Chpt 35 lecture Anthophyta Morphology and Physiology: A Comprehensive Guide to Flowering Plants

Classification and Identification of Flowering Plants

• Flowering plants are categorized within a single phylum of the plant kingdom known as the Anthophyta.
• While the technical phylum name is Anthophyta, flowering plants are frequently discussed in terms of their major evolutionary groups based on distinct body forms.
• The phylum is subdivided into four sub-phyla, though the focus is primarily on two major groups: Monocots and Dicots (or Eudicots).
• The prefix "eu-" in Latin translates to "true," meaning Eudicots refers to "true dicots."

Comparative Morphology: Monocots vs. Dicots

• Seed Structure (Cotyledons):
  • A seed contains an embryo and packaged food resources known as a cotyledon.
  • Monocots: Possess a single (one) cotyledon.
  • Dicots: Possess two ($2$) cotyledons.
• Leaf Venation:
  • Monocots: Exhibit parallel veins (vascular bundles) that run the length of the leaf.
  • Dicots: Exhibit a network-like or net pattern of veins.
• Stem Vascular Arrangement:
  • Monocots: Vascular bundles are scattered throughout the interior of the stem in no regular pattern.
  • Dicots: Vascular bundles are arranged in a distinct ring on the outer edge of the stem.
• Root Systems:
  • Monocots: Characterized by a fibrous root system that grows in a wide, sprawling network.
  • Dicots: Typically feature a taproot system (e.g., a carrot), which consists of one main vertical root.
• Microscopic Root Cross-Sections:
  • Dicots: The xylem cells in the center of the root are arranged in an "X" pattern.
  • Monocots: The arrangement differs significantly and lacks the central "X" formation.
• Floral Parts:
  • Monocots: Flower petals occur in multiples of three ($3, 6, 9, \dots$).
  • Dicots: Flower petals occur in multiples of four or five ($4, 8, 12, \dots$ or $5, 10, 15, \dots$).

Root Physiology and Symbiosis

• Primary Functions: Roots anchor the plant and absorb water and nutrients from the soil.
• Surface Area Optimization: Survival depends on maximizing the surface area of the root system.
• Root Hairs: These are small extensions of the epidermal cells on the root surface where the vast majority of absorption occurs.
• Mycorrhizae (Fungal Partnerships):
  • Researchers found that many root systems do not have sufficient surface area to support the plant's body alone.
  • Most plants form a mutualistic relationship with soil fungi called mycorrhizae.
  • Fungal bodies are made of hyphae, which are thread-like structures one cell wide and many cells long.
  • In this arrangement, the fungi provide the plant with absorbed water and nutrients, while the plant provides the fungi with sugars produced via photosynthesis.
  • This partnership essentially extends the reach of the plant's root system and has existed since early plant evolutionary history.
• Modified Roots:
  • Prop Roots: Specialized roots that support a plant and prevent it from tipping over.
  • Storage Roots: Roots that swell to store resources (e.g., carrots and beets store sugars/carbohydrates produced in the leaves).

Anatomy of the Shoot System and Stems

• The shoot system consists of the plant parts above ground, primarily stems and leaves.
• Stem Terminology:
  • Node: The specific point on a stem where leaves or branches attach.
  • Internode: The space on the stem located between two nodes.
• Meristematic Tissues: These are specific areas of the plant body where cell division occurs for growth.
  • Apical Meristem: Located at the apex (tip) of a branch or root, responsible for vertical growth.
  • Axillary Bud: A pocket of meristematic tissue located at a node; it can grow into a new leaf or a new stem branch.
  • Terminal Bud: Another term for the bud located at the apex/end of the stem.
• Modified Stems:
  • Rhizomes/Stolons/Runners: Horizontal stems that grow along the ground (e.g., strawberries, ferns). They allow the plant to spread sideways and sprout new individual plants, expanding access to light and soil resources.
  • Tubers: Swollen horizontal stems used for resource storage. A potato is a tuber (a stem), not a root.

Leaf Morphology and Microscopic Anatomy

• Leaf Structure:
  • Blade: The wide, flat section of the leaf designed for capturing light for photosynthesis.
  • Petiole: The stalk-like structure that attaches the leaf blade to the stem node.
• Simple vs. Compound Leaves:
  • Compound Leaf: A single leaf unit composed of several smaller blades called leaflets.
  • Identification: A botanist determines if a structure is a single leaf or a leaflet by looking for an axillary bud at the base of the petiole. If there is no bud at the base of an individual leaflet, it is part of a compound or doubly compound leaf.
• Internal Tissue Layers (Mesophyll):
  • Parenchyma Cells: Generalized plant cells inside the leaf responsible for photosynthesis.
  • Palisade Mesophyll: Upper layer of internal leaf cells arranged in tight columns to maximize light absorption.
  • Spongy Mesophyll: Lower layer of internal cells with large air spaces between them to facilitate gas exchange.

Plant Tissue Types and Specialized Cells

• Three Major Tissue Systems:
  • Epidermal Tissue: The outermost covering of the plant body; often includes a waxy cuticle to prevent water loss.
  • Vascular Tissue: Conducting tissues (xylem and phloem) that transport water and nutrients.
  • Ground Tissue: Any tissue that is not epidermal or vascular. Includes the cortex (tissue between the epidermis and vascular bundles) and the pith (tissue in the center of the stem).
• Primary Cell Types:
  • Parenchyma Cells: Most common cells; thin-walled, greenish, and responsible for metabolic functions like photosynthesis and storage.
  • Collenchyma Cells: Have thicker cell walls and provide flexible structural support (e.g., the "strings" in celery).
  • Sclerenchyma Cells: Have extremely thick cell walls reinforced with lignin; their primary role is rigid structure. Lignin typically takes on a red color when treated with botanical stains.

Vascular System: Xylem and Phloem

• Vascular bundles always contain xylem and phloem packed side-by-side.
• Xylem:
  • Transports water and minerals.
  • Cells are dead at functional maturity, leaving empty cell walls that act as "pipes."
  • In monocot vascular bundles, larger xylem cells often create an arrangement that resembles a "face."
• Phloem:
  • Transports sugars and sap.
  • Cells remain alive but lose their nucleus at functional maturity to clear space for fluid flow.
  • Companion Cells: Adjacent cells with nuclei that control the functions of the enucleated phloem cells.

Plant Growth and Lifespans

• Life Expectancy Categories:
  • Annuals: Complete their entire life cycle (growth to seed production) and die in $12$ months or less, usually before the cold season.
  • Biennials: Live for more than $12$ months but less than two ($2$) years; they often require a cold season to stimulate reproduction.
  • Perennials: Live for more than two ($2$) years.
• Indeterminate Growth:
  • Perennial plants exhibit indeterminate growth, meaning they do not have a genetically preprogrammed lifespan ("old age") like animals.
  • Unlike humans, whose lifespans are linked to the shortening of telomeres during chromosome copying, perennial meristematic tissues can theoretically produce new cells indefinitely.
  • Perennials typically die due to environmental damage or disease, not senescence.
• Growth Patterns:
  • Primary Growth: Vertical growth (increase in length) occurring at the tips of stems and roots.
  • Secondary Growth: Lateral growth (increase in circumference/girth) making the stem wider.