13.1 Growth

• Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell
• Growth is accompanied by metabolic process i.e. Anabolic and catabolic (where anabolism > catabolism), that occur at the expense of energy
• Plants retain capacity for unlimited growth throughout their life
• Cells of meristems have the capacity to divide and self-perpetuate
• The product loses the capacity to divide and makes up the plant body
• When new cells are always being added to the plant body by the activity of the meristem is called the open form of growth
• Growth, at a cellular level, is a consequence of increase in the amount of protoplasm; increase in protoplasm is difficult to measure
• One maize root apical meristem can give rise to more than 17,500 new cells per hour
• Cells in a watermelon may increase in size by up to 3,50,000 times
• The growth of a pollen tube is measured in terms of its length
• An increase in surface area denotes the growth in a dorsiventral leaf
• The period of growth is divided into three phases; meristematic, elongation and maturation
• The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth
• Meristematic cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections
• Meristematic cells are rich in protoplasm and possess large conspicuous nuclei.
• Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells of elongation region
• The cells next to meristematic zone are in the phase of elongation
• Proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation
• Cells of the maturation zone, attain their maximal size in terms of wall thickening and protoplasmic modifications
• In arithmetic growth only one daughter cell continues to divide while the other differentiates and matures
• Lt = L0 + rt

  Lt = length at time ‘t’

  L0 = length at time ‘zero’

  r = growth rate / elongation per unit time.
• Give the equation of arithmetic growth

  Lt = L0 + rt
• In geometric growth, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to a stationary phase.
• A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant
• W1 = W0 e^rt

  W1 = final size (weight, height, number etc.)

  W0 = initial size at the beginning of the period

  r = growth rate
• Measurement and the comparison of total growth per unit time is called the absolute growth rate
• The growth per unit time as percentage of initial size is the Relative Growth Rate
• Relative growth rate = ?

  Relative growth rate = (Growth / time x initial time) x 100
• Conditions of growth are water, oxygen, nutrients, temperature, light and gravity