Roots and Stems

Roots usually grow under the soil. A root grows out from the stem and spreads through the soil. The first root that grows from a seed is called the primary root, or main root. Thin roots that branch out from the primary root are called secondary roots. Very fine, thread-like hairs on the surface of a root are called root hairs. These hairs soak up water and minerals from the soil. At the very tip of a root, a small cap of tissue called the root cap protects the growing tip as it pushes through the soil.

Roots do two main jobs for a plant. First, they hold the plant firmly in the soil so wind and rain do not wash it away. Second, they soak up water, minerals, and nutrients from the soil and send these up to the stem, from where they travel to every other part of the plant.

Root hairs sound like a small detail and they are not. Water can only enter a root through its surface, so the amount a plant can drink depends entirely on how much root surface is touching wet soil. A smooth root would touch very little. Covering it in thousands of fine hairs, each one poking into the spaces between soil particles, multiplies that surface enormously without the plant having to grow any bigger.

You have met this idea before. In the topic on cells, we saw that everything a cell needs has to come in through its surface, which is exactly why cells stay small. A root has the same problem and solves it the opposite way: instead of staying small, it grows a huge amount of surface into a small space. Lungs do the same thing with air, and the lining of the intestine does it with food. Whenever a living thing needs to take a lot of something in, look for a surface that has been folded, branched, or covered in hairs.

This also explains something gardeners know. When a seedling is dug up and moved, it usually droops for a day or two even if it is watered straight away. Root hairs are extremely delicate and most of them tear off in the soil when the plant is pulled out. Until the plant grows new ones it simply cannot drink properly. That is why seedlings are moved with a lump of soil around the roots whenever possible, and why they are shaded for the first few days.

Not all plants have the same kind of root. Plants with one seed leaf, called monocots, such as grass, wheat, and onion, grow a bunch of thin roots of about the same size. This kind of root is called a fibrous root. Plants with two seed leaves, called dicots, such as mustard and mango, grow one thick main root with thinner roots branching from it. This kind of main root is called a tap root.

These are not just two names to learn. They are two different ways of solving the same problem, and each one wins in a different situation. A fibrous root spreads wide but stays shallow, which means it grips a broad patch of topsoil and catches light rain the moment it falls, before the water has time to sink away. A tap root drives straight down, which means it can reach water that is deep underground long after the surface has dried out.

This is why grass is planted along the edges of terraced fields and on landslide-prone slopes across the hills of Nepal. It is not because grass is pretty. Its fibrous roots weave through the top layer of soil and hold it in place when heavy rain would otherwise carry it downhill. A single mango tree, with its deep tap root, could not do that job at all, even though it is far larger.

Some plants have roots with an unusual shape because they do a special extra job. Roots that have changed their shape to do a special job are called modified roots.

Notice that all four are answers to real difficulties. Maize grows tall and thin on soft ground, so it needs extra legs. A banyan spreads branches so wide and heavy that they would snap under their own weight, so it grows props to stand on. A carrot has to survive a dry season, so it packs food away underground. A dodder plant has almost no green in it and cannot make its own food, so it takes somebody else's. A modification is never decoration. It is always a problem being solved.

The part of a plant that grows above the soil and holds up the leaves, flowers, and fruits is called the stem. The point on a stem where a leaf or branch grows out is called a node. The part of the stem between two nodes is called an internode. A small growth at a node, called a bud, can develop into a new branch, leaf, or flower. In some plants, such as sugarcane, potato, and rose, a piece of stem with a bud can be planted to grow into a whole new plant, another example of vegetative propagation.

The transport row is worth reading twice, because a stem is doing two opposite journeys at the same moment. Water and minerals are travelling upwards from the root, and the food made in the leaves is travelling downwards to the root and everywhere else. Those two streams move in separate sets of tubes, side by side, all day and all night, and neither can stop without the plant being in serious trouble.

Just like roots, some stems change shape to do a special job. Potato, ginger, and turmeric have thick underground stems that store food. A cactus has a thick, green stem that carries out photosynthesis because it does not have proper leaves, and often grows sharp spines instead of leaves to protect itself from being eaten.

Read that first sentence again. A potato is a stem. So is ginger, and so is turmeric. They grow underground and look nothing like the stem of a plant you can see, which is exactly why this is the most commonly missed question in the whole topic. Being underground does not make something a root, and being fat does not either. So how do you actually tell?

The test is the buds. Leave a potato in a dark corner of the kitchen for a few weeks and pale shoots push out of its eyes. Those eyes are buds, buds sit at nodes, and only a stem has nodes. A carrot left in the same corner does not do that, because it is genuinely a root. One kitchen shelf settles the whole question, and it is worth trying rather than believing.