What is Growth?

Growth is an irreversible, permanent increase in the size of an organ, a part, or a whole cell/organism, usually accompanied by an increase in dry weight and in the amount of protoplasm. It is one of the most fundamental characteristics of a living being.

Key features

  • Growth in plants is due to the meristems - regions with cells that retain the power to divide. Because plants can grow throughout their life at these localised regions, plant growth is called open growth (indeterminate).
  • Growth = cell division (formation of new cells) + cell enlargement. The most conspicuous, permanent feature is the increase in cell size by water uptake into vacuoles.
  • Growth is measurable - it can be recorded as an increase in length, area, volume, fresh weight, dry weight or cell number. For example, a pollen tube is measured by length, a leaf by area, and a dividing culture by cell number.

The three phases of growth (along a root/shoot)

At a growing tip the cells pass through three zones:

  • 1. Meristematic phase (phase of formation) - the cells of the root/shoot apex; they are constantly dividing, have rich protoplasm, large nuclei, thin primary cellulosic walls with abundant plasmodesmata.
  • 2. Phase of elongation - cells just behind the meristem; they show increased vacuolation, cell enlargement and new cell-wall deposition.
  • 3. Phase of maturation - farthest from the apex; the enlarged cells differentiate and mature to attain their final size and wall thickening.

One-liners: growth = irreversible/permanent increase; plants show open growth via meristems; the three phases are meristematic -> elongation -> maturation; growth is measured by length/area/volume/weight/cell number.

Conditions for Growth

Growth needs both internal (genetic/hormonal) and external factors:

  • Water - essential for cell enlargement (turgor), for enzyme activity and as a medium; cell elongation depends on turgidity.
  • Oxygen - required for respiration, which releases the energy (ATP) needed for growth.
  • Nutrients (macro- and micro-) - supply the raw material for building protoplasm and the energy for growth.
  • Optimum temperature - each plant grows within a range; away from the optimum, growth slows or is damaged.
  • Light and gravity - environmental signals that affect the form of the growing plant.

Growth is measurable - the parameters

Since growth shows up as increases in different quantities, it is quantified per unit time using parameters such as increase in cell number, length, area, volume or fresh/dry weight - whichever suits the system being studied.

NEET tip: Growth in higher plants is open (occurs at meristems throughout life); the very first requirement for the elongation of a cell is water (for turgor-driven enlargement); oxygen is needed because growth is energetically expensive (respiration provides ATP).