Structure of the stem

Plants have parts both underground and overground. Stems connect the roots to the other plant organs such as leaves and flowers. Think of the stem as a plant “road”, which allows for transport of nutrients between the roots and the rest of the plant. Water and minerals travel up the stem from the roots to the rest of the plant. A simple sugar produced in the leaves, glucose, travels down to the roots.

But what are stems made up of? The surface of the stem is usually covered in a waxy, protective coating, which repels water. This coating is the cuticle, and its role is to reduce the amount of water loss from the plant. It also protects the inner tissues from infection and physical injury. The cuticle is produced by the outermost tissue layer of the stem.

Woody plants, such as trees and bushes don’t have the cuticle, but instead have an extra outer layer: the bark. The outermost tissue that secretes the cuticle, or is directly beneath the bark, is called the epidermis. It is transparent and consists of long, flat cells. Inwards from the epidermis, is the cortex. There are three types of tissue in the cortex of the stem.

The outermost tissue of the cortex is made up of cells which are elongated and have thick walls. This tissue strengthens the stem. Further inwards, is the second tissue of the cortex, which is made up of irregularly shaped cells. These cells have thin walls and are loosely packed, creating air pockets within the stem. Cells in this tissue store food and water.

The innermost layer of the cortex consists of cells with thickened walls that serve as strengthening tissue. Finally, at the very centre of the stem, there are vessels that transport water and nutrients around the plant. These are called vascular bundles. In different types of plants, vascular bundles are arranged differently. In one type, the vascular bundles are arranged in several circles that form a ring around the edge of the cortex.

The vessels that transport water face the inside of the stem. The vessels that transport nutrients face the outside of the stem. In the other type of plants, vascular bundles aren’t arranged in any special pattern, but are scattered throughout the cortex. But stems don’t only act as “roads” connecting roots to leaves and flowers. Stems also support the leaves and flowers.

Stems of some plants, such as cacti, can even store large amounts of food and water! Stems are very important parts of plants, and it’s their internal structure that makes stems so efficient.