(i) Growth rate refers to the expression of increased growth per unit time. 

(ii) It can be arithmetic or geometric increase. 

(iii) In arithmetic growth, following mitotic cell division only one daughter cell continues to divide while the other differentiates and mature. 

(iv) The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate. 

(v) On plotting length of the organ at different times a linear curve is obtained.

It is expressed as:

L_t = L₀ + rt

Here, 

L_t = Length at time 't'

L₀ = Length of time Zero

r = growth rate per unit time.

(vi) In geometrical growth, the initial growth is slow (lag phase) and rapidly at an exponential rate (log or exponential phase). 

(vii) In this growth, both the progeny cells formed by mitotic cell division retain the ability to divide and continue to do so. 

(viii) However, with limited nutrient supply, the growth slows down (stationary phase). 

(ix) If we plot a parameter of growth against time, we get a typical S-shaped or sigmoid curve. 

(x) The exponential growth can be expressed as:

W₁ = W₀e^rt

Here, W₁ = final size (Weight, height, number etc.) 

W₀ = initial size 

r = growth rate 

t = time of growth 

e = base of natural logarithms.