Plant’s growth comprises cell division, cell enlargement and cell differentiation. When growth is measured per unit time, it is called growth rate. If we consider the increase in cell number as growth it can be the result of arithmetic or geometric growth.
Arithmetic growth refers to the division of one daughter cell obtained during mitosis while the other daughter cell enlarges and differentiates. Elongation of the root is arithmetic growth . A linear curve is obtained for arithmetic growth.
Geometric growth, on the other hand, refers to the division of both daughter cells obtained during mitosis. This type of growth is seen at the early stage of plant development, that is, development of the zygote to embryo. We can plot a graph for geometric growth by taking the length of the plant part on the Y-axis and the time taken for growth on the X-axis. An ‘S’ shaped sigmoidal curve is obtained. The curve depicts the different phases of growth in living organisms under natural conditions. Growth is slower in the initial phase and is called the lag phase. Later, growth is rapid, at an exponential rate, and is called the elongation or log phase. Finally, growth slows and it is called the stationary phase.
The growth rate obtained here is called the relative growth rate. Thus, we can see geometric growth during the development of the zygote to embryo, and arithmetic growth in the later stages of development of the seed into a mature plant. Growth in plants can be quantitatively compared using absolute growth rate and relative growth rate. Absolute growth rate refers to the total growth of a plant part per unit time, while relative growth rate refers to the growth of a plant part per unit initial parameter. The difference in the relative growth rate may be due to the difference in growth conditions. Plants require specific conditions for growth. These include water, oxygen, nutrients, temperature, light and gravity. Water acts as a medium for metabolic reactions required for growth. Oxygen is required by plants for the release of energy in food. Macro- and micro-nutrients are used as energy sources and in the synthesis of cellular contents like protoplasm. The growth of plants depends on light as they manufacture food by photosynthesis. Gravity determines the direction of shoot and root growth in plants.