How Trees Grow?

Start outside. Walk the class past the trees on the street and in the nearest park and make them name the shapes: the wide flat top of a rain tree, the upright vase of an ashok, the sideways sprawl of a banyan holding itself up on its own props. No two are the same, because each one added cells at its own rate and in its own pattern, year after year. Growth happens in two directions at once. The tips of shoots and roots push outwards and upwards, carrying leaves towards the light — that is height and spread. Separately, the trunk and branches thicken, so they can carry the weight of what they have grown. Underneath all the variety, every tree is the same machine: one central trunk, a framework of branches, an outer layer of leaves, and a root system that widens as the tree above it does. In a mature tree most of the cells in the trunk, branches and roots are dead or inactive. New growth happens at very few places — the tips of shoots and roots, and a thin layer of cells just inside the bark.

Find a tree that has dropped its leaves and look closely at a bare twig. Right at the tip is the terminal bud: a small hard packet holding next season's shoot, already built and waiting. Along the sides are lateral buds, which will become side branches. Buds are made months before they open and then sit dormant. In most of India the signal to open is not the end of cold — it is the return of water. Buds break with the pre-monsoon heat and the first rain. Say this plainly, or students will spend the lesson looking for a spring that never arrives.

Now run a finger back down the twig. You will reach a ring of thickened bark going right round it. That is the terminal bud scar, left where last season's terminal bud sat before it grew away. The distance from that scar to the present terminal bud is exactly one season's growth. Go further back and you find the scar before it, and the one before that. Each gap is one year. A twig is a ruler the tree made of itself, and with it students can age a branch and compare a good year with a bad one.

Height and thickness come from different places. Height comes from the buds at the tips. Thickness comes from the cambium, a thin layer of living cells between the bark and the wood, wrapped around the whole tree. Show it with paper cups. Turn one upside down, stack a second over it, then a third. Each cup is one year. The narrow closed end pointing up is the terminal bud; each new cup adds height at the top and a layer of thickness all the way down; the outside surface of the cups is the cambium. Stack enough and you have the shape of a real trunk — a very long cone, thickest at the base, because the base has been growing longest.

How tall a tree gets, and how fast, is partly its conditions and partly its genes. Give two species the same sunlight and water and they still stop at very different sizes; no amount of care turns a bottlebrush into a sal. A tree can keep growing for as long as it lives, so its final height depends on how fast it grows and how long it lasts. Fast, short-lived species stay small. Slow, long-lived ones — sal, teak, mahua, the big figs — get large because they get old. Shape is even more open to circumstance: how much room the tree had, how much sun and water reached it, how fast its tip grew compared with its side branches, and what happened to it along the way — lightning, fire, wind, goats, borers, disease. A tree's shape is its biography.

The crown is built one year at a time, in whatever direction the light is. Trees that get above their neighbours take most of the sun and are called dominant. Trees stuck underneath are suppressed, and they grow slowly. Some species cannot stand this for long and die in the shade. Others are shade-tolerant and can simply wait. A young sal can sit half-grown under the canopy for years doing almost nothing, then shoot upwards the moment a neighbour falls and the light comes through. In a forest, patience is a strategy.

Height and a wide crown buy sunlight, and they cost stability. A big spreading crown catches wind, so the tree answers by pushing roots deeper and wider to hold itself down. Some trees solve the problem another way: the banyan drops aerial roots from its branches, and where they reach the ground they thicken into props, so one tree can carry a crown the size of a playing field. High ground favours narrow, pointed crowns — the deodar and pine of the Himalaya shed snow and take wind off their shoulders. In the plains the common form is broad and rounded: mango, banyan, peepal, rain tree, all built to spread sideways and shade the ground under them.

One more structure matters — the flower, or the cone on a conifer. These make the seeds. Some species flower within a few years of germinating; others take decades; and a suppressed tree in deep shade may never flower at all, because it never reaches the size flowering needs. When it does, the cycle closes. Think of the amaltas going solid yellow just before the monsoon, or the mango flowering in the cold and fruiting in the heat — the same event each year, timed to a calendar the tree has been keeping far longer than the school has. The seeds then travel by wind, by water, or inside an animal, and the very few that land somewhere workable begin the whole journey again.