Topic7A Plant Growth and Transport: Physiology and Cell Biology Study Notes

Topic7A Plant Growth and Transport

Introduction to Plant Growth and Transport

Plant growth and transport are fundamental aspects of plant physiology and cell biology. Understanding how plants grow in height and thickness, and how they transport water, minerals, and sugars, is essential for comprehending plant adaptation and survival on land.

Why Are Some Plants Small and Others Tall?

Primary vs. Secondary Growth

Plants exhibit two main types of growth: primary growth (increase in length) and secondary growth (increase in thickness). Primary growth is driven by apical meristems, while secondary growth is driven by lateral meristems.

• Primary Growth: Responsible for elongation of roots and shoots.

• Secondary Growth: Responsible for thickening of stems and roots, mainly in woody plants.

Example: Sequoia sempervirens can grow over 100 meters tall due to extensive secondary growth, while mosses remain small due to the absence of vascular and supportive tissues.

Adaptations to Life on Land

Advantages and Disadvantages

Transitioning to terrestrial life provided plants with new opportunities and challenges.

• Advantages:

  • Abundant sunlight for photosynthesis

  • Greater availability of CO2 and O2

• Disadvantages:

  • Water scarcity and risk of desiccation

  • Need for structural support against gravity

  • Separation of resources (water/nutrients in soil, gases in air)

  • Challenges in gamete dispersal

Key Adaptations:

• Development of vascular tissues (xylem and phloem)

• Regulated stomata to control water loss

• Thicker cell walls with lignin for support

• Protective structures for reproduction (seeds, pollen)

Primary Growth: Growing Tall

Apical Meristems

Primary growth occurs at the tips of roots and shoots due to the activity of apical meristems, which are regions of undifferentiated, actively dividing cells.

• Shoot Apical Meristem: Produces new cells for stem and leaf elongation.

• Root Apical Meristem: Produces new cells for root elongation.

Root Apical Meristem: Structure and Function

The root tip contains several zones:

• Root Cap: Protects the meristem as the root pushes through soil.

• Zone of Cell Division: Includes the apical meristem and primary meristems (protoderm, ground meristem, procambium).

• Zone of Elongation: Cells elongate, pushing the root tip forward.

• Zone of Differentiation: Cells mature into specialized tissues (dermal, ground, vascular).

Shoot Apical Meristem: Structure and Function

The shoot apical meristem is responsible for the upward growth of stems and the formation of leaves and axillary buds.

• Leaf Primordia: Small outgrowths that develop into leaves.

• Axillary Bud Meristems: Can give rise to branches or flowers.

Apical Dominance

Apical dominance is the phenomenon where the main, central stem of the plant is dominant over (i.e., grows more strongly than) other side stems; this is regulated by plant hormones such as auxin.

• Excurrent Growth: Strong apical dominance, resulting in a single, tall trunk (e.g., conifers).

• Decurrent Growth: Weaker apical dominance, resulting in a more branched form (e.g., many angiosperms).

Secondary Growth: Growing Thick

Lateral Meristems

Secondary growth increases the thickness of stems and roots and is driven by two types of lateral meristems:

• Vascular Cambium: Produces secondary xylem (wood) and secondary phloem.

• Cork Cambium: Produces cork cells, contributing to the bark.

Structure of Woody Stems

Woody stems show distinct layers due to secondary growth:

• Secondary Xylem: Forms the bulk of wood.

• Secondary Phloem: Part of the bark, transports sugars.

• Cork and Phelloderm: Protective outer layers.

Tree Rings and Dendrochronology

Annual rings in wood are formed due to seasonal variations in growth rate, with early wood (spring) being lighter and late wood (summer/fall) being denser. The study of these rings is called dendrochronology and is used to determine the age of trees and past climate conditions.

Vascular vs. Non-Vascular Plants

Key Differences

Feature	Vascular Plants	Non-Vascular Plants (e.g., Mosses)
Vascular Tissue	Present (xylem & phloem)	Absent
Support	Lignin in cell walls	Only cellulose
Size	Can grow tall	Remain small
Water Transport	Efficient, long-distance	Short-distance, by diffusion

Example: Mosses lack vascular tissue and lignin, limiting their size and ability to transport water efficiently.

Water and Sugar Transport in Plants

Water Transport

Water is absorbed by roots and transported upward through the xylem, driven by transpiration (evaporation from leaves) and water potential gradients.

• Xylem: Conducts water and minerals from roots to shoots.

• Transpiration: Loss of water vapor from leaves, creating a pull that draws water upward.

Sugar Transport

Sugars produced in leaves (source) are transported to other parts of the plant (sink) via the phloem.

• Phloem: Conducts sugars and other organic molecules throughout the plant.

• Source to Sink: Movement from areas of production (leaves) to areas of storage or use (roots, fruits).

Summary Table: Primary vs. Secondary Growth

Growth Type	Location	Meristem	Result
Primary Growth	Root and shoot tips	Apical meristem	Increase in length
Secondary Growth	Stems and roots (woody plants)	Lateral meristems (vascular & cork cambium)	Increase in thickness

Key Terms

• Meristem: Region of undifferentiated cells capable of division and growth.

• Apical Meristem: Located at tips of roots and shoots; responsible for primary growth.

• Lateral Meristem: Includes vascular and cork cambium; responsible for secondary growth.

• Xylem: Vascular tissue that transports water and minerals.

• Phloem: Vascular tissue that transports sugars and organic nutrients.

• Auxin: Plant hormone involved in apical dominance and growth regulation.

• Dendrochronology: Study of tree ring patterns to determine age and environmental history.