READING 5 - TISSUES AND PRIMARY GROWTH OF STEMS (1)

Tissues and the Primary Growth of Stems

Learning Objectives

  • Evolution of Land Plants: Students will summarize the evolution of land plants.

  • Plant Examples: Identify examples of herbs and woody plants.

  • Plant Cell Classes: Define the three classes of plant cells: parenchyma, collenchyma, sclerenchyma.

  • Basic Stem Organization: Describe the components of basic stem organization and modifications.

  • Internal Organization: Recall components of the internal organization of stems.

Basic Types of Cells and Tissues

  • Parenchyma: Thin primary walls, metabolically active, found in soft parts of plants (e.g., leaves, fruits).

  • Collenchyma: Unevenly thickened walls, provides flexible support, especially in young, elongating tissues.

  • Sclerenchyma: Has primary and thick secondary walls, many are dead at maturity; provides rigid support; consists of fibers and sclereids.

External Organization of Stems

  • Stems consist of three main parts: leaves, stems, roots.

  • Structure: Each stem has nodes (where leaves attach) and internodes (spaces between nodes).

  • Stem Functions: Support, transport nutrients, photosynthesis (through leaves), storage (sugars, water).

  • Modified Structures: Cacti have reduced leaves to prevent water loss; other species exhibit unique adaptations.

Internal Organization of Stems: Primary Tissues

  • Epidermis: Protective layer, prevents water loss, communicates with the environment through stomata.

  • Cortex: Located below the epidermis; may be involved in storage and photosynthesis.

  • Vascular Tissues:

    • Xylem: Transports water and minerals; includes tracheids and vessel elements (which have perforations).

    • Phloem: Distributes sugars; comprises sieve tube members and companion cells.

  • Pith: Located at the center of stems in eudicots, typically parenchyma.

Stem Growth and Differentiation

  • Shoot Apical Meristem: Site for new cell production via mitosis; growth pushes stem upward, leading to new leaves and vascular tissue.

  • Types of Xylem: Protoxylem forms first and is extensible; metaxylem forms after elongation ceases and is stronger.

  • Differentiation of Phloem: Protophloem is transient and stops functioning quickly; metaphloem is more stable and conducts sugars effectively.

Vascular Bundles

  • In Eudicots: Vascular bundles arranged in a ring around the pith.

  • In Monocots: Scattered vascular bundles throughout the pith and cortex.

Plants vs. Animals: Growth Differences

  • Plants exhibit indeterminate growth, continuously producing new cells, leading to an indefinite size.

  • Animals demonstrate determinate growth, reaching a maximum size as determined by genetic factors, with a fixed number of organs.

Summary of Key Concepts

  1. Plant bodies consist of three parts (roots, stems, leaves) organized by cell type and function.

  2. Stems function in transport and support, with various internal modifications.

  3. Understanding parenchyma, collenchyma, and sclerenchyma is essential to grasp plant structure.

  4. Vascular tissues (xylem and phloem) play critical roles in moving materials throughout the plant.

  5. Plants extensively modify structure (e.g., cacti) to adapt to environmental stresses (like drought).