Growth is a permanent increase in size, mass or cell number. When we measure a growth parameter (such as height, length or dry mass) and plot it against time, we get a growth curve. The shape of the curve tells us how fast the organism grows at each stage.
The sigmoid (S-shaped) curve
Most organisms follow a sigmoid growth curve. It has four phases:
- Lag phase — slow growth at the start while the organism prepares (for example a seed absorbing water and mobilising food).
- Exponential (log) phase — rapid growth; the rate is at its highest and the curve rises steeply.
- Deceleration phase — growth slows as limiting factors (space, nutrients) take effect.
- Plateau phase — growth stops or is balanced by loss; the curve is flat.
Key idea
A sigmoid curve is S-shaped: slow (lag) → fast (exponential) → slowing (deceleration) → level (plateau).
Intermittent (stepped) growth
Some animals with a hard exoskeleton, such as insects and crustaceans, cannot enlarge continuously. They must moult (shed the old exoskeleton) before the body can expand. Their growth curve is a series of steps — a sudden increase at each moult, then a flat period. If measured by mass instead of length the underlying trend is still roughly sigmoid.
Example
A grasshopper's length stays constant between moults, then jumps up each time it sheds its exoskeleton, producing a staircase-shaped curve. A bean plant, by contrast, gives a smooth S-shaped curve.
Growth rate
The growth rate is the change in the parameter divided by the time taken. On a sigmoid curve the rate is greatest during the exponential phase (steepest slope) and near zero during lag and plateau phases (flat parts). Comparing curves lets scientists judge which conditions favour growth.
Remember
A flat part of a growth curve means a low growth rate, not necessarily a small organism — the organism may already be large but no longer growing.