Plant Growth and Development

Chapter 30: Plant Growth and Development

Core Concepts Overview

1. Shoot and Root Apical Meristems:

   • Shoot apical meristems produce new cells for stem lengthening, leaf formation, and axillary bud development.

   • Root apical meristem allows roots to grow downwards, facilitating water and nutrient absorption.

2. Lateral Meristems:

   • Allow plants to increase in diameter, improving stability and vascular system transport.

3. Plant Hormones:

   • Are chemical signals influencing cell growth and differentiation.

4. Growth Responses:

   • Plants adapt to environmental cues (light, gravity, wind) affecting internode elongation and organ development.

Unique Plant Growth Characteristics

• Fundamental Differences from Animals:

  • Plants have meristems (totipotent cells) where growth and cell division occur throughout life, contrasting with animal growth patterns.

• Response Mechanism:

  • Plants do not move but modify their shape and size in response to environmental stimuli.

Shoot Growth

• Primary vs. Secondary Growth:

  • Both types of growth originate at meristems. Shoot growth is characterized by modularity and regeneration through the shoot apical meristem.

• Elongation Process:

  • Cells express meristem identity genes and grow in a zone beneath the shoot apical meristem, primarily through cell elongation.

• Leaf Evolution:

  • Initial plants had photosynthetic stems; flat leaves evolved for improved photosynthesis and various functions.

Leaf Structure and Function

• Attachment to Stems:

  • Young leaves (primordia) emerge from the shoot meristem and initially absorb nutrients via diffusion before developing connections to vascular systems (xylem and phloem).

• Vascular Tissue Formation:

  • Procambial cells form strands connecting leaves to vascular tissue, enabling fluid and nutrient transport.

Root Characteristics and Structure

• Root Function:

  • Roots secure water and nutrients necessary for plant survival, enabling adaptation from wet to dry habitats.

• Growth Mechanisms:

  • Roots grow from the apical meristem, protected by a root cap. The structure aids in nutrient absorption through endodermis and the Casparian strip.

• Pericycle Role:

  • It allows for new root formation, enabling flexibility influenced by nutrient availability.

  • Roots vary primarily in growth patterns between monocots and plants with secondary growth.

Comparison of Roots and Stems

Secondary Growth and Meristem Types

• Lateral Meristem Role:

  • Vascular Cambium: Produces new xylem and phloem.

  • Cork Cambium: Maintains the outer protective bark layer.

• Growth rings indicate environmental conditions (wider rings for better growth conditions).

Plant Hormones and Their Functions

1. Auxin:

   • Influences development and arrangement of vascular bundles and leaf formation.

2. Gibberellins:

   • Stimulates stem elongation, beneficial for light competition.

3. Cytokinins:

   • Promotes branching and supports leaf production through hormone interactions.

4. Ethylene:

   • Affects growth in response to environmental pressures.

5. Abscisic Acid:

   • Responds to stress and regulates growth timing.

Environmental Influences on Growth

• Tropism:

  • Directional growth in response to environmental stimuli (e.g., roots positively gravitropic).

• Phototropism:

  • Plants bend towards light due to differential growth rates caused by auxin concentration.

• Mechanism of Light:

  • Plants measure day length and respond to seasonal changes (photoperiodism).

  • Different strategies involve vernalization, influencing flowering based on prior temperature exposure.

Conclusion

Plants exhibit a complex interplay of meristematic growth, hormone signaling, and environmental responses to develop and adapt through their lifecycle.