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If your textbook says a plant grows, the exam question that follows is usually: grows in what? Length, weight, cells? NCERT's Class 11 chapter Plant Growth and Development answers this in one short section, with two memorable examples: a maize root tip and a watermelon.
How is plant growth measured?
NCERT starts at the cell. Growth at the cellular level is principally a consequence of an increase in the amount of protoplasm. Protoplasm is difficult to measure directly, so one generally measures some quantity that is more or less proportional to it (NCERT, section 13.1.2). The textbook names six such parameters, introducing them as "some of which are":
fresh weight
dry weight
length
area
volume
cell number
The same chapter defines growth as an irreversible permanent increase in size of an organ, its parts or even an individual cell. That one word, irreversible, matters when you choose a parameter.
What do the maize root tip and watermelon examples show?
They show that bigger can mean two different things. According to NCERT, 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.
In the same paragraph, NCERT says cells in a watermelon may increase in size by up to 3,50,000 times. Here growth is expressed as an increase in the size of the cell, not in how many cells there are.
A quick sense check: if that rate held for 24 hours, 17,500 cells an hour would mean more than 4,20,000 new cells a day from one root tip. If an exam asks which example shows growth by cell division and which by cell enlargement, maize is division and watermelon is enlargement.
Which parameter should you use for which plant part?
Pick the quantity that actually changes in the part you are studying. NCERT adds two more examples: the growth of a pollen tube is measured in terms of its length, and an increase in surface area denotes growth in a dorsiventral leaf (NCERT).
Dividing cells at a root or shoot apex: cell number (the maize example).
Fruit cells enlarging as they mature: cell size or volume (the watermelon example).
Pollen tube: length.
Flat, dorsiventral leaf: surface area.
Stem or root thickening in secondary growth: girth, which NCERT links to the lateral meristems, vascular cambium and cork cambium.
Whole seedling or plant: weight. In classical growth analysis, relative growth rate is calculated from plant dry weights (Hoffmann and Poorter, Annals of Botany, 2002).
Why dry weight? Fresh weight includes water, which can rise and fall with watering, much like the swelling wood. Dry weight counts the material the plant has actually built.
Our explainer on the three phases of growth at the root tip shows where the dividing, elongating and maturing zones sit along a root.
What is the difference between absolute and relative growth rate?
The comparison of total growth per unit time is called the absolute growth rate, and growth per unit time expressed per unit initial parameter is called the relative growth rate (NCERT, section 13.1.4). NCERT's Figure 13.7 makes the point with two leaves: leaf A grows from 5 cm² to 10 cm², and leaf B from 50 cm² to 55 cm² in the same time. Work it through:
Absolute growth: both leaves added 5 cm², so their absolute growth rates are equal.
Relative growth: A added 5 on a base of 5, so 5 ÷ 5 = 1, a 100% increase. B added 5 on a base of 50, so 5 ÷ 50 = 0.1, a 10% increase.
Answer: leaf A has the higher relative growth rate, ten times B's on this simple measure.
Log version: researchers calculate relative growth rate as (ln W₂ − ln W₁) ÷ (t₂ − t₁) (Hoffmann and Poorter, 2002). Applied to the leaf areas, A gives ln(10/5) = ln 2 ≈ 0.69 per period and B gives ln(55/50) = ln 1.1 ≈ 0.095. A still wins, by about seven times.
The log form gives a smaller ratio because it treats growth as compounding, as in NCERT's exponential growth equation W₁ = W₀e^rt, where r is the relative growth rate. For more on steady versus compounding growth, see arithmetic vs geometric plant growth.
How can you practise measuring growth at home?
Soak a few seeds, record root length daily and plot it; our daily plant growth log with a ruler has a format. Ask two questions: which of the six parameters is this, and could the change reverse?
If a term in this chapter feels shaky, such as meristem or protoplasm, the Fermi Concept Library covers every concept in biology, each with its own lesson. For growth-rate numericals, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
