chapitre 2 botanic General Botany Notes on Spermaphytes and Stems

General Overview of Vegetative Apparatus of Spermaphytes

1. Definition of Spermaphytes

• Spermaphytes derived from Greek:

  • Sperma = seed

  • Phyte = plant

• Spermaphytes are commonly known as seed plants.

• They include two sub-branches:

  • Angiosperms:

  • Derived from Greek; aggeion = urn or container, Sperma = seed.

  • Gymnosperms:

  • Derived from Greek; Gymnos = naked, Sperma = seed. These are plants with seeds not protected by a fruit.

2. Vegetative Apparatus Definition

• The vegetative apparatus includes all organs responsible for nutrition and growth, excluding reproductive functions.

• Spermaphytes are classified as cormophytes due to their structure, which consists of a corm (roots, stems, leaves).

The Stem

1. Definition of the Stem

• The stem is an aerial axis extending from the root and carrying leaves. During reproduction, it holds the reproductive organs.

• It's defined as:

  • "Any aerial or underground organ, green or brown, erect, creeping, or climbing, which bears leaves, is a stem.”

• A plant lacking a stem is termed a stemless plant.

2. Origin of the Stem

• The stem originates from the gemmule, which gives rise to the stem and leaf assembly.

• Growth occurs:

  • Opposite to gravitational pull (negative geotropism)

  • Towards light (positive phototropism)

3. Role of the Stem

• The stem has a dual role:

  • Provides support and resistance against wind forces.

  • Carry two types of sap:

  • Raw: unprocessed nutrients

  • Elaborated: processed nutrients

4. Differences Between Stem and Root

• Presence of nodes: Stems have distinct nodes, while roots do not.

• Presence of internodes: Stems contain segments between nodes.

• Absence of protective terminal cap: Roots have a root cap; stems do not.

5. Morphology of the Stem

A. Parts of the Stem:

1. Main Axis:

   • Typically erect; the top part is the terminal bud (apical bud), which is the primary growth point.

2. Nodes:

   • Swollen parts along the stem where leaves are attached.

3. Between Nodes:

   • Segments between nodes. If short, the plant is referred to as stemless with rosette leaves.

4. Buds:

   • Composed of:

     • Meristematic cells (growing points)

     • Undifferentiated leaves covered by bracts (modified leaves) during dormancy.

   • Types of Buds:

     • Terminal Bud: Responsible for stem growth.

     • Axillary Buds: Located in leaf axils; can develop into either leafy or flowering branches.

6. Types of Stems

A. Aerial Stems:

• Most common; can be upright, lying, creeping, or climbing.

• Categories:

  1. Erect Stems:

  • Grow vertically without support.

  • Types:

    • Herbaceous Stems: Annual or biennial.

    • Woody Stems: Perennial; include trees and shrubs.

    • Names include:

      • Trunk: Stem of trees/shrubs; strong and robust.

      • Stipe: Stem of palm trees, lacking lateral stems.

      • Thatch: Hollow herbaceous stem in grasses (e.g., wheat).

  1. Creeping Stems (Stolons):

  • Slender, soft stems growing horizontally.

  • Examples: Strawberry, melon, watermelon.

  1. Climbing Stems:

  • Flexuous stems that ascend by clinging to supports.

  • Types:

    • With Crampons: Attach with adventitious roots (e.g., ivy).

    • With Thorns: Use thorns/hooks for attachment (e.g., rose bush).

    • With Tendrils: Use slender axes to cling (e.g., vine).

    • Twining: Wrap around support (e.g., bindweed).

  1. Modified Aerial Stems:

  • Three examples:

    • Fleshy Stems: Characteristic of succulent plants (e.g., prickly pear).

    • Leaf-like Stems (Cladodes): Flattened branches acting as leaves (e.g., little holly).

    • Spur Stems: Modified into thorns for survival in arid environments (e.g., Genet).

B. Underground Stems:

• May resemble roots externally but are anatomically stems.

• Types:

  1. Rhizomes: Horizontal, nutrient-rich stems producing roots below and stems above (e.g., bamboo, ginger).

  2. Tubers: Underground stems store reserves and have axillary buds known as “eyes” (e.g., potato).

  3. Bulbs: Composed of fleshy scales surrounding a short stem (e.g., onion, garlic, tulip).

Anatomy of the Stem

1. Types of Structures in Stems

• Two structural types identified:

  • Primary Structure: From the primary meristem; characterizes monocotyledons.

  • Secondary Structure: From the secondary meristem (AGSP, AGLL); found in dicotyledons.

2. Anatomical Structure of Monocots

• Cross-section shows two main parts: bark and a central cylinder.

  • Bark:

  • Thinner than root bark; includes:

    • Epidermis: Primary protective tissue, single layer with stomata, covered in cutin.

    • Cortical Parenchyma: Composed of large cells with spaces (meatus).

    • Sclerenchyma: Thick-walled support tissue forming a ring.

  • Central Cylinder:

  • Larger part with criblo-vascular bundles arranged in circles.

  • Bundles include two conductive tissues:

    • Xylem: Transports water/minerals.

    • Phloem: Transports organic nutrients.

  • Xylem positioned below phloem in a V-shape.

  • Primary xylem (protoxylem) contains small cells, while metaxylem has larger cells.

  • Development of xylem is centrifugal.

  • Remaining space is filled by marrow.

3. Anatomical Structure of Dicotyledons

• Dicot stems develop secondary structures allowing increase in thickness over time.

• Secondary Meristem: Consists of two layers:

  • Subero-Phellodermal Generative Layer (AGSP) or phellogen.

  • Libero-Woody Generative Layer (AGLL) or cambium.

• In dicot stems:

  • Cross-sections reveal bark and central cylinder:

  • Bark:

  • New layers including suber or cork (secondary protective tissue) and phelloderm.

  • Central Cylinder:

  • Fewer cribrovascular bundles arranged in a single circle.

  • Cambial zone divides to produce secondary phloem (bast) outwards and secondary xylem (wood) inwards in aging stems.

Comparative Anatomy: Monocot vs. Dicot Stems

Summary of Primary Structure of Stems and Roots

Instructed by: Mettai Mhamed

Maitre-assistant in Botanique