Plant Structure and Function

Plant Organs: Structure and Function

This lecture discusses the structure and function of plant organs, including roots, stems, leaves, flowers, fruits, and seeds. It emphasizes the importance of plants to humans, plant evolution through domestication, monocots versus dicots, and the basic structure of plant organs.

Why We Care About Plants

Plants are essential for various reasons:

• Food: Plants are a primary food source. Even meat consumption relies on animals that eat plants.

• Fuel: Plants are used directly as wood for burning and indirectly as fossil fuels (oil and coal), which are formed from ancient plant matter.

• Fibers: Plants provide fibers like cotton, bamboo, and hemp for clothing and other textiles.

• Construction: Plants are used as building materials for houses, furniture, and other structures.

• Aesthetics: Plants enhance the beauty of living spaces.

• Medicine: Many medicines are derived from plants, including pharmaceuticals (e.g., morphine from poppies) and traditional remedies.

Plant Evolution and Domestication

Domestication is the process of adapting wild plants and animals for human use. This process involves genetic changes. All domestication, cultivation, and adaptation reflects genetic changes.

• Domestication reflects genetic changes that lead to observable differences in plant traits.

• Artificial Selection: Humans interfere with crop evolution through artificial selection, but humans are part of the natural world.

• Domestication is mutually beneficial for both plants and humans, enhancing the plant's reproductive success.

Plant Evolution Stages

1. Mosses: The first plants to colonize land.

2. Club Mosses: Early vascular plants.

3. Ferns: Seedless vascular plants.

4. Gymnosperms (Conifers):$400,000,000$ years ago. The first plants with seeds, providing a significant evolutionary advantage. Examples include pine trees (used for Christmas trees, paper, and softwood furnishings) and pine nuts (from Pinus edulis). Ginkgo is used in traditional Chinese medicine and marketed as a memory aid.

5. Angiosperms (Flowering Plants): Arose about $50,000,000$ years ago. They possess flowers and fruits, contributing to their success. They comprise the vast majority of plant species and almost all crops.

   • Flowers attract pollinators, increasing genetic diversity.

   • Fruits aid in seed dispersal.

Monocots vs. Dicots (Eudicots)

Flowering plants are divided into monocots and dicots (eudicots).

• Monocots: Have a single cotyledon (embryonic leaf) in the seed. Examples include grasses like wheat, barley, oats, and rice.

• Dicots (Eudicots): Have two cotyledons in the seed. Examples include canola and legumes.

Distinguishing Characteristics

Plant Structure

Plants are organized into cells, tissues, and organs with specialized functions.

• Organs: Leaves, stems, roots, flowers, fruits, and seeds.

• Shoot System: Above-ground parts, mainly for photosynthesis.

• Root System: Below-ground parts, mainly for nutrient and water acquisition.

Meristems

Meristems are regions of plant tissue where new cells are formed. They are similar to stem cells in animals.

• Shoot Apical Meristem: Located at the tip of the stem, responsible for growth in height.

• Root Apical Meristem: Located at the tip of the root, responsible for growth in length.

• Cambiums: Located in the stem, responsible for growth in width.

Meristematic cells are undifferentiated and can develop into any cell type in the plant.

Types of Meristems

• Primary Meristems: Increase plant length (shoot and root apical meristems).

• Secondary Meristems (Cambiums): Increase plant width.

Tissue Systems

Plants have three tissue systems:

• Dermal Tissue: Outer protective layer (like epidermis).

• Vascular Tissue: Transports water and nutrients (xylem and phloem).

• Ground Tissue: Tissue in between dermal and vascular tissues.

Cell Types

• Parenchyma Cells: Perform metabolic functions, such as photosynthesis and storage. (e.g., mesophyll cells, storage cells in potatoes)

• Collenchyma Cells: Provide support in stems and leaves. (e.g., celery sticks)

• Sclerenchyma Cells: Provide structural support with thick, rigid cell walls. (e.g., hard pits in unripe pears).

Plant Organs: Roots

Roots provide water and nutrient uptake, anchorage, and food storage. They also facilitate beneficial microbial interactions.

Dicots have a taproot system, while monocots have a fibrous root system.

Root Structure

• Root Cap: Protects the growing root tip.

• Meristematic Zone: Actively dividing cells.

• Elongation Zone: Cells elongate and increase in size.

• Maturation Zone: Cells differentiate and mature.

Roots maximize surface area through lateral roots and root hairs.

Root Hairs

Root hairs microscopic extensions of epidermal cells that increase the surface area for water and nutrient absorption.

Lateral Roots

Lateral roots originate from within the primary root and connect to the vascular tissue.

Cell Structure of Roots

Vascular tissue is located in the center of the root. The phloem transports sugar and the xylem transports water.

Plant Organs: Stems

Stems provide support and height for leaves, facilitate new growth, and transport water and sugars.

Stems act as highways for xylem and phloem.

Stem Structure

Stems are composed of phytomers, which include a leaf, a bud, an internode, and a node.

• Monocot Stems: Scattered vascular bundles.

• Dicot Stems: Vascular bundles arranged in a ring.

Vascular Cambium

The vascular cambium is responsible for secondary growth and produces new xylem and phloem.

• Pith: Central core of the stem.

• Cortex: Outer layer of the stem.

• Vascular Bundles: Connect to form the vascular cambium.

Plant Organs: Leaves

Leaves are the primary site of photosynthesis.

Leaf Structure

• Blade (Lamina): The surface of the leaf.

• Petiole: Connects the leaf to the stem.

• Midrib: The main vein in the middle of the leaf.

• Venation: The pattern of veins in the leaf.

Monocot Leaf Structure

• Leaf wraps around the stem, providing support.

• Ligule: Small collar at the base of the leaf.

• Sheath: The part of the leaf that wraps around the stem.

• Auricles: Small arm-like structures that wrap around the stem.

Leaf Anatomy

Leaves are thin and horizontal to maximize photosynthesis. They contain specialized mesophyll cells full of chloroplasts. A cuticle on the outside is a waxy layer to prevent water loss. The underside has pores called stomata for gas exchange.

Stomata

Stomata facilitate gas exchange, allowing carbon dioxide to enter and water and oxygen to exit the leaf. They are made of two guard cells and open and close depending on moisture and light levels.