What growth is

A plant's life, from the zygote to its death, is one long process of development: the sum of growth and differentiation. It usually starts when a seed germinates in favourable conditions.

Growth is an irreversible, permanent increase in size of an organ, its parts, or a single cell. Like your height, it does not go back down. It usually uses energy from metabolism. A leaf expanding is growth. Wood swelling when soaked in water is not, because it is not permanent and no living process is involved.

Plants differ from animals in one big way: their growth is generally indeterminate: they can keep growing all their lives. This is because they have meristems: groups of cells that keep dividing and also keep themselves going. This is called the open form of growth.

• Apical meristems at the tips of roots and shoots make the plant longer: primary growth.
• Lateral meristems (the vascular cambium and cork cambium) in dicots and gymnosperms make it thicker (wider in girth): secondary growth.

At cell level, growth means more protoplasm, which is hard to measure directly. So we measure it indirectly: by fresh or dry weight, length, area, volume or number of cells. The tip of a maize root can make more than 17,500 new cells an hour, while each cell of a watermelon can grow up to 3,50,000 times in size.