Section 2B Concise

Common Types of Plant Cells

  • Differentiation leads to specialization of plant cells.

  • Major types include:

    • Parenchyma

    • Collenchyma

    • Sclerenchyma

    • Xylem (tracheids, vessel elements)

    • Phloem (sieve tube elements, companion cells)

Parenchyma Cells

  • Thin, flexible primary walls; lack secondary walls.

  • Large central vacuole in mature cells.

  • Functions: metabolic activities, photosynthesis, starch storage, retain ability to divide.

Collenchyma Cells

  • Support young plant shoots.

  • Elongated cells with thicker, uneven walls.

  • Provide flexible support.

Sclerenchyma Cells

  • Rigid with thick, lignin-reinforced secondary walls.

  • Dead at maturity; two types:

    • Sclereids: short, irregular, thick-walled.

    • Fibres: long, slender, grouped in strands.

Water-Conducting Cells of the Xylem

  • Types: tracheids and vessel elements; both dead at maturity.

  • Tracheids are in nearly all vascular plants; vessel elements in most angiosperms.

  • Water movement through pits and perforation plates.

Sugar-Conducting Cells of the Phloem

  • Sieve-tube elements: alive but lack organelles.

  • Sieve plates facilitate fluid flow.

  • Each element has a companion cell for support via plasmodesmata.

Primary Growth in Plants

  • Directly from apical meristems.

  • Occurs in roots and shoots:

    • Root tip protected by root cap; secretes polysaccharides for soil lubrication.

    • Growth zones: cell division, elongation, maturation.

Organization of Primary Tissues in Roots

  • Epidermis, ground tissue, vascular tissue produced.

  • Stele is central vascular cylinder in angiosperm roots.

  • Eudicots have star-shaped xylem with phloem in between.

Development of Lateral Roots

  • Originate in pericycle, grow through cortex and epidermis.

Primary Growth of Shoots

  • Shoot apical meristem produces leaves and axillary bud meristems for lateral branches.

Tissue Organization of Stems

  • Vascular bundles arranged in ring (eudicots) or scattered (monocots).

Tissue Organization of Leaves

  • Leaf structure includes:

    • Epidermis with stomata for gas exchange.

    • Mesophyll with palisade and spongy layers for photosynthesis.

    • Vascular bundles (veins) for transport.

Secondary Growth in Woody Plants

  • Involves vascular and cork cambium.

  • Vascular cambium produces secondary xylem (wood) and phloem.

  • Cork cambium creates protective cork layer.

Growth Ring Analysis

  • Early spring wood: large diameter, thin walled.

  • Late wood: thick walled for support.

  • Dendrochronology studies growth rings for climate change analysis.