How plants grow

A plant grows in three ways:

• in length: the stem gets taller and the roots go deeper;
• in girth: the stem gets thicker;
• by regrowth: it grows back after being cut or grazed.

All three need meristematic tissue: cells that keep dividing.

The onion experiment. Grow onion roots in two jars. In Jar A, let them grow. In Jar B, cut off the root tips on day 3. Jar A's roots keep growing; Jar B's stop. So roots grow only from their tips. The growing zones at the tips of roots and shoots are the apical meristems (apex = tip). They make the plant longer.

Here is a surprise: hammer a nail into a tree trunk at your height. Years later, it is still at your height. A tree grows taller from its tips, not from the middle.

Stems of many plants (dicots) also get thicker. A ring of dividing cells inside the stem, the lateral meristem (lateral = at the side), adds new cells inwards and outwards.

Look at a cut tree trunk and you will see annual growth rings, roughly one for each year. A wide ring means a good growing year; a narrow ring means a hard one. Count the rings to find the tree's age, and read their widths to learn about past climate.

Now cut the tip off a young stem. It stops growing taller, but new branches sprout from the nodes: the points where leaves or branches grow out. The part of the stem between two nodes is an internode. Just above each node, at the base of the internode, is the intercalary meristem. That is why a hedge grows bushy after trimming and grass grows back after mowing.

In short:

• Apical meristem: at the tips, makes the plant longer.
• Lateral meristem: in a ring, makes the stem thicker.
• Intercalary meristem: near the nodes, helps regrowth and branching.