Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 35.2 Meristems and Primary Growth Allow Roots and Shoots to Grow Throughout a Plant’s Life
Plants grow continuously thanks to specialized tissues called meristems, which contain undifferentiated cells that can divide and generate new organs and tissues. This indeterminate growth allows roots and shoots to extend throughout the plant's life, supporting survival, reproduction, and adaptation.
Types of Plant Growth
- Indeterminate growth: Continuous growth throughout life; typical of roots and stems.
- Determinate growth: Organs like leaves, thorns, and flowers grow to a certain size and then stop.
Life Cycle Types in Flowering Plants
- Annuals: complete their life cycle in one year (e.g., wheat, rice)
- Biennials: require two growing seasons (e.g., carrots, turnips)
- Perennials: live many years (e.g., trees, shrubs, grasses)
Types of Meristems
1. Apical Meristems
- Found at tips of roots and shoots
- Responsible for primary growth (lengthening)
- Give rise to primary meristems:
- Protoderm → dermal tissue
- Ground meristem → ground tissue
- Procambium → vascular tissue
2. Lateral Meristems
- Responsible for secondary growth (thickening)
- Two types:
- Vascular cambium: adds secondary xylem (wood) and secondary phloem
- Cork cambium: forms the periderm (replaces epidermis in woody plants)
Growth Zones and Differentiation
- Cells in meristems divide actively during growing seasons
- Some daughter cells remain as stem cells (formerly called initials)
- Other cells differentiate into mature tissues
- In woody plants:
- Apical buds contribute to height (primary growth)
- Lateral meristems increase girth (secondary growth), especially after elongation stops
Visual Clues to Growth: The Winter Twig
- Each year's growth is visible in twig structure:
- Apical bud scars: mark seasonal growth
- Leaf scars and axillary buds: indicate past leaf locations and potential new branches
- Bud scales: protected the apical meristem during dormancy
In a Nutshell
Plant growth depends on meristems: apical meristems enable upward and downward extension, while lateral meristems increase thickness. These tissues ensure that plants can adapt their form to environmental conditions, replacing lost tissues and growing new ones for light capture, support, or nutrient absorption.