Biology XI: 15 Plant Gro & Dev(2 Growth Rate)

Growth Rate Types of Growth Rate Growth rate: Increase in growth per unit time Plants show two types of growth—Arithmetic and Geometric—according to the increase shown by the growth rate Arithmetic growth Only one daughter cell continues to divide while others differentiate or mature. Example − root elongating at a constant rate Mathematically, Lt= L0 + rtWhere: Lt = Length at time, tL0 = Initial length r = Growth rate On plotting length against time, a linear curve is obtained. Geometric Growth Initial growth is slow (lag phase), followed by a rapid increase in growth (log/exponential phase), followed by a phase where growth slows down (stationary phase) Example − all cells, tissues and organs typically show this type of growth Mathematically, W1 = W0 ertW1 = Final sizeW0 = Initial sizer = Growth ratet = Time of growthe = Base of natural algorithms On plotting the size or weight of the organ against time, a sigmoid or S-shaped curve is obtained. Ways to compare growth quantitatively Two ways to compare growth quantitatively: By measuring (i) absolute growth rate and (ii) relative growth rate Absolute growth rate: Measurement of total growth per unit time In the given figure, an absolute increase is shown in the areas of the leaves A and B to form leaves A1 and B1. Relative growth rate: Growth of a given system per unit time expressed on a common basis; e.g., per unit of initial parameter In the given figure, both leaves increase by 10 cm2, but a relatively greater growth has occurred in leaf A. Conditions for Growth Include: water, oxygen, nutrients Water: Cell enlargement requires water. Water provides medium for enzymatic activities. Oxygen: Releases metabolic energy needed for growth. Nutrients: Source of energy Required for synthesis of protoplasm Temperature: Plays an important role in growth. Any deviation from the optimum temperature hampers growth. Environment signals (light and gravity)