Anatomy of Flowering Plants

A plant tissue is an organized group of cells having a common origin, similar structural organization, and cooperating to perform a specialized physiological function. Plant tissues are broadly grouped into meristematic tissues (actively dividing, immature cells) and permanent tissues (mature cells that have lost the power of cell division).

Meristematic cells are characterized by thin primary cellulosic cell walls, dense granular cytoplasm, prominent nuclei, absence of large vacuoles, no intercellular spaces (compactly arranged), and high respiratory and metabolic rates.

• Classification based on Location:
  • Apical Meristems: Situated at the growing tips of roots and shoots (Root Apical Meristem - RAM and Shoot Apical Meristem - SAM). They produce primary tissues and are responsible for linear growth (increase in length) of the plant body.
  • Intercalary Meristems: Situated between mature permanent tissues, commonly at the base of internodes and leaf sheaths in monocots (e.g., grasses, bamboo, mint). They are separated portions of apical meristems left behind during primary growth; they regenerate plant parts consumed by grazing herbivores.
  • Lateral Meristems: Cylindrical meristems arranged longitudinally along the lateral perimeter of roots and stems. They divide periclinally to produce secondary tissues that increase the plant's girth/thickness (e.g., fascicular vascular cambium, interfascicular cambium, and cork cambium / phellogen).
• Classification based on Origin:
  • Primary Meristem: Derived directly from embryonic promeristem, present from the beginning of plant life (Apical & Intercalary meristems).
  • Secondary Meristem: Arise later in life from differentiated permanent cells by the process of dedifferentiation (e.g., interfascicular cambium formed from medullary ray parenchyma, and cork cambium / phellogen formed from outer cortical cells).

Simple permanent tissues are homogeneous, composed of only one type of structurally and functionally identical cells:

• Parenchyma (মৃদূতক):
  • Structure: Living, isodiametric, spherical, oval, or polygonal cells with thin cellulosic walls and prominent central vacuoles. Usually possess conspicuous intercellular spaces.
  • Functions: Forms the bulk ground tissue (pith, cortex, mesophyll). Special modifications include Chlorenchyma (parenchyma packed with chloroplasts performing photosynthesis) and Aerenchyma (parenchyma with large continuous air cavities providing buoyancy and aeration in hydrophytes like Eichhornia and Hydrilla).
• Collenchyma (স্থূলকোণ কলা):
  • Structure: Living cells, elongated with tapered ends, characterized by localized, uneven cell wall thickening at cell corners composed of pectin, cellulose, and hemicellulose. Intercellular spaces are completely absent.
  • Occurrence: Occurs in 3-4 layers in the hypodermis of dicotyledonous herbaceous stems and leaf petioles. Completely absent in monocot stems and all roots.
  • Function: Provides flexible mechanical support (tensile strength without brittleness) to young growing aerial organs, allowing bending without breaking during windstorms.
• Sclerenchyma (দৃঢ়োতক কলা):
  • Structure: Dead at maturity, devoid of protoplasm. Characterized by exceptionally thick, rigid, secondary cell walls heavily impregnated with lignin, bearing narrow lumens and simple or bordered pits.
  • Types of Sclerenchyma:
    1. Sclerenchymatous Fibres: Greatly elongated, slender, needle-like cells with pointed, overlapping ends, occurring in longitudinal bundles (e.g., bast fibres in phloem, xylary fibres). Provide immense tensile rigidity.
    2. Sclereids (Stone Cells): Extremely thick-walled, spherical, oval, or cylindrical cells with highly reduced, branched lumens (pit canals). Found in the gritty pulp of fruits (guava, pear, sapota), hard shells of nuts (walnut, coconut endocarp), and seed coats of legumes.

Complex permanent tissues are heterogeneous, composed of more than one type of cell cooperating as a structural and functional unit to conduct water, mineral nutrients, and organic food solutes throughout the plant body.

Xylem conducts water and dissolved mineral ions unidirectionally from roots to aerial parts and confers mechanical support. It consists of 4 distinct elements:

• Tracheids (ট্রাকিড): Elongated, tube-like dead cells with tapering, chisel-like closed ends and lignified walls bearing bordered pits. Devoid of protoplasm. Water moves from tracheid to tracheid through lateral pits. Main conducting element in gymnosperms and pteridophytes.
• Vessels (বাহিকা / Xylem Tracheae): Long cylindrical tubes formed by the end-to-end coalescence of multiple vessel members separated by dissolved or perforated transverse end walls (perforation plates). Possess wider lumens than tracheids, lignified walls with various thickening patterns (annular, spiral, reticulate, pitted). Key Angiosperm Hallmark: Vessels are universally present in angiosperms but completely absent in gymnosperms (except Gnetales like Ephedra, Gnetum) and pteridophytes.
• Xylem Fibres: Highly elongated, dead, lignified cells with obliterated central lumens; provide mechanical rigidity.
• Xylem Parenchyma: The only living element of xylem. Cells possess thin cellulosic walls and prominent nuclei; function in storing food reserves (starch, fats) and tannins. Ray parenchyma cells perform radial conduction of water.

Phloem transports organic photoassimilates bidirectionally from source (leaves) to sinks (roots, fruits, buds). It consists of 4 elements:

• Sieve Tube Elements (চালনী উপাদান): Elongated tubular conducting cells placed end-to-end. Their transverse end walls are perforated by numerous sieve pores forming sieve plates. Cytological Hallmark: A mature functional sieve tube element possesses a peripheral cytoplasm and a large central vacuole, but completely lacks a nucleus. Its metabolic and transport activities are governed by the nucleus of the adjacent companion cell.
• Companion Cells (সঙ্গী কোশ): Specialized, narrow, nucleated parenchymatous cells intimately associated with sieve tube elements. Both arise from the same mother cell (sister cells) and remain connected through fine plasmodesmatal bridges in pit fields. Companion cells maintain the hydrostatic pressure gradient in sieve tubes. (In gymnosperms, companion cells are absent and functionally replaced by albuminous cells).
• Phloem Parenchyma: Living, elongated parenchymatous cells that store organic resins, latex, and mucilage. Absent in most monocotyledons.
• Phloem Fibres (Bast Fibres): The only dead element in primary/secondary phloem. Highly elongated, unbranched sclerenchymatous fibres with thick lignified walls. Commercially harvested from jute (Corchorus capsularis), flax (Linum usitatissimum), and hemp (Cannabis sativa).

Based on their structural location and functional specialization within the plant body, Julius von Sachs (1875) classified plant tissues into three integrated tissue systems: Epidermal, Ground (Fundamental), and Vascular (Fascicular).

Forms the outermost protective boundary of all primary plant organs:

• Epidermis: A continuous single layer of tightly packed parenchymatous cells devoid of intercellular spaces. The outer tangential wall is covered by a waterproof waxy layer called the cuticle (composed of cutin), which prevents excessive transpirational water loss. (Cuticle is completely absent in roots).
• Stomata & Stomatal Apparatus: Microscopic pores present in leaf and young stem epidermises for gaseous exchange ($CO_2/O_2$) and transpiration. Each stoma is bounded by two specialized guard cells:
  • Kidney-shaped / Bean-shaped: In dicotyledons, with thick inner concave walls and thin outer convex walls.
  • Dumb-bell-shaped: In monocotyledons (grasses, wheat, maize), with narrow middle and swollen bulbous ends.
  • Specialized epidermal cells surrounding the guard cells are termed subsidiary cells. The stomatal aperture, guard cells, and subsidiary cells together constitute the Stomatal Apparatus.
• Epidermal Appendages:
  • Trichomes: Multicellular epidermal hairs on stems; can be branched/unbranched, soft/stiff, or glandular/secretory. They reduce water loss by trapping humid air and deter herbivores.
  • Root Hairs: Unicellular tubular extensions of epiblema cells in the maturation zone of roots; absorb soil water and minerals.

Constitutes all internal tissues except epidermal and vascular systems:

• Cortex: Multi-layered region between epidermis and pericycle. In dicot stems, it is zoned into Hypodermis (collenchymatous), General Cortex (parenchymatous), and Endodermis.
• Endodermis: The innermost single layer of the cortex. In dicot stems, cells store abundant starch grains, termed the Starch Sheath. In roots, radial and transverse cell walls are impregnated with impermeable bands of suberin called Casparian Strips, which block the apoplastic pathway and force water through the symplast.
• Pericycle: Layer(s) situated immediately internal to the endodermis. In dicot roots, it gives rise to lateral roots and part of the vascular cambium. In dicot stems, it forms alternating semilunar patches of sclerenchyma (hard bast).
• Pith (Medulla) & Medullary Rays: Central core of parenchymatous cells. Medullary rays run radially between vascular bundles.

Consists of coordinated strands of xylem and phloem organized into vascular bundles:

• Radial Vascular Bundles (অরীয় সংবহন বান্ডিল): Xylem and phloem lie on separate alternate radii along the perimeter, separated by non-vascular parenchymatous conjunctive tissue. Diagnostic characteristic of all roots (dicot and monocot roots).
• Conjoint Vascular Bundles (যুক্ত সংবহন বান্ডিল): Xylem and phloem occur together on the same radius. Found in stems and leaves:
• Conjoint Collateral Open: A strip of secondary meristematic intrafascicular cambium lies sandwiched between outer phloem and inner xylem. Capable of secondary growth in thickness. (Characteristic of Dicotyledonous stems).
• Conjoint Collateral Closed: Cambium is completely absent between xylem and phloem. Incapable of secondary growth. (Characteristic of Monocotyledonous stems and leaves).
• Bicollateral: Phloem occurs on both outer and inner sides of central xylem, with two cambium strips (outer and inner cambium). (e.g., Cucurbitaceae family: cucumber, pumpkin).

Roots absorb water and dissolved minerals while anchoring the plant body. Although dicot and monocot roots share basic tissue zoning, critical differences exist in their vascular bundle number, pith development, and secondary growth capacity.

1. Epiblema (Piliferous Layer): Outermost single layer of compactly arranged, thin-walled tabular cells. Cuticle and stomata are completely absent. Some cells prolong outwards as unicellular root hairs for water uptake.
2. Cortex: Broad multi-layered zone of loosely arranged, thin-walled parenchymatous cells with intercellular spaces, functioning in storage and radial water transport.
3. Endodermis: Distinct single layer of barrel-shaped cells devoid of intercellular spaces. The radial and tangential walls possess hydrophobic suberin deposits forming Casparian strips. Thin-walled passage cells occur opposite protoxylem points.
4. Pericycle: Thick-walled parenchymatous layer beneath endodermis. It is fully meristematic: it initiates all lateral roots and contributes to the formation of secondary vascular cambium and cork cambium.
5. Vascular Cylinder (Stele):
   • Arrangement: Radial with exarch xylem (protoxylem towards periphery, metaxylem towards center).
   • Number of Bundles: Typically Diarch to Hexarch (2 to 6 xylem and phloem bundles alternating with each other).
   • Conjunctive Tissue: Parenchymatous tissue lying between xylem and phloem strands that later dedifferentiates into cambium.
6. Pith: Extremely small, reduced, or completely absent/inconspicuous.

Possesses epiblema, broad cortex, endodermis with Casparian strips, and pericycle similar to dicot roots, but with definitive diagnostic differences:

• Polyarch Vascular Bundles: Possesses numerous vascular bundles—always more than 6 (usually 8 to 20 or more) xylem and phloem strands arranged in a prominent ring.
• Pith: Exceptionally large, conspicuous, and well-developed parenchymatous pith in the center.
• Secondary Growth: Completely absent. Monocot roots lack cambium and never undergo secondary thickening.

Stems and leaves exhibit pronounced structural divergence between dicotyledonous and monocotyledonous lineages, reflecting adaptations in mechanical support, water distribution, and photosynthetic gas exchange.

• Epidermis: Single layer of cutinized cells bearing multicellular unbranched/branched trichomes and occasional stomata.
• Hypodermis: 3-5 layers of collenchymatous cells with pectin corner thickenings immediately below epidermis, providing mechanical flexibility and tensile strength.
• Endodermis (Starch Sheath): Single undulating layer rich in prominent starch grains.
• Pericycle: Heterogeneous; composed of alternating semilunar patches of sclerenchyma (hard bast) above each vascular bundle and intervening parenchyma.
• Vascular Bundles: Arranged in a neat broken ring (Eustele). Each bundle is conjoint, collateral, open (possessing intrafascicular cambium), and endarch (protoxylem pointing inwards).
• Medullary Rays & Pith: Radial columns of parenchymatous cells (medullary rays) connect cortex with the large central pith.

• Epidermis: Cutinized single layer; multicellular trichomes are completely absent.
• Hypodermis: 2-3 layers of rigid, lignified sclerenchymatous cells.
• Ground Tissue: Undifferentiated mass of continuous parenchymatous cells. No distinction into cortex, endodermis, pericycle, medullary rays, or pith.
• Vascular Bundles: Numerous and scattered irregularly throughout ground tissue (Atactostele). Peripheral bundles are smaller, younger, and closely packed; central bundles are larger, older, and widely spaced.
• Bundle Anatomy: Conjoint, collateral, closed (no cambium, no secondary growth), endarch. Each bundle is encased in a prominent sclerenchymatous bundle sheath. Phloem parenchyma is absent. Protoxylem elements disintegrate to form a characteristic water-containing lysigenous / schizolysigenous cavity.

Secondary growth is the increase in the girth (thickness) of plant axes resulting from the activity of secondary lateral meristems: the vascular cambium (producing secondary vascular tissues) and the cork cambium (phellogen) (producing protective periderm). It occurs typically in gymnosperms and arborescent dicotyledons, but is absent in monocots.

1. Formation of the Continuous Cambial Ring: In a young dicot stem, the intrafascicular cambium is already present within vascular bundles. The medullary ray parenchyma cells adjacent to it dedifferentiate to form strips of interfascicular cambium. Together, they form a continuous, undulating circular ring of vascular cambium.
2. Bifacial Division: The cambial ring divides periclinally: cells cut off towards the interior differentiate into Secondary Xylem (Wood), while cells cut off towards the exterior differentiate into Secondary Phloem. Because the cambium is far more active internally, secondary xylem is produced at a rate 8-10 times faster than secondary phloem. Primary xylem remains intact near the center, while primary and older secondary phloem get crushed against the expanding wood cylinder.

• Spring Wood (Early Wood / বসন্ত কাঠ): In spring, climatic conditions are favorable, and the cambium is highly active. It produces numerous xylem vessels with wide lumens and thin walls. Spring wood is light in color and has low density.
• Autumn Wood (Late Wood / শরৎ কাঠ): In winter/autumn, cambial activity drops due to cooler temperatures. It produces fewer, narrow xylem vessels with narrow lumens and thick lignified walls. Autumn wood is dark in color and has high density.
• Annual Rings (Growth Rings / বর্ষবলয়): The alternating concentric bands of light spring wood and dark autumn wood formed in a single seasonal year constitute one annual growth ring. By counting the number of annual rings in a basal trunk cross section, the exact chronological age of a tree can be determined—a scientific discipline known as Dendrochronology (বৃক্ষকালানুক্রমিকবিদ্যা).

As the stem expands due to secondary xylem accumulation, the outer epidermis and cortex rupture. To replace them, a new secondary lateral meristem develops from collenchymatous or parenchymatous outer cortex cells, termed the Cork Cambium (Phellogen / ফেলোজেন).

• Activity of Phellogen: Divides on both sides:
  • Outwards: Cuts off cells that differentiate into Cork (Phellem / ফেল্লেম). Cork cells are dead at maturity, compactly arranged without intercellular spaces, and their walls are heavily impregnated with suberin, rendering them impermeable to water and gases.
  • Inwards: Cuts off living parenchymatous cells forming the Secondary Cortex (Phelloderm / ফেল্লোডার্ম).
• Periderm (পেরিডার্ম): The collective anatomical term for all three layers: $$\text{Periderm} = \text{Phellem (Cork)} + \text{Phellogen (Cork Cambium)} + \text{Phelloderm (Secondary Cortex)}$$
• Bark (বাকল / ছাল): A non-technical commercial term referring to all tissues exterior to the vascular cambium, including both secondary phloem and periderm ($$\text{Bark} = \text{Periderm} + \text{Secondary Phloem} + \text{Primary Phloem}$$).
• Lenticels (লেন্টিসেল): Lens-shaped aerating ruptures in the cork where phellogen produces loosely arranged, thin-walled parenchymatous complementary cells instead of suberized cork cells. They permit direct respiratory gas exchange ($O_2/CO_2$) between internal woody living tissues and the atmosphere.