What is a Stem?
Stems are one of the three organs of a plant. The primary function (most important job) of a stem is mechanical support. Mechanical support means the plant’s stem supports the plant and holds it upright to help it grow toward sunlight. Stems also connect the other organs of the plant. Both the leaves and roots are connected to the stem! The stem transports water and nutrients up from the roots all the way to the leaves, and the stem transports sugars from the leaves to the rest of the plant. Stems support leaves, flowers, and fruits. Stems are the main transportation routes of water from the roots to the leaves. Food made in the leaves travels through stems to the rest of the plant.
Stems respond to air movement, to the shortage and surplus of nutrients and water, and they harvest sunlight.
The Huntington collects plants from all over the world. Some of the plants at The Huntington are not adapted to windy environments. When a big storm hit The Huntington, several plants were not able to survive the wind. What do you notice about the plants that died during the windy storm? Do they share any characteristics?
Many of the plants that died during the windy storm were tall trees. These trees had stems (trunks) that were not able to respond to the air movement (the wind). The air movement caused the stem to break. Can you identify where on each plant the stem broke?
They’re all green! Why? Because these stems make food from sunlight!
Plants make food from sunlight when they photosynthesize. On most plants, leaves are the organ responsible for photosynthesis. But not on all plants. Some plants use a different organ for photosynthesis.
Some plants use their stem for photosynthesis before the leaves have developed, and stop using their stem for photosynthesis once the leaves have grown. Other plants use their stems for photosynthesis throughout their whole lives! All these plants use their stems to gather sunlight and convert it into food.
Look at the stems above. How are they different from these non-photosynthesizing stems? How are they similar?
Supporting the Plant
Both of the plant’s other organs, the leaves and roots, attach to the stem. The stem supports the leaves. In some plants, the stem also supports branches (which are also part of the stem), flowers, fruits, cones, armor, and other parts! Look at the stems below. Some stems support epiphytes. Epiphytes are plants that grow on other plants instead of in the soil. Look at the stems below. What is each stem supporting?
Underwater stems aren’t nearly so stiff! Have you ever been in a pool or a lake? If you lift your feet from the floor, you don’t fall down. Why? The water helps support you. The same is true for aquatic plants. Aquatic plants are plants that live in the water. The water helps support the plant so the stem doesn’t need to do all the heavy lifting. Because of this, underwater stems are usually softer and more flexible.
Protecting the Plant
Some stems have a special adaptation: wood! Most tree stems have wood, and a few other plants have wood as well. In this picture, we can see a tree whose stems have been split open. Near the bottom we can see a cross-section of a stem, and near the top we can see the stem from the side. What do you notice? How many different colors can you see? Each of the parts of the woody stem are a different color in this plant.
Bark is an adaptation that helps a plant in many ways.
Bark helps protect the plant’s resources. All living things need water and nutrients and sugars to grow and survive. Plants use their stems to store and move water, nutrients, and sugar. Bark is the part that moves sugars. Many animals (including many insects) and fungi want to eat the tasty sugars, water, and nutrients inside the stem. Bark helps a plant protect its water, nutrients, and sugar by making it harder for animals (including insects) to get to the inside of the stem.
Most bark also prevents water from escaping the plant as a gas, though some bark has special pores that allow gasses into and out of the stem through the bark.
Bark also keeps plants safe in extreme temperatures. Thick bark can help a plant survive very cold or very hot environments by protecting the inside of the stem. Here we can see three redwood trees that are still alive and growing even though they were in a fire! Do you see where the fire burned the bark? Do you see anywhere on the trees that doesn’t look burned?
Here we can see several maple trees covered in snow. These trees have bark, which helps them survive the cold winter. Bark provides insulation to the tree. This means bark helps keep the cold out and the warmth in. Trees move water and other liquids like sap through their stems, so it is important that the trees do not get too cold. If they get too cold, the liquids could freeze.