Anatomy of Flowering Plants – Tissue-Level Study Notes

Meristematic Tissues / Meristems

• Definition: “Group of similar or dissimilar cells having common origin and performing a specific function.”

• General features

  • Cells immature, young, with capacity for repeated division.

  • No inter-cellular spaces.

  • Homogeneous, thin cellulose wall.

  • Large nucleus + abundant cytoplasm.

  • Metabolically very active; no long-term food storage.

  • Plastids absent; only small protoplastids present.

  • Dense cytoplasm contains numerous, still-developing mitochondria.

  • Vacuoles absent → turgid, isodiametric cells.

Classification by Origin & Development

• Promeristem / Primordial meristem

  • Directly from embryo ⇒ earliest, embryonic meristem.

  • Located where any new organ is initiated.

• Primary meristem

  • Derived from promeristem; retains meristematic nature.

  • Found in apices of roots, stems, leaf primordia.

• Secondary meristem

  • Arise later in permanent tissue; do not possess a promeristem.

  • Permanent cells dedifferentiate & regain divisional ability (e.g., cambium, cork-cambium).

Classification by Position

• Apical meristem

  • Situated at growing tips of main & lateral shoots + roots.

  • Responsible for linear (primary) growth.

• Intercalary meristem

  • Detached pieces of apical meristem left behind during organ elongation.

  • Positions: base of node (e.g., Mentha viridis), base of internode (monocot stems – wheat, grasses, Equisetum), base of leaf (Pinus).

• Lateral meristem

  • Cylindrical, parallel to axis (e.g., vascular cambium, cork cambium).

  • Cell division mainly periclinal ⇒ increases girth (secondary growth).

Classification by Function (Haberlandt, 1890)

• Protoderm → epidermal tissue system.

• Procambium → primary vascular tissues (protoxylem, protophloem).

• Ground meristem → ground tissues (hypodermis, cortex, endodermis, pericycle, pith, medullary rays).

Classification by Plane of Division

• Mass meristem: anticlinal divisions in all planes ⇒ 3-D masses (spores, cortex, pith, endosperm).

• Plate meristem: anticlinal in two planes ⇒ 2-D plates/laminae (leaf epidermis, leaf blade growth).

• Rib/File meristem: anticlinal in one plane ⇒ files or rows (lateral root formation).

Permanent Tissues

• Mature cells, lost power of division; attain definitive size, shape, function via differentiation.

• May be living or dead; thin- or thick-walled.

Simple Permanent Tissues (cells alike in origin, form, function)

1. Parenchyma

   • Most primitive, unspecialised; thin walls, living, isodiametric.

   • Functions: photosynthesis, storage, secretion, regeneration.

2. Collenchyma (term by Schleiden, 1839)

   • Living cells, uneven cellulose/hemicellulose/pectin thickening at corners.

   • Tissue of primary body; provides plastic support to growing organs.

3. Sclerenchyma (coined by Mettenius, 1805)

   • Thick-walled, lignified, dead cells (fibres & sclereids).

   • Great mechanical strength; shapes vary.

Special / Secretory Tissues

• Function: produce & store secretions (resins, gums, oils, latex, etc.).

Laticiferous Tissues

• Thin-walled, elongated, extensively branched, multinucleate (coenocytic) tubes containing latex (colourless, milky, yellow).

• Two structural types

  1. Latex cells (simple/non-articulated laticifers)

  • Single, highly branched cells ramifying through ground tissue.

  • Examples: Calotropis, Nerium, Vinca, Euphorbia, Ficus.

  1. Latex vessels (compound/articulated laticifers)

  • Formed by fusion of many cells into a network.

  • Examples: Argemone, Papaver, Sonchus, Hevea (rubber), Manihot (tapioca).

Glandular Tissues

• Highly specialised secretory/excretory structures.

• External glands – on epidermis (glandular hairs) e.g., Plumbago, Boerhaavia.

• Internal glands – embedded inside organs

  • Oil glands (Citrus, Eucalyptus), resin ducts (Pinus), etc.

Tissue Systems (Sachs, 1875)

• A tissue system = collection of tissues performing same general function.

• Three major systems: Epidermal, Ground (Fundamental), Vascular.

1. Epidermal Tissue System

• Origin: outermost layer of apical meristem (protoderm).

• Components

  • Epidermis: single-layered, tightly packed.

  • Cuticle & wax (cutin)

  • Deposited on aerial organs; thickness varies.

  • Minimises transpirational water loss.

  • Stomata

  • Minute pores bounded by two kidney-shaped guard cells (bean-shaped in dicots; dumb-bell in grasses).

  • Absent on roots.

  • Distribution types

    1. Apple–Mulberry type: stomata mostly on lower surface (hypostomatic) – Oxalis, Mulberry, Apple.

    2. Potato type: greater no. on lower surface of dorsiventral leaves – pea, bean, tomato.

    3. Oat type: almost equal on both sides (amphistomatic) – erect isobilateral leaves of grasses/cereals.

    4. Nymphaea type: only on upper surface of floating leaves – Nelumbo, Nymphaea, water-lily.

    5. Potamogeton type: stomata absent in submerged leaves – Hydrilla, Vallisneria, Potamogeton.

  • Trichomes

  • Uni-/multicellular epidermal outgrowths; roles in protection, dispersal, secretion.

  • Root hairs

  • Tubular extensions of specialised epiblema cells (trichoblasts).

  • Occur in the root-hair zone; greatly increase absorptive surface.

2. Ground / Fundamental Tissue System

• All tissues other than epidermal & vascular.

• Origin: ground meristem; forms bulk of plant body.

Principal Regions

1. Cortex (between epidermis & pericycle)

   • Clear in dicots; indistinct in monocots (merges with pith).

   • Sub-zones

     • Hypodermis: collenchymatous (dicot stem) or sclerenchymatous (monocot stem); mechanical support.

     • General cortex: parenchymatous; food storage & limited photosynthesis/transport.

     • Endodermis (starch sheath): single layer of barrel-shaped, compact cells rich in starch.

2. Pericycle

   • Single or multiple cell layers just inside endodermis, outside vascular bundle.

   • Variable composition: thin-walled parenchyma, sclerenchyma bands (Sunflower), many sclerenchyma layers (Cucurbita stem).

   • Gives rise to lateral roots, part of vascular cambium, cork cambium.

3. Pith / Medulla

   • Central region (dicot stem; monocot root).

   • Parenchymatous; stores food & sometimes aerenchyma.

   • In dicot root largely obliterated by metaxylem; in dicot stem radial files between bundles = primary medullary rays.

3. Vascular Tissue System

• Central cylinder (stele) comprised of xylem, phloem, (cambium in secondary growth) + supporting parenchyma/fibres.

• Vascular bundles = discrete strands of xylem–phloem.

Types of Vascular Bundles

1. Radial

   • Xylem & phloem in separate, alternating radii; parenchyma between.

   • Characteristic of roots.

2. Conjoint (xylem & phloem together in same radius) – stems, leaves.

   • Collateral: phloem external, xylem internal (ex. Sunflower).

   • Bicollateral: phloem both outside & inside xylem (ex. Cucurbita, Pumpkin).

Significance & Connections

• Meristems underpin primary growth (length) & secondary growth (girth) – foundation for organogenesis & adaptation.

• Simple tissues illustrate progression from living, flexible support (collenchyma) to rigid, dead mechanical tissue (sclerenchyma).

• Secretory structures represent evolutionary innovation for defence (latex with alkaloids), attraction, and storage.

• Tissue-system concept integrates cell types into functional networks; understanding it is key for fields like plant physiology, pathology, biotechnology (e.g., cambial manipulation for increased timber; latex extraction for natural rubber).

Quick Reference of Examples

• Intercalary meristem: \textit{Mentha}, wheat, grasses, Equisetum, Pinus leaf.

• Simple laticifers: Calotropis, Ficus.

• Compound laticifers: Hevea brasiliensis (rubber).

• Hypodermis collenchyma: dicot stem (Helianthus).

• Hypodermis sclerenchyma: monocot stem (grasses).

• Bicollateral bundle: Cucurbita stem (pumpkin, gourd).