Class 11 Biology Plant Growth Development Growth rates

Growth rates

  • The increased growth per unit time is termed as growth rate.
  • An organism, or a part of the organism can produce more cells in a variety of ways, such as
  • Arithmetic
  • Geometric
  • Stages during embryo development showing geometric and arithematic phases
  • In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures.
  • On plotting the length of the organ against time, a linear curve is obtained.

Mathematically, it is expressed as

Lt = L0 + rt

Lt = length time at t

L0 = length at time 0

r = growth rate / elongation per unit time.

  • In geometrical growth, the initial growth is slow (lag phase), and it increases rapidly thereafter at an exponential rate, where both the progeny cells following mitotic cell division retain the ability to divide and continue to do so.
  • With limited nutrient supply, the growth slows down leading to a stationary phase.
  • If we plot the parameter of growth against time, we get a typical sigmoid or S-curve.
  • A sigmoid curve is a characteristic of living organism growing in a natural environment.
  • The exponential growth can be expressed as

W1 = w0 ert


W1 = final size

W0 – initial size

r = growth rate

t = time of growth

e = base of natural logarithms

  • r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index.
  • The final size of W1 depends on the initial size, W0.
  • Quantitative comparisons between the growth of living system can also be made in two ways:
  • measurement and the comparison of total growth per unit time is called the absolute growth rate.
  • the growth of the given system per unit time expressed per unit initial parameter is called the relative growth rate.


Fig. Sigmoid curve

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