The growth is measurable : (a) The growth (at a cellular level) is basically a consequence of increase in the amount of protoplasm.
(b) We cannot measure growth directly, it is measured by some quantity that is more or less proportional to it. So the growth is measured by a variety of parameters like increase in fresh weight; dry weight; length; area; volume and cell number etc.
(c) One single maize root apical meristem may give rise to more than 17,500 new cells per hour.
(d) The cell in a watermelon can increase in size by upto 3,50,000 times.
(e) Therefore, growth may be expressed as increase in cell number or as increase in size of cell.
(f) The growth of a pollen tube is measured in terms of length.
(g) Aar increase in surface area measures growth in a dorsiventral leaf or dicot leaf.