Primary and secondary growth, and measuring growth

1Taller, then wider

A maize plant grows from a seed to over two metres tall in a single season, yet its stem never becomes much thicker than your thumb. A tree also grows taller each year, but it grows wider as well: its trunk thickens for decades and records each season as a ring of wood. Growing taller and growing wider are two different processes, carried out by different groups of dividing cells. This lesson describes them, and then shows how biologists measure growth and turn the measurements into growth rates.

2Primary growth: the tips

At each root and shoot tip, an apical meristem keeps making new cells. Behind it, the new cells take up water into their vacuoles and grow many times longer, pushing the tip forward; further back they stop growing and differentiate into xylem, phloem, epidermis and root hairs (Figure 1). All plants have primary growth.

Figure 1. The zones of primary growth in a root tip (simplified, not to scale).

3Secondary growth: the cambium

In woody dicots and conifers, the vascular cambium, a thin cylinder of meristem between the xylem and the phloem, divides throughout the plant's life. It makes new secondary xylem towards the inside and new secondary phloem towards the outside. The xylem builds up year after year as wood, while the old phloem is crushed. As the stem widens, the epidermis would split, so a second lateral meristem, the cork cambium, makes layers of waterproof cork (bark) on the outside. Monocots such as maize, sorghum and grasses have no vascular cambium, so they show little or no secondary growth.

Figure 2. A woody stem after four growing seasons (simplified). The cambium makes xylem inwards and phloem outwards.

Where seasons differ, the cambium makes large, thin-walled vessels when water is plentiful (early wood) and smaller, thick-walled cells as growth slows (late wood). The sharp change from late wood to the next season's early wood makes a visible growth ring. In temperate climates there is one ring a year, so rings give a tree's age. In the tropics, rings follow wet and dry seasons and are not always yearly, so they must be counted with care. A wide ring means a good growing season; narrow rings mean drought or crowding.

4Measuring growth

Two rates can be calculated from the measurements:

$$\text{absolute growth rate}=\frac{\text{increase in size}}{\text{time taken}}\text{relative growth rate}=\frac{\text{absolute growth rate}}{\text{size at the start}}\backslash text\{absolute\; growth\; rate\}\; =\; \backslash frac\{\backslash text\{increase\; in\; size\}\}\{\backslash text\{time\; taken\}\}\; \backslash qquad\; \backslash text\{relative\; growth\; rate\}\; =\; \backslash frac\{\backslash text\{absolute\; growth\; rate\}\}\{\backslash text\{size\; at\; the\; start\}\}$$

Plotted against time, the size of most organisms, organs and populations gives an S-shaped, sigmoid curve (Figure 3): a slow lag phase, a rapid exponential phase in which growth is proportional to the size already reached, a slowing phase as resources or maturity limit it, and a plateau when growth stops. Annual plants then decline as they set seed and die.

Figure 3. A sigmoid growth curve and the absolute growth rate it gives.