The Function of Stems

A plant’s stems are almost invariably upstaged by its colourful flowers and leaves. However, stems are fundamental to plants and superbly adapted to the role they play.

One of the main functions of a stem is to pass water and dissolved minerals from the roots to the leaves. These substances pass through tubes, or vessels, in the stem known as the xylem. A second, closely-related function is to transport food materials manufactured in the leaves to all other parts of the plant. Food materials pass through tubes in the stem known as the phloem.

Stems are also important as supporting structures. They hold up the leaves so that they can spread out and catch the light, which is needed for photosynthesis. They also support the flowers so that they can be easily pollinated by insects or the wind.

In some cases, for example in some cacti, the stems are also adapted to store food.

Stem, shoot and trunk The terms ‘stem’, ‘shoot’ and ‘trunk’ do not refer to different structures on a plant, but are used for structures at different stages of development. They all refer to those supporting structures that are above soil or compost level, although there are a few exceptions to this. For example, rhizomes are stems that can grow underneath the ground.

A shoot is a young, immature stem, usually, though not always, incorporating a leaf or leaves. As it grows it develops into a stem, which in most plants — from small annuals to twining climbers — links the roots, leaves and flowers.

A trunk is the woody main stem of a tree, which is able to support a great weight of branches and leaves.

Shrub or tree?stem of a redwood tree

The terms ‘shrub’ and ‘tree’ are often confused, but in fact they are distinguished by the way their stems grow. A tree has a clear stem, usually 1.2-1.8m (4-6ft) in height, before it branches out. A shrub has many woody stems (branches) that arise at soil level.

Plant stems are much more varied than roots, and take a number of different forms. Plants destined to grow into large trees have tough, strong stems. Other plants have climbing or trailing stems.

Some plants with long stems can either trail, climb or sprawl. These plants have thicker stems than plants that will only trail. They include climbers such as Devil’s Ivy (Epipremnun aureum).

Most plants that live totally submerged in water do not need strong stems to support their leaves, as the water keeps the leaves buoyant. However, plants submerged in fast-flowing streams need stems that are stronger and more pliable than those that live in still water. Aquatic plants with leaves that rise above the water also need strong stems, both to support the leaves and to resist water flow.

Some stems are covered with spines or thorns, or have thick bark to protect them against water loss, plant-eating animals or fire.

Making new plants from stems

There are several ways you can increase your plants by using their stems.

Stem-tip cuttings consist of a piece of stem and a few leaves. The bottom of the stem is trimmed below a leaf joint and the lower leaves removed. The base of the cutting is inserted 2.5-5cm (1— 2in) into compost.

Layering is an easy way to increase woody-stemmed plants. Long stems close to the ground are partly severed and pegged into a V-shaped hole. When roots have formed the new plant can be severed from its parent and planted out separately.

Log cuttings, thick pieces of stem 8-10cm (3-4 in)long and bare of leaves, can be inserted vertically into compost and kept warm and moist to encourage rooting. Suitable plants include Draceanas and Cordylines.

Air-layering can be used on certain plants, such as the Rubber Plant (Ficus elastica). Plants become unsightly when their bases are bare of leaves, and by partly severing the stem below the lowest leaf, packing moist peat round the incision and wrapping in polythene, you can induce the top part to produce fresh roots. When rooted, the stem is completely severed and the top part of the plant potted in fresh compost.

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