For children: A new book answers every question that a child might have about trees and their lives

How do trees stand so straight and tall when they don’t even have backbones?

Good question! About 500 million years ago, plants, which had until then grown only in water, evolved to live on land. But it took them 130 million more years to figure out the engineering required to create a trunk that was strong enough to support the weight of an entire tree while “standing up”. Here are some cool innovations that trees came up with to make this possible.

They built their own “backbones”

Trees grow in two ways. They grow longer at the tips of their roots (searching for water) and at the tips of their branches (searching for the sun). They also grow thicker around their trunks. Each year, when the weather is favourable, the trunk grows a new ring of cells and gets fatter. This new ring consists of xylem cells, which take over the job of moving the water and minerals from the roots to the leaves. The older ring of the xylem dies, and joins the even older rings inside the trunk to become what is called the heartwood, the strong, solid centre of the tree. (PS: It isn’t quite accurate to call the heartwood of a tree its backbone, but it certainly does for the tree what our backbones do for us – keeps it upright.)

They figured out an anti-gravity transport system

To move water and minerals from their roots to their highest leaves, trees have to work very hard against gravity. We have a heart that works as a pump to move our blood from our feet to our chests and even higher, but trees don’t. How do they do it, then? They use something called capillary action, a process in which liquids rise by themselves in very narrow tubes. It is capillary action that allows a paper towel or a sponge to suck up a spill, and capillary action that trees use to “lift” water and minerals through the xylem to the height of their highest leaves.

Now, we know that when a sponge is full of liquid, it will not absorb any more until you have squeezed it dry. What do trees do to “squeeze themselves dry” so that new water and nutrients can come in? They “transpire”, or throw out extra water from their leaves continuously, in the form of vapour. Simple!

They grew bark

The hard, protective outer covering of a tree, bark is made up of layers and layers of dead cells that keep insects out, resist fire damage, and make sure the tree loses as little water as possible. Just inside the outer bark is the inner bark, made up of phloem, which transports sugar from the leaves to all parts of the tree (see why you need to be very careful not to cut into the bark of a tree?). Phloem only lives for about a year, after which the tree grows a new ring of phloem. The old, dead phloem gets pushed outwards, making the bark thicker and harder. This thick, hard bark also adds to the strength of the trunk. Double whammy!

Can you really tell the age of a tree by counting its rings?

Sometimes. But not always.

Here’s why.

When does a tree grow a ring of new cells inside its trunk? When the weather is favourable. In the world’s temperate regions, where winters are harsh, trees grow in spring and summer. In their trunks, those seasonal growth spurts show up as a light-coloured ring of ‘earlywood’ when the trunk is fattening rapidly, and a dense, narrow, darker-coloured ring of ‘latewood’ when the weather starts getting cold and the trunk is fattening more slowly. (It’s this darker border that makes growth rings so easy to see and count.) Then the tree stops growing thicker until the weather is favourable again.

What happens to trees that grow in the world’s tropical regions, including much of India? They grow through the year, slowly and steadily, because the weather is always favourable. And that means… that’s right, no dark latewood, and therefore, no clear rings! It’s almost impossible to tell the age of trees like these, by looking at a cross-section of their trunks.

Even when a tree has rings, the number of rings may not be the most accurate way of telling its age. How come? Often, many annual rings are fused together, which makes it difficult to tell how many years they represent. There’s another problem as well. In particularly dry years, when there isn’t enough rainfall, or in years when the weather conditions are extreme—too hot or too cold— the tree may not grow thicker at all. Which means… yes, missing growth rings!

Excerpted with permission from Let’s Talk about Trees, Roopa Pai, illustrated by Barkha Lohia, Juggernaut.