(a) Arithmetic growth
In arithmetic growth, one of the daughter cells continues to divide, while the other differentiates into maturity following mitotic cell divsion. The rate of growth is constant and increase in growth occurs in arithmetic progression, i.e., 2,4,6,8,10,..
Example: Elongation of roots at a constant rate
(b) Geometric growth
In geometric growth,growth is slow in the initial stages (lag phase) whereas rapid during the later stages (log or exponential phase). The daughter cells formed as a result of mitosis retain the ability to divide, but slow down due to limited supply of nutrients. The size or number increases in multiplicative fashion, i.e., 2,4,6,8,16,32,..
(c) Sigmoid growth curve
It is an S-shaped graph produced on plotting growth of living organisms in their natural environment against time. This curve is divided into three phases: lag phase, log phase or exponential phase of rapid growth, and stationary phase.
Exponential growth can be expressed as:
W1 = W0 e^rt
where,
W1 = Final size
W0 = Initial size
r = Growth rate
t = Time of growth
e = Base of natural logarithms
(d) (i) Absolute growth rate: It refers to the measurement and comparison of total growth per unit time.
(ii) Relative growth rate: It refers to the growth of a given system per unit time, expressed on a common basis.