Meristems: where a plant grows

Watch any plant for long enough and you will see three different kinds of growth. A seedling shoots up and its roots push deeper. A trunk gets fatter year after year. And grass that a cow has chewed down to stubble is green and tall again a few weeks later.

None of this happens all over the plant. Each kind of growth comes from particular spots where cells keep dividing. Together those cells form meristematic tissue, or a meristem. Tap each glowing zone to find the three kinds.

Here is an experiment you can design yourself. Stand two onion bulbs on two jars of water so that only their bases touch the water. Measure the roots on days 1, 2 and 3. On day 3, snip about 1 cm off the root tips of the bulb in Jar B only. Keep measuring both for four more days and record the lengths in a table like Table 3.1.

Run it below. Drag the days, or press play, and watch the graph build itself.

The roots in Jar A keep getting longer. The roots in Jar B, once their tips are gone, do not grow any more. So a root does not lengthen along its whole length: it grows only from its tip, where cells divide nonstop. You saw those dividing cells yourself in the onion root tip slide of Chapter 2.

Shoot tips have the same kind of dividing cells, which push the stem upwards. The growing zones at the tips of roots and shoots are called apical meristems (apex means tip). They make a plant grow in length.

A young dicot stem is thin and green. Years later the same stem can be a trunk you cannot wrap your arms around. Saw across a trunk and you see the record of that growth: rings, one inside another. Each one is an annual growth ring, about one ring for each year.

Rings are not all the same width. In a year with good growing conditions the tree adds a wide ring; in a hard year, a narrow one. Grow a tree below, year by year.

So counting rings estimates the age of the tree, and their widths hint at the climate it lived through.

Where do the new layers come from? Inside the stem is a ring of actively dividing cells. They add new cells both towards the inside and towards the outside, in circles around the centre, so the stem gets wider. This ring is the lateral meristem (lateral means at the side).

Pinch out the tip of a young stem and it stops getting taller. But the plant does not give up: new branches sprout from the nodes, the points on the stem where leaves and branches arise. Between one node and the next lies a length of bare stem, the internode.

A gardener uses this every time a hedge is trimmed: more branches come up and the hedge turns bushy. Grass does it even better. Mow a lawn or let goats graze it, and the blades are back in a few weeks. Try it.

So a plant has three meristems, and between them they account for every kind of growth: the apical one at root and shoot tips (length), the lateral one around the stem (girth), and the intercalary one at the base of plants such as grasses (regrowth and branching).

Put a meristem cell next to a mature plant cell and the differences jump out.

Picture a meristem cell: tiny, thin walled, with a big, prominent nucleus and dense cytoplasm crowded with organelles. It usually has no vacuole, and its neighbours press against it with little or no space in between. Everything about it is set up for fast, repeated division.

Each division adds cells. Some stay meristematic and keep dividing; others lose that ability. Those cells change shape and contents and take up a single job, such as support, transport or storage. They become permanent tissue, and the process that turns meristem into permanent tissue is called differentiation.