Growth of a whole organ under natural constraints often traces a sigmoid or S-shaped curve when cumulative size is plotted against time. A lag phase begins with a relatively slow increase; a rapid exponential or log phase follows, then the growth rate declines as maturation and limits approach. A final stationary region can show little further change in the measured size.

The slope of the cumulative size curve indicates absolute rate of change at that moment. The steepest region has the largest absolute rate; a flat plateau has approximately zero rate of increase in the chosen measure, not necessarily a dead plant. The plant may still respire, photosynthesise or reproduce while one organ has reached near-final length.

If a graph instead plots rate directly against time, its peak and plateau have different meanings. Always inspect the vertical axis before naming phases. A sigmoid pattern describes a common trajectory under limited growth, not a guaranteed exact shape for every cell and organ in all environments.

Worked example: On a cumulative length curve, a steep middle segment signals faster extension than the early lag or late plateau.

NCERT trap: A stationary size plateau does not prove all physiological activity stopped.

NEET method: Read the y-axis; use the local slope to locate maximum size-increase rate.

Reading estimate; allow another 3–5 minutes for each set of three checks.

Plant Growth and Development

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