Q.Explain sigmoid growth curve with the help of diagram.

When the total growth (size or weight) of an organ is plotted against time, the curve rises slowly in the lag phase, steeply in the exponential phase and flattens in the stationary phase.

Step 1. Why the rate changes. The growth of an organ runs through three phases that correspond to the three phases of cell growth. In the lag phase (cell division/formation) the rate of growth is slow; in the exponential or log phase (cell enlargement) the rate is faster and reaches its maximum; in the stationary phase (cell maturation) the rate gradually slows down as cells stop enlarging and differentiate.

Step 2. The curve. Because the rate first rises and then falls, a graph of the total growth - the size or weight of the organ on the vertical axis - against time on the horizontal axis starts almost flat, curves steeply upward through the middle and then bends over to a plateau. The result is an elongated 'S', hence the name sigmoid curve.

Step 3. Diagram. Draw the horizontal axis as Time and the vertical axis as Size/weight of the organ. Mark the gently rising foot of the curve as the lag phase, the steep middle as the exponential phase and the flat top as the stationary phase, as in the graph above.

Step 4. Grand period of growth. The total time required for all the phases to occur - from the start of the lag phase to the plateau of the stationary phase - is called the grand period of growth. The sigmoid curve is typical of the growth of cells in culture and of most plant organs, and the same idealised curve is used in the NCERT/CBSE Class 11 treatment of plant growth.

Connect the three growth phases to the three segments of the S-curve, and describe the plot (Section 7.5).

• Confusing the sigmoid curve (growth rate over all three phases) with the simple exponential curve (geometric growth alone)
• Forgetting to describe the stationary (levelling-off) phase, leaving the curve looking like a simple upward exponential