces and contain a higher density of chloroplasts aligned along the cell wall contours. Imparts a dark green color to the upper leaf surface.
Spongy Parenchyma: Located adjacent to the abaxial epidermis. Composed of 3–5 rows of loosely arranged, irregularly shaped cells with large intercellular spaces. Some spaces form sub-stomatal chambers directly above stomatal pores. Acts primarily as a ventilating tissue with fewer chloroplasts, imparting a light green color to the lower surface.
Leaves with distinct adaxial and abaxial surfaces are designated as bifacial leaves.
Vascular Bundles (Veins):
Embedded within the mesophyll as veins; conduct water, minerals, and organic food while providing mechanical support to the lamina.
Veins decrease in thickness from the leaf base toward the apex and margins.
Almost spherical in cross-section with xylem and phloem arranged side-by-side along the same radius.
Type: Conjoint, collateral, closed (lacks cambium).
Orientation: Xylem is adaxial; phloem is abaxial. Protoxylem is adaxial and metaxylem is abaxial (endarch xylem orientation as protoxylem points away from phloem).
Bundle Sheath & Extensions:
Enclosed by a specialized layer of mesophyll called border parenchyma or bundle sheath.
Adaxial and abaxial bundle sheath cells divide to form bundle sheath extensions reaching the epidermis.
Bundle sheath extensions often become collenchymatous, granting tensile strength to the leaf lamina and veins, while conducting water to mesophyll cells and organic solutes to vascular tissues.
Collenchymatous hypodermal patches may also exist along leaf margins.
Pseudo-isobilateral Leaf Variation:
In plants like Nerium, palisade parenchyma and multiple epidermis are present on both adaxial and abaxial sides. Such leaves are termed pseudo-isobilateral leaves.
Anatomy of Monocot (Isobilateral / Unifacial) Leaf
General Organization:
Exemplified by maize (Zea mays).
Differentiated into epidermis, mesophyll, and vascular bundles.
Tissues are distributed equally on adaxial and abaxial sides, making the leaf isobilateral and unifacial (homogeneously green on both surfaces).
Epidermis:
Single-layered, composed of compactly arranged barrel-shaped cells covered by a cutin cuticle layer. Each cell contains a single nucleus and a vacuolated protoplast lacking chloroplasts.
Epidermal hairs are typically absent.
Stomata are distributed equally on both adaxial and abaxial epidermal layers (amphistomatic condition). Each stoma is surrounded by two dumbbell-shaped guard cells.
Bulliform Cells (Motor Cells):
Modified upper epidermal cells located along the veins in grasses.
Enlarged, thin-walled, fan-shaped water-storing cells with vacuolated protoplasts lacking chloroplasts.
Turgor Action: When bulliform cells absorb water and become turgid, the leaf surface unrolls and exposes the lamina. Under water stress, they lose water and become flaccid, causing the leaf surface to roll inward to decrease transpiration loss.
Function: Provides mechanical protection to internal tissues, facilitates gas exchange, and regulates transpiration.
Mesophyll:
Located between the adaxial and abaxial epidermis.
Undifferentiated tissue composed of cylindrical, spherical, or subspherical loosely arranged cells with intercellular spaces.
All mesophyll cells contain an equal number of chloroplasts, resulting in uniform green coloration across both surfaces.
Vascular Bundles:
Arranged in parallel venation, reflected by near-similar sizes of vascular bundles across the lamina.
Type: Conjoint, collateral, and closed.
Orientation: Protoxylem is adaxial and metaxylem is abaxial; xylem is adaxial and phloem is abaxial.
Bundle Sheath:
Enclosed by a single layer of border parenchyma (bundle sheath).
Bundle sheath cells contain Calvin cycle enzymes and actively participate in starch synthesis.
Plasmodesmata connect bundle sheath cells directly to mesophyll cells and conducting vascular elements for efficient material transport.
Adaxial and abaxial bundle sheath cells divide to form bundle sheath extensions extending to the epidermis. These extensions become sclerenchymatous and provide mechanical strength to the leaf.