This MCQ module is based on: Growth Measurement Phases
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Growth Measurement Phases
🎓 Class 11
Biology
CBSE
Theory
Ch 13 – Plant Growth and Development
⏱ ~14 min
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Growth — Its Nature, Measurement and Phases
A tree adds wood for two hundred years; a leaf on that same tree reaches a fixed size in a few weeks and then falls. Both are growing, yet the two processes are not the same kind of thing. This chapter begins by defining growth precisely enough to tell such cases apart.
13.1 Growth
Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. But what exactly is it?
Growth can be defined as an irreversible permanent increase in size of an organ or its parts, or even of an individual cell. Generally, growth is accompanied by metabolic processes — both anabolic and catabolic — that occur at the expense of energy.
So the expansion of a leaf is growth.
In-text question — how would you describe the swelling of a piece of wood placed in water? This is not growth, and the definition tells you why on two separate counts. First, the swelling is reversible: dry the wood and it shrinks back. Growth is an irreversible permanent increase. Second, the wood is dead — the swelling is the purely physical imbibition of water by cell walls, involving no metabolic processes and no expenditure of energy. The correct term is imbibition or swelling, not growth. The question is a good test of whether you have read the definition as a whole or only its first half, since “increase in size” alone would wrongly admit it.
13.1.1 Plant growth generally is indeterminate
Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability is due to the presence of meristems at certain locations in their body. The cells of such meristems have the capacity to divide and self-perpetuate. The product, however, soon loses the capacity to divide, and such cells make up the plant body.
Open form of growth. This form of growth, wherein new cells are always being added to the plant body by the activity of the meristem, is called the open form of growth.
In-text questions — what would happen if the meristem ceased to divide? Does this ever happen? If a meristem stopped dividing, no new cells would be added and growth of that axis would stop permanently — the growth would become determinate rather than indeterminate. And yes, this does happen, routinely. A leaf grows to a fixed size and stops; so do a flower and a fruit. When a shoot apex converts into a floral apex, its indeterminate vegetative growth ends. Seed dormancy is another case, in which meristematic activity is suspended rather than abolished. This is exactly why the chapter says plant growth is generally indeterminate — and why Section 13.2 will conclude that growth in plants “can be indeterminate or determinate”.
The meristems and the two kinds of growth
You have already studied the root apical meristem and the shoot apical meristem. They are responsible for the primary growth of the plants and principally contribute to the elongation of the plants along their axis.
In dicotyledonous plants and gymnosperms, the lateral meristems — vascular cambium and cork cambium — appear later in life. These are the meristems that cause the increase in the girth of the organs in which they are active. This is known as the secondary growth of the plant.
13.1.2 Growth is measurable
Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm. But since increase in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less proportional to it.
Parameters of growth. Growth is measured by a variety of parameters, some of which are: increase in fresh weight, dry weight, length, area, volume and cell number.
Two striking figures from the chapter show why no single parameter will do:
- One single maize root apical meristem can give rise to more than 17,500 new cells per hour. Here growth is expressed as an increase in cell number.
- Cells in a watermelon may increase in size by up to 3,50,000 times. Here growth is expressed as an increase in size of the cell.
And the parameter must fit the organ. The growth of a pollen tube is measured in terms of its length, while an increase in surface area denotes the growth in a dorsiventral leaf.
| Organ or system | Appropriate parameter | Why |
|---|---|---|
| Maize root apical meristem | Cell number | Over 17,500 new cells per hour — division, not enlargement, is what changes |
| Watermelon fruit cells | Cell size / volume | Cells may enlarge up to 3,50,000 times |
| Pollen tube | Length | It grows as a narrow tube in one direction only |
| Dorsiventral leaf | Surface area | It is flat — growth is expansion in two dimensions |
| Whole plant / crop | Fresh or dry weight | Dry weight excludes water content, so it reflects real material gain |
13.1.3 Phases of growth
The period of growth is generally divided into three phases, namely meristematic, elongation and maturation. The easiest place to see all three at once is a root tip.
1. Meristematic phase. The constantly dividing cells, both at the root apex and the shoot apex, represent this phase. The cells in this region are rich in protoplasm, possess large conspicuous nuclei, and their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.
2. Phase of elongation. The cells proximal (just next, away from the tip) to the meristematic zone represent this phase. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.
3. Phase of maturation. Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of the axis undergoing maturation. The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications. Most of the tissues and cell types you have studied in earlier classes represent this phase.
📐 Activity 13.1 — The parallel line technique
What to do. Germinate a few gram or maize seeds on moist cotton until the radicle is 2–3 cm long. Take one seedling and, with a fine waterproof marker, draw equally spaced parallel lines across the root, starting from the very tip and working back — label them A, B, C, D, E, F, G. Return the seedling to the moist chamber and leave it for 24 hours. Then measure the spacing between consecutive lines again.
Predict: will all the gaps widen equally? If not, which ones will widen most, and which region of the root does that identify?
Observation. The gaps do not widen equally. The zones A, B, C, D immediately behind the apex have elongated most, while the gaps further back (E, F, G) are almost unchanged, and the extreme tip itself has moved forward without much stretching.
What it proves. It locates the phase of elongation, and shows that growth in length is not spread along the whole root but is concentrated in a short zone just proximal to the meristem. The meristem itself adds cells rather than length; the elongation zone converts those cells into length through increased vacuolation, cell enlargement and new cell wall deposition; and by the maturation zone the cells have attained their maximal size, so no further stretching occurs there.
Why the marks stay put behind the zone. This is also a neat demonstration of why a nail driven into a tree trunk stays at the same height for years — elongation happens only at the growing regions, never uniformly throughout the axis.
🎯 Interactive: Identify the phase or meristem
Phase / type of growth: Meristematic phase — primary growth
Constantly dividing cells, rich in protoplasm, with large conspicuous nuclei and thin primary cellulosic walls carrying abundant plasmodesmatal connections. It contributes to elongation of the plant along its axis.
🎯 Competency-Based Questions
Scenario: A student grows four systems and records one measurement for each after two days: (i) a maize root tip, counting cells; (ii) a germinating pollen grain on a slide; (iii) a young dorsiventral leaf traced on graph paper; (iv) a dry wooden block left in a beaker of water, weighed before and after.
Q1. For each of (i) to (iii), state whether the parameter chosen is appropriate and why. L3 Apply
All three are appropriate. (i) Cell number suits a root apical meristem, where a single maize root apical meristem can give rise to more than 17,500 new cells per hour — the change is in number, not size. (ii) Length suits a pollen tube, since the growth of a pollen tube is measured in terms of its length; it extends as a narrow tube in one direction. (iii) Area suits a dorsiventral leaf, because an increase in surface area denotes the growth in a dorsiventral leaf — it is flat and expands in two dimensions.
Q2. The wooden block in (iv) gained 30% in weight. Has it grown? Defend your answer with the definition. L4 Analyse
No. Growth is an irreversible permanent increase in size, generally accompanied by metabolic processes, both anabolic and catabolic, that occur at the expense of energy. The block's gain fails both tests: it is reversible (drying restores the original weight) and it involves no metabolism and no energy expenditure, being the purely physical imbibition of water by dead cell walls. The example also warns that fresh weight alone can mislead — which is precisely why dry weight is often the safer parameter.
Q3. Fill in the blanks: The three phases of growth are ______, ______ and ______. Cells of the first are rich in ______ with large ______ and thin ______ walls; cells of the second show increased ______, cell ______ and new ______ deposition. L1 Remember
meristematic, elongation, maturation; protoplasm; conspicuous nuclei; primary / cellulosic; vacuolation; enlargement; cell wall.
Q4. A nail is driven into a tree trunk 1 m above the ground. Twenty years later the tree is far taller, yet the nail is still 1 m above the ground — but it is now buried deep in the wood. Explain both observations. L4 Analyse
The two observations separate the two kinds of growth. The nail stays at 1 m because increase in height comes from the shoot apical meristem, which contributes to elongation of the plant along its axis at the tip; the mature tissue at 1 m has already completed its phase of maturation and does not elongate further, as the parallel line technique shows. The nail becomes buried because the vascular cambium and cork cambium, lateral meristems that appear later in life in dicots and gymnosperms, cause the increase in girth — secondary growth — laying new wood outside the nail year after year.
Q5. “Plants grow forever, animals do not — so plants are immortal in a way animals are not.” Evaluate this claim. L5 Evaluate
The first half is close to the chapter's position but overstated; the second half is a leap. What is correct: plant growth is unique because plants retain the capacity for unlimited growth throughout their life, owing to meristems whose cells can divide and self-perpetuate, giving the open form of growth in which new cells are always being added. Animals, by contrast, have no equivalent permanent embryonic tissue in most organs. What is overstated: the chapter says plant growth is generally indeterminate, and immediately notes that the products of the meristem soon lose the capacity to divide. Leaves, flowers and fruits are all determinate, reaching a fixed size and often falling. Section 13.2 states plainly that growth in plants can be indeterminate or determinate. As for immortality: the claim confuses capacity for continued growth with survival. An annual plant completes its whole life in one season; and even in a long-lived tree, most of the body is dead support tissue. A fair statement: plants possess an unlimited capacity for adding new body through retained meristems — which is a real and profound difference from animals — but that capacity is neither uniform across all organs nor the same thing as escaping death.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Plant growth is described as an open form of growth.
Reason (R): New cells are always being added to the plant body by the activity of the meristem, whose cells can divide and self-perpetuate.
Answer: A. Both are true and the reason is exactly the definition the chapter gives for the open form of growth.
Assertion (A): Growth at the cellular level is measured directly as the increase in protoplasm.
Reason (R): Growth is measured by parameters such as fresh weight, dry weight, length, area, volume and cell number.
Answer: D. The assertion is false — growth is principally a consequence of increase in protoplasm, but increase in protoplasm is difficult to measure directly, so one measures a proportional quantity instead. The reason correctly lists those proportional quantities.
Assertion (A): In the parallel line technique, the zones immediately behind the root apex elongate most.
Reason (R): The cells proximal to the meristematic zone show increased vacuolation, cell enlargement and new cell wall deposition.
Answer: A. Both are true and the reason explains the assertion: those zones are the phase of elongation, and the three cellular changes named are precisely what converts newly formed cells into increased length.
Coming next. Part 2 takes up Sections 13.1.4 and 13.1.5 and then Section 13.2 — arithmetic and geometric growth with their equations, the sigmoid curve, absolute versus relative growth rates, the conditions necessary for growth, and then differentiation, dedifferentiation and redifferentiation.
Frequently Asked Questions - Growth, Its Measurement and Phases
How is growth defined in plants?
Growth is an irreversible permanent increase in size of an organ or its parts, or even of an individual cell. It is generally accompanied by metabolic processes, both anabolic and catabolic, that occur at the expense of energy.
Is the swelling of a piece of wood in water an example of growth?
No. The swelling is reversible, since drying restores the original size, and it involves no metabolic processes or energy expenditure because the wood is dead. It is imbibition, not growth.
Why is plant growth called indeterminate and open?
Because plants retain the capacity for unlimited growth throughout life, due to meristems whose cells can divide and self-perpetuate. Since new cells are always being added to the plant body by meristem activity, this is called the open form of growth.
Which meristems cause primary and secondary growth?
The root apical and shoot apical meristems are responsible for primary growth and principally contribute to elongation along the axis. In dicotyledonous plants and gymnosperms the lateral meristems, vascular cambium and cork cambium, appear later in life and cause the increase in girth, which is secondary growth.
Why is growth measured by different parameters?
Because growth at the cellular level is principally an increase in protoplasm, which is difficult to measure directly, so one measures a proportional quantity instead. Fresh weight, dry weight, length, area, volume and cell number are all used, and the right choice depends on the organ.
Give examples of growth measured as cell number and as cell size.
One single maize root apical meristem can give rise to more than 17,500 new cells per hour, where growth is expressed as increase in cell number. Cells in a watermelon may increase in size by up to 3,50,000 times, where growth is expressed as increase in cell size.
What are the three phases of growth?
Meristematic, elongation and maturation. The meristematic phase has constantly dividing cells rich in protoplasm with large nuclei and thin primary cellulosic walls. The elongation phase shows increased vacuolation, cell enlargement and new cell wall deposition. In the maturation phase cells attain maximal size with wall thickening and protoplasmic modifications.
What does the parallel line technique demonstrate?
It locates the zone of elongation. When equally spaced lines are drawn across a root and measured again later, the zones immediately behind the apex have elongated most, showing that growth in length is concentrated in a short region just proximal to the meristem rather than spread along the whole root.
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