01 · Explore
The Nature of Plant Growth
Growth is a fundamental characteristic of living beings, defined as an irreversible permanent increase in the size of an organ, its parts, or even an individual cell.
In plants, growth is unique because it is generally indeterminate. This means plants retain the capacity for unlimited growth throughout their life due to the presence of meristems. Meristems are localized regions of cells that have the capacity to divide and self-perpetuate. This 'open form of growth' allows new cells to be constantly added to the plant body.
Primary growth occurs at the root and shoot apical meristems, leading to the elongation of the plant along its axis. In dicotyledonous plants and gymnosperms, lateral meristems like vascular cambium and cork-cambium appear later, causing an increase in girth, known as secondary growth.
Growth is measurable through various parameters because direct measurement of protoplasm increase is difficult. Common parameters include increase in fresh weight, dry weight, length, area, volume, and cell number. For example, a single maize root apical meristem can produce over 17,500 new cells per hour.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
What is the 'open form of growth' in plants?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeQuestion
How does primary growth differ from secondary growth?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 1, 2, 3.
02 · Explore
Phases and Conditions of Growth
The period of growth in plants is divided into three distinct phases: meristematic, elongation, and maturation.
The meristematic phase occurs at the root and shoot apices where cells are constantly dividing. These cells are rich in protoplasm, have large nuclei, and thin cellulosic primary walls with abundant plasmodesmata. The phase of elongation follows, characterized by increased vacuolation, cell enlargement, and new cell wall deposition.
The final phase is maturation, where cells attain their maximal size and undergo protoplasmic modifications to perform specific functions. This phase results in the various tissues seen in a mature plant body.
Growth requires specific environmental conditions. Water is essential for cell turgidity and enzymatic activity. Oxygen is required for releasing metabolic energy. Nutrients (macro and micro elements) provide the building blocks for protoplasm and energy. Additionally, plants require an optimum temperature range and respond to signals like light and gravity.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
What are the cellular characteristics of the elongation phase?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 3, 4, 6.
03 · Explore
Mathematical Expressions of Growth Rates
Growth rate is the increased growth per unit time, which can be expressed as arithmetic or geometric growth.
In arithmetic growth, only one daughter cell continues to divide after mitosis, while the other differentiates. This results in a linear growth curve. A common example is a root elongating at a constant rate.
Geometric growth is characterized by both daughter cells retaining the ability to divide. This typically follows a sigmoid or S-curve, consisting of a slow lag phase, a rapid exponential (log) phase, and a stationary phase when nutrients become limited. The sigmoid curve is typical for most living organisms in natural environments.
Arithmetic Growth Calculation
Lt = L0 + rt
To find the length of a root (Lt) after 5 hours (t) growing at a rate (r) of 2 mm/hr, starting from an initial length (L0) of 10 mm: Lt = 10 + (2 * 5) = 20 mm.
Geometric Growth Expression
W1 = W0 * e^(rt)
W1 is the final size, W0 is the initial size, r is the relative growth rate (efficiency index), and t is time. This shows that final size depends heavily on the initial size.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
What does a sigmoid growth curve represent?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 4, 5.
04 · Explore
Differentiation, Dedifferentiation, and Redifferentiation
Plant cells undergo complex transitions in their ability to divide and specialize.
Differentiation is the process where cells from meristems mature to perform specific functions, involving structural changes like the loss of protoplasm in tracheary elements to form water-conducting tubes. Dedifferentiation occurs when specialized, living cells regain the capacity to divide, such as parenchyma cells forming interfascicular cambium.
Redifferentiation is the final stage where cells produced by dedifferentiated meristems lose their ability to divide again and mature into specialized tissues. This flexibility shows that differentiation in plants is 'open' and often determined by the cell's location within the organ.
Development is the sum of growth and differentiation. Plants also exhibit plasticity, the ability to follow different pathways to form different structures in response to the environment. An example is heterophylly, where juvenile leaves differ from adult leaves (as in larkspur) or terrestrial leaves differ from aquatic leaves (as in buttercup).
The Developmental Cycle of a Plant Cell
- 1
Meristematic Cell
The starting point; a cell capable of continuous division.
- 2
Plasmatic Growth
Increase in protoplasm and cell expansion/elongation.
- 3
Differentiation
Structural and functional specialization leading to a mature cell.
- 4
Maturation
The process of reaching full functional maturity following expansion.
- 5
Senescence
The period of aging leading toward the end of the life cycle.
- 6
Death
The final stage of the developmental process.
Sequence of developmental processes in a plant cell from division to senescence as shown in Figure 13.8.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
What is an example of dedifferentiation?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8, 9.
05 · Explore
Plant Growth Regulators: Auxins and Gibberellins
Plant Growth Regulators (PGRs) are small molecules that control growth and development, categorized as promoters or inhibitors.
Auxins (e.g., IAA, NAA, 2,4-D) are produced at growing apices and migrate to action sites. They promote flowering in pineapples, initiate rooting in cuttings, and induce parthenocarpy in tomatoes. A key effect is apical dominance, where the apical bud inhibits lateral buds. 2,4-D is also used as a herbicide to kill dicot weeds without affecting monocots.
Gibberellins (GAs) are acidic promoters. They are used to increase the length of grape stalks and improve the shape of fruits like apples. In the brewing industry, GA3 speeds up the malting process. They also promote 'bolting' (rapid internode elongation) in rosette plants like cabbage and can increase sugarcane yield by increasing stem length.
| PGR Group | Primary Functions | Agricultural Application |
|---|---|---|
| Auxins | Cell division, apical dominance, rooting | Herbicides (2,4-D), rooting of stem cuttings |
| Gibberellins | Internode elongation, bolting, delay senescence | Increasing sugarcane yield, malting in brewing |
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
How is apical dominance managed in tea plantations?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 9, 10, 11.
06 · Explore
Cytokinins, Ethylene, and Abscisic Acid
These regulators manage cell division, fruit ripening, and stress responses.
Cytokinins (e.g., kinetin, zeatin) promote cytokinesis (cell division). They are synthesized in regions of rapid division like root apices. They help overcome apical dominance, promote nutrient mobilization, and delay leaf senescence. They are essential for producing new leaves and chloroplasts.
Ethylene is a gaseous PGR produced by ripening fruits and senescing tissues. It is unique for its role in fruit ripening, where it causes a 'respiratory climactic' (rise in respiration rate). It breaks seed dormancy and is used commercially as Ethephon to ripen tomatoes and apples and to increase female flowers in cucumbers.
Abscisic Acid (ABA) acts as a general growth inhibitor and is known as the 'stress hormone.' It stimulates stomatal closure to prevent water loss and induces seed dormancy to help seeds survive unfavorable conditions. It often acts as an antagonist to Gibberellins.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
Why is Abscisic Acid called the stress hormone?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeQuestion
What is the 'respiratory climactic'?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 11, 12, 13.