Table of Contents
Types of Plant Growth
Plant growth is not just “getting bigger”; it follows specific patterns and rules.
Primary and Secondary Growth
- Primary growth
- Lengthening of roots and shoots.
- Occurs at apical meristems (root and shoot tips).
- Produces the primary plant body: young leaves, primary xylem and phloem, cortex, pith.
- Responsible for plants reaching into new soil and air spaces.
- Secondary growth
- Increase in thickness (girth) of stems and roots.
- Occurs mainly in cambial meristems of many gymnosperms and dicotyledonous angiosperms.
- Produces the secondary plant body: wood (secondary xylem) and secondary phloem.
- Explains tree rings and the formation of bark.
Not all plants display strong secondary growth. Many herbaceous plants retain mostly primary tissues.
Determinate vs. Indeterminate Growth
- Indeterminate growth
- Growth that can continue throughout life.
- Typical of shoot and root apices: new organs (leaves, roots, flowers) can arise as long as the meristem remains active.
- Determinate growth
- Growth stops after an organ reaches a certain size or after a specific developmental program is completed.
- Examples:
- Individual leaves.
- Flowers and fruits.
- Some plant species or organs where apical meristems are converted to flowering meristems and then cease further vegetative growth.
This combination allows plants to continuously produce new organs, while each organ itself has a defined “endpoint” in its development.
Meristems and Zones of Growth
Plant growth is based on populations of cells called meristems, regions of active cell division.
Apical Meristems
At both root and shoot tips, meristems show distinct zones:
- Zone of cell division
- Contains small, densely cytoplasmic cells that repeatedly divide.
- Defines the meristem proper.
- Zone of elongation
- Cells stop dividing and elongate strongly, often several times their original length.
- Most of the increase in length of roots and shoots occurs here.
- Zone of differentiation (maturation)
- Cells acquire their final structures and functions.
- Root hairs, functional xylem vessels, and phloem elements appear in this zone.
These zones are not rigid boundaries but gradual transitions.
Lateral Meristems
In plants with secondary growth:
- Vascular cambium
- A cylinder of dividing cells between primary xylem and phloem.
- Produces:
- Secondary xylem (wood) inward.
- Secondary phloem outward.
- Leads to thickening of stems and roots.
- Cork cambium (phellogen)
- Produces cork (phellem) to the outside and sometimes phelloderm to the inside.
- Contributes to the formation of bark and protects the plant body as it thickens.
Cell Division, Expansion, and Growth Direction
Plant growth relies on the coordinated sequence:
- Cell division (in meristems).
- Cell expansion (mainly in elongation zones).
- Differentiation (maturation of structure and function).
Cell Wall Properties and Expansion
Unlike animal cells, plant cells are encased in cell walls, which:
- Resist unlimited expansion.
- Must be loosened and restructured for cell enlargement.
Key aspects:
- Primary cell wall plasticity
- Cellulose microfibrils + matrix materials (hemicelluloses, pectins) allow controlled stretching.
- Turgor pressure
- Water uptake into the vacuole generates internal pressure.
- If the wall yields in specific directions, the cell elongates.
Directional (Anisotropic) Growth
Growth is usually directional, not random:
- The orientation of cellulose microfibrils in the wall largely determines growth direction:
- Microfibrils arranged circumferentially → cell elongates in length.
- More random orientation → more uniform expansion.
Thus, plants can create long fibers or strongly elongated cells (e.g., xylem vessels, root hairs) by orienting their cell wall structures.
Differentiation: From Meristem Cells to Specialized Cells
Differentiation is the process whereby initially similar meristematic cells become structurally and functionally distinct.
Main Tissue Systems
Meristematic cells give rise to three major tissue systems:
- Dermal tissue system
- Outer protective tissues:
- Epidermis in young organs (with cuticle, stomata, trichomes).
- Periderm (cork) in older organs with secondary growth.
- Ground (fundamental) tissue system
- Fills much of the plant body:
- Parenchyma (storage, photosynthesis, regeneration).
- Collenchyma (flexible support).
- Sclerenchyma (rigid support, fibers, sclereids).
- Vascular tissue system
- Transport of water, minerals, and organic substances:
- Xylem (vessels, tracheids, fibers, parenchyma).
- Phloem (sieve elements, companion cells, fibers, parenchyma).
Examples of Specialized Cells
- Xylem vessels and tracheids
- Elongated, lignified, dead at maturity.
- Thick secondary walls with characteristic patterns (rings, spirals, networks).
- Conduct water and dissolved minerals.
- Sieve tube elements (phloem)
- Living but lacking nucleus at maturity.
- Connected end-to-end via sieve plates.
- Supported by companion cells in angiosperms.
- Guard cells
- Paired cells controlling stomata.
- Regulate gas exchange and transpiration.
- Root hair cells
- Epidermal cells that form tubular extensions.
- Greatly increase root surface area for water and mineral uptake.
Each cell type arises from undifferentiated meristem cells through specific gene expression patterns and structural changes.
Pattern Formation in Plant Organs
Growth and differentiation follow spatial patterns, giving rise to characteristic plant forms.
Root Architecture
- Primary root (often the taproot) arises from the root apical meristem.
- Lateral roots
- Originate from pericycle cells inside the root.
- Maintain continuity with vascular tissues.
- The pattern of branching impacts water and nutrient uptake and anchorage.
Shoot and Leaf Patterns
- Shoot apical meristem regularly produces:
- Leaf primordia in characteristic arrangements (phyllotaxis):
- Opposite, whorled, spiral patterns.
- Axillary buds
- Form in the axils of leaves.
- Can develop into lateral branches or flowers.
- Create the hierarchical branching pattern of shoots.
- Leaf differentiation
- Different regions arise:
- Blade (lamina) for photosynthesis.
- Petiole for attachment and orientation.
- Sometimes stipules and other appendages.
Formation of Wood and Growth Rings
In plants with conspicuous secondary growth:
- Vascular cambium activity changes with seasons (in many climates):
- Faster growth → larger, thin-walled early wood cells.
- Slower growth → smaller, thick-walled late wood cells.
- This produces annual rings, used to estimate age and study past environmental conditions (dendrochronology).
Plasticity, Regeneration, and Totipotency
Plants show remarkable developmental plasticity: the ability to change growth and differentiation in response to internal and external cues.
Regeneration
- Many plants can:
- Regrow roots or shoots after injury.
- Heal wounds by forming new protective tissues (e.g., callus, cork).
- This is common in vegetative propagation (cuttings, layering).
Totipotency of Plant Cells
- Many differentiated plant cells retain the ability to revert to a less specialized state and form entire new plants under suitable conditions.
- In culture:
- Callus tissue (undifferentiated mass) can be induced.
- From this, roots, shoots, and complete plants can be regenerated.
This property is foundational for micropropagation and various biotechnological applications.
Hormonal and Environmental Control of Growth and Differentiation
Plant growth and differentiation are coordinated by plant hormones and environmental signals. Their detailed mechanisms are treated elsewhere; here we focus on their roles in shaping growth patterns.
Roles of Plant Hormones (Overview of Effects)
- Auxins
- Promote cell elongation in shoots.
- Involved in apical dominance: the main shoot suppresses lateral buds.
- Direct root initiation and bending responses (tropisms).
- Cytokinins
- Stimulate cell division.
- Promote shoot formation in tissue culture when in high ratio to auxin.
- Delay leaf senescence.
- Gibberellins
- Stimulate stem elongation.
- Influence seed germination and flowering in some species.
- Abscisic acid (ABA)
- Generally restrains growth under stress.
- Promotes dormancy in buds and seeds.
- Ethylene
- A gaseous hormone.
- Affects cell expansion patterns, fruit ripening, and responses to mechanical stress (e.g., thicker, shorter stems when repeatedly bent by wind).
It is often the ratio of hormones, not just their absolute amounts, that determines specific growth and differentiation outcomes.
Environmental Influences
Environmental factors interact with hormones to direct growth:
- Light
- Direction: causes phototropism (shoots bending toward light).
- Quantity and quality: affect leaf thickness, chloroplast development, and shade vs. sun leaf types.
- Day length: influences flowering time and transitions between vegetative and reproductive growth.
- Gravity
- Roots grow with gravity (positive gravitropism).
- Shoots grow against gravity (negative gravitropism).
- Specialized sensing cells help orient growth direction.
- Mechanical stimuli
- Touch or wind can lead to thigmomorphogenesis:
- Shorter, thicker stems.
- Enhanced mechanical strength.
- Water and nutrient availability
- Influence overall growth rate and allocation:
- Under low nutrients, more growth may be directed to roots.
- Under abundant water and nutrients, more shoot and leaf growth.
Transition from Growth to Maturation and Reproduction
Growth and differentiation ultimately prepare the plant for reproduction and survival.
- Juvenile vs. adult phases
- Juvenile plants often cannot flower, even under favorable conditions.
- After reaching a certain size or age, they enter an adult phase with potential for flowering.
- Vegetative vs. reproductive growth
- Vegetative meristems produce leaves and stems.
- After specific internal and external signals, some meristems become floral meristems, producing flowers instead of further vegetative organs.
- This is a key differentiation step at the level of the whole shoot system.
Once reproductive structures are formed and seeds develop, some plants die (annuals, biennials), while others return to vegetative growth cycles (perennials), continuing the interplay of growth and differentiation over many years.