Growth is a permanent increase in size or mass, usually brought about by an increase in cell number and cell size. Temporary changes, such as a plant cell swelling as it takes up water, are not growth. To study growth we need a measurable variable, recorded over time.Common measures:
• Length or height: easy and non-destructive, but ignores growth in width and branching.
• Fresh (wet) mass: easy and non-destructive, but the water content changes with drinking, eating, watering or transpiration, so it can be unreliable.
• Dry mass: the mass after all water is removed by heating (for example at about 100 °C) until the mass stops changing. It is the most reliable measure because it records only the organic and mineral material that has been added, but the organism is killed, so a large sample of similar organisms is needed, with a few removed at each time.
• Leaf surface area, or cell number counted with a haemocytometer for microorganisms.Results can be shown in three ways. Absolute growth is the total size plotted against time. Absolute growth rate is the change in size per unit time. Relative growth rate is the growth rate divided by the size at the start of the period, which allows organisms of different sizes to be compared.relative growth rate = change in size ÷ (initial size × time) Worked example: a seedling's dry mass rises from 2.0 g to 2.6 g in 5 days. Absolute growth rate = 0.6 ÷ 5 = 0.12 g per day. Relative growth rate = 0.12 ÷ 2.0 = 0.06 g per g per day.
• Length or height: easy and non-destructive, but ignores growth in width and branching.
• Fresh (wet) mass: easy and non-destructive, but the water content changes with drinking, eating, watering or transpiration, so it can be unreliable.
• Dry mass: the mass after all water is removed by heating (for example at about 100 °C) until the mass stops changing. It is the most reliable measure because it records only the organic and mineral material that has been added, but the organism is killed, so a large sample of similar organisms is needed, with a few removed at each time.
• Leaf surface area, or cell number counted with a haemocytometer for microorganisms.Results can be shown in three ways. Absolute growth is the total size plotted against time. Absolute growth rate is the change in size per unit time. Relative growth rate is the growth rate divided by the size at the start of the period, which allows organisms of different sizes to be compared.