Studies conducted on growth in unicellular organisms, or organs (in multicellular organisms) like leaf, stem root etc have revealed the existence of a characteristic pattern. This pattern of growth rate resulting in increase of height, weight, volume etc has a perfect correlation when considered in the context of time.
The results obtained by the study of growth pattern when plotted against time graphically would yield what is known as the growth curve. Studies conducted by plant physiologists have shown that the growth curve is always sigmoid or ‘S’ shaped in all the living organisms without any exception. The growth is slow initially and then is followed by a rapid growth rate and finally settles down to a slow rate or steady state in all the organisms.
These three phases of growth can be marked on the growth curve as lag phase, log phase and the steady state phase. The lag phase is characterised by a slow rate of growth (cell division) followed by the log phase when there is geometric increase of growth (cell elongation) to be followed by a steady state phase (cell maturation) when the curve flattens out.