Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 35.3 Primary Growth of Roots Produces the Epidermis, Ground Tissue, and Vascular Tissue
Primary growth in roots allows plants to penetrate deeper into the soil by elongating cells and developing new tissues. This process occurs in distinct zones and produces the epidermis, ground tissue, and vascular tissue, forming a complete system for nutrient absorption and transport.
Root Primary Growth Overview
Root growth originates from the root apical meristem, which:
- Continuously produces new cells
- Is protected by a root cap that secretes lubricating slime
- Occurs in three zones:
- Zone of cell division: includes stem cells and initial new cells
- Zone of elongation: cells elongate rapidly, pushing the root tip forward
- Zone of differentiation (maturation): cells specialize into distinct tissues
Three Primary Meristems and Their Tissues
1. Protoderm → Epidermis
- Outer layer with root hairs (increase surface area for absorption)
- Root hairs are short-lived but make up 70–90% of the root’s surface area
2. Ground Meristem → Ground Tissue (Cortex)
- Composed mostly of parenchyma cells
- Functions:
- Stores carbohydrates
- Transports water/salts from root hairs inward
- Allows extracellular diffusion between cells
- Endodermis: innermost cortex layer, acts as a selective barrier into vascular tissue
3. Procambium → Vascular Cylinder (Stele)
- Contains xylem and phloem
- Surrounded by the pericycle, where lateral roots originate
- In eudicots: xylem has a starlike shape, phloem fills spaces
- In monocots: vascular tissue forms a ring around parenchyma core
Lateral Root Formation
- Lateral roots arise from pericycle cells inside the endodermis
- New roots push through outer tissues to emerge
- Branching enables the plant to explore resource-rich soil regions
Primary Growth of Shoots
- Shoot apical meristem generates new leaves and stems
- Surrounded by leaf primordia and protected by young leaves
- Produces protoderm, ground meristem, and procambium
- Axillary buds form lateral shoots; apical dominance suppresses them unless the apical bud is removed
Shoot Anatomy
- Epidermis: single layer with cuticle, guard cells, and trichomes
- Ground tissue:
- Parenchyma cells for general support
- Collenchyma and sclerenchyma provide structural strength
- Vascular bundles:
- In eudicots: arranged in a ring (xylem inside, phloem outside)
- In monocots: scattered in ground tissue (no distinct cortex/pith)
Leaf Growth and Structure
- Leaves arise from leaf primordia at the shoot tip
- Epidermis has a cuticle and stomata for gas exchange
- Stomata consist of pores and guard cells to regulate opening
- Mesophyll (ground tissue):
- Palisade layer: rich in chloroplasts for photosynthesis
- Spongy layer: loosely packed with air spaces for gas diffusion
- Veins (vascular tissue):
- Bring xylem and phloem close to mesophyll
- Surrounded by bundle sheath cells, especially in C₄ plants
In a Nutshell
Primary growth enables root and shoot elongation through organized zones of cell division, elongation, and differentiation. Root tissues develop from meristems to form the epidermis, cortex, and vascular cylinder, while shoots grow via apical meristems producing stems, leaves, and lateral buds. This growth underpins a plant’s ability to explore its environment, transport resources, and support new growth.