A seed can develop into a mature plant with roots, stems, leaves, flowers and fruits. This orderly process involves both growth and differentiation. Plant development is influenced by internal chemical signals and external conditions such as water, light and temperature.
Table of Contents
Seed Germination
Germination begins when a viable seed encounters suitable conditions. It absorbs water, resumes metabolic activity and starts growing into a seedling. Water, oxygen and a suitable temperature are important for germination; some seeds also have particular light requirements.
A seed may remain dormant when conditions are unsuitable. When favourable conditions return and dormancy requirements are met, growth can resume.
What Is Plant Growth?
Growth is a permanent, irreversible increase in the size of a cell, organ or whole plant. It can be measured in different ways, including an increase in length, area, volume, mass or cell number.
Many plants keep growing throughout their lives because they retain actively dividing regions called meristems:
- Apical meristems at root and shoot tips contribute mainly to growth in length.
- Lateral meristems, such as vascular cambium, contribute to an increase in girth where they are present.
Three Phases of Growth
Growth near a root or shoot tip can be understood in three phases.
| Phase | Main feature |
|---|---|
| Meristematic phase | Cells divide actively |
| Elongation phase | Newly formed cells increase in size |
| Maturation phase | Cells reach mature size and develop specialised features |
These phases follow one another along the growing region of a plant organ.
Growth Rate
Growth rate means the increase in a growth measure per unit time. Growth may follow different patterns:
- Arithmetic growth: Increase occurs at a roughly constant rate. A graph of size against time is approximately a straight line.
- Geometric growth: Dividing cells continue to produce more dividing cells, allowing rapid increase when resources are plentiful.
- Sigmoid growth: Under natural conditions, growth commonly shows a slow initial phase, a rapid phase and then a slower or stationary phase. The graph is S-shaped.
Absolute growth rate measures the total increase per unit time. Relative growth rate compares that increase with the starting size, making it useful when comparing organs of different sizes.
Conditions Needed for Growth
Plants require water, oxygen and nutrients for growth. Water supports cell expansion and provides a medium for chemical reactions. Oxygen helps release energy through respiration, while nutrients are needed to build new cell material. Temperature, light and gravity also influence growth and development.
Differentiation and Development
Differentiation occurs when a cell develops structures suited to a particular function. For example, a developing xylem cell acquires features that allow it to help conduct water.
Some mature living plant cells can regain the ability to divide. This is called dedifferentiation. Cells formed from such dividing tissue may later become specialised again, a process called redifferentiation.
Development includes the changes a plant undergoes through its life cycle. It can be understood broadly as growth plus differentiation. Plant development is flexible: the final form of an organ can be influenced by both the plant’s internal state and its environment.
Plant Growth Regulators
Plant growth regulators (PGRs) are chemical substances that influence plant growth and development. The five major groups in this chapter are auxins, gibberellins, cytokinins, ethylene and abscisic acid (ABA).
A regulator may have several effects, and its response depends on the plant part and conditions.
Auxins
Auxins help regulate cell elongation, rooting in cuttings and apical dominance. Apical dominance occurs when the growing shoot tip suppresses the growth of some lateral buds. Removing the tip can encourage branching.
Gibberellins
Gibberellins promote processes such as stem elongation and bolting, the rapid elongation of a stem before flowering in certain plants. They can also influence seed germination and other developmental responses.
Cytokinins
Cytokinins are associated with cell division. They help promote lateral shoot growth and can delay the ageing of leaves, known as senescence.
Ethylene
Ethylene is a gaseous plant growth regulator. It is especially known for promoting fruit ripening. It also influences senescence, the shedding of some plant parts and other growth responses.
Abscisic Acid (ABA)
ABA is strongly associated with responses to stress. It helps induce stomatal closure during water shortage and supports seed dormancy. It generally acts as a growth inhibitor.
| Regulator | A key point to remember |
|---|---|
| Auxin | Cell elongation and apical dominance |
| Gibberellin | Stem elongation and bolting |
| Cytokinin | Cell division and delayed leaf senescence |
| Ethylene | Fruit ripening |
| ABA | Stress response and stomatal closure |
Quick Revision
- Growth: Permanent increase in size.
- Meristem: Region containing actively dividing cells.
- Differentiation: Development of cells for specialised functions.
- Dedifferentiation: Mature cells regain the ability to divide.
- Redifferentiation: Newly produced cells become specialised again.
- Development: Growth and differentiation through the life cycle.
- PGRs: Chemical signals that regulate plant responses.
Check Your Understanding
- Name the three phases of growth.
- Which meristems mainly increase a plant’s length?
- What is the difference between differentiation and dedifferentiation?
- Which plant growth regulator is especially associated with fruit ripening?
- Which regulator helps close stomata during water stress?
Answers: (1) Meristematic, elongation and maturation. (2) Apical meristems. (3) Differentiation makes cells specialised; dedifferentiation allows some mature cells to divide again. (4) Ethylene. (5) ABA.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is plant growth?
Plant growth is an irreversible increase in size or mass due to cell division and enlargement.
What is development in plants?
Development includes growth, differentiation and other changes throughout a plant’s life.
What conditions are needed for most seeds to germinate?
Suitable water, oxygen and temperature are generally needed; some seeds have additional light or dormancy requirements.
What are the main phases of plant growth?
Meristematic cell division, cell elongation and cell maturation.
What is a meristem?
It is a region of actively dividing, relatively undifferentiated plant cells.
What is differentiation?
It is the process by which cells acquire specialised structures and functions.
What is dedifferentiation?
It is the return of specialised living cells to a state capable of division.
What is redifferentiation?
It is the specialisation of cells formed from dedifferentiated cells.
What is the difference between arithmetic and geometric growth?
Arithmetic growth adds a roughly constant amount per unit time; geometric growth increases in proportion to existing size.
What is a sigmoid growth curve?
It is an S-shaped curve with lag, rapid growth and slowing or stationary phases.
What are plant growth regulators?
They are small organic molecules that influence growth and developmental processes.
What is the main role of auxin?
Auxin commonly promotes cell elongation and helps regulate apical dominance and tropic growth.
What do gibberellins do?
They can promote stem elongation and help break dormancy or stimulate germination in some seeds.
What is a major role of cytokinins?
They promote cell division and can delay leaf senescence.
How do ethylene and ABA differ?
Ethylene promotes processes such as fruit ripening; abscisic acid (ABA) commonly promotes dormancy and stress responses.