In this lesson, we’ll explore plant stems, what their functions and structures are, and why they are important to a plant.
We’ll look at how stems grow and what shoots are.
What Are Plant Stems?
Stems are the main trunk of a plant, or the primary axis that develops buds and shoots. Stems are divided into nodes and internodes.
Nodes hold at least one leaf and buds that can grow into branches. Internodes are the spaces between nodes.Stems and shoots are often confused. Shoot refers to new plant growth, whether it is a stem, a leaf, or even a flower. Stems are usually above ground, but there are some plants that have stems underground, such as bulbs or tubers. Stems usually have branches and leaves.
Stems are generally round like a stick and may be herbaceous or woody.Herbaceous stems are green and will generally bend. Most herbaceous plants are annuals, which need to be planted each year, as they die over the winter, such as petunias, or perennials, like peonies that die back in winter, but sprout again in spring.Woody stems are covered by bark, and they often will not bend without tremendous force. These are generally trees and they tend to live for a long time.
The functions of stems include:
- Support of leaves, flowers, and fruits. Keeping the leaves in sunlight is important for the plant to undergo photosynthesis.
Stems also provide places for flowers and fruits and keep them off the ground, which allows for pollination and seed distribution.
- Transportation of water and minerals and the products of photosynthesis in the xylem and phloem.
- Storage of food and/or water.
- Production of new tissue. Plant cells have a life span of one to three years. Stems have specialized cells called meristems that generate new tissue on an annual basis.
The apical meristem cells elongate the plant, while the lateral meristem cells widen.
The function of the stem is reflected in the internal structure. These internal structures are pretty much the same in all plants, although the arrangement of these internal structures will differ between plants. There are three types of plant cells and three types of tissue.These are the three types of plant cells:
- Parenchyma cells form the bulk of non-woody plants, like the inside part of an apple. This is the most abundant and least specialized type of plant cell.
The cortex is made up of these types of cells.
- Collenchyma cells support the growing parts of plants. For example, the strings in a stalk of celery. These cells have thicker walls for supporting the weight of the plant.
- Sclerenchyma cells support the non-growing parts of plants. These cells contain chemicals known as lignin.
These cells are tough and elastic; they allow the plant to bend with the wind and still stand straight up. They make up the fibers in linen and flax. Flax is a plant that is made into a fabric called linen. They also give pears their gritty texture and hardness to peach pits and walnut shells.
- Dermal tissue covers the outside of the plant and acts as a raincoat, security, and controls gas exchange. These cells are regular in shape and have a waxy covering.
- Ground tissue makes up the majority of the plant body and contains all three cell types. In stems, ground tissue develops support cells to hold the plant upright. This is also called the cortex and is located outside and/or around the vascular bundles.
Pith is the center of the stem and is made up of parenchyma cells.
- Vascular tissue forms longitudinal vascular bundles, which run the length of the stem and form veins in leaves.
Xylem and phloem are the two types of vascular tissue. This tissue transports water and minerals, as well as the products of photosynthesis. Vascular tissue is arranged in bundles, and it also supports the stem.
Have you noticed that when a plant needs water it droops? That’s because the vascular tissue isn’t full of water.So, how does a plant grow?
In plants, the areas of meristems are devoted almost entirely to the formation of new growth.There are three types of meristems:
- Apical meristem is located at the tips of stems and increases in length. This is called primary or non-woody growth. The apical meristem is responsible for formation of bud scales, leaves, and flowers or cones.
- Intercalary meristem is located between the tip and the base of stems and leaves and increases length between nodes.
These are found in grasses and bamboo and allow them to regrow quickly after being cut, which is why your grass needs cutting so often.
- Lateral meristem is located on the sides of stems and increases the diameter of a plant. This is called secondary growth, and it gives rise to bark and other woody growth.
The tip of the stem is the growing point.
Cell division takes place directly behind the tip. Apical meristem cells cause elongation of the stem. As they grow, the cells begin to change.
The cells on the outside develop into epidermal cells and cover the growing point. Other cells become the cortex, and still others will become vascular tissue. This is referred to as primary growth.Secondary growth involves increasing the plant’s width. It’s most obvious in woody plants, like trees.
Extra vascular tissue and additional strengthening tissue are required as the plant becomes larger and wider. A new layer of meristem tissue between the primary xylem and phloem develops. New xylem cells are added toward the inside of the stem, and new phloem cells are added toward the outside.The plant stem grows thicker when these new cells divide. In the spring, large xylem cells are produced, when sap is rising. In the fall, the cells become smaller as the sap flow stops.
The larger xylem tissue builds pressure on the phloem tissue, ultimately crushing it. This is how the annual rings are developed, because spring wood and fall wood are very different due to the changes in the plant cells and tissues.
Stems are the main trunk of a plant. We often confuse stems and shoots. Shoots refers to new plant growth, whether it is a stem, a leaf, or even a flower.
Stems are usually above ground, but there are some plants that have stems underground, such as bulbs or tubers. Stems usually have branches and leaves.Herbaceous stems are green and will generally bend.
Woody stems are covered by bark, and they will often not bend without tremendous force.The functions of stems include support; transport of water, minerals, and the products of photosynthesis; storage of food and/or water; and growth of new tissue. The apical meristem cells elongate the plant, while the lateral meristem cells widen.The three types of plant cells are parenchyma, collenchyma, and sclerenchyma. They make up the three types of plant tissue: dermal, ground, and vascular.
There are three types of stem growth: apical meristems, which elongate the plant, lateral meristems, which widen the plant, and intercalary, which lengthens the space between nodes.Primary growth focuses on elongating the plant, while secondary growth focuses on widening the plant.
Vocabulary Words & Their Definitions
|Stems||the main trunk of a plant or the primary axis that develops buds and shoots|
|Shoot||new plant growth, whether it is a stem, a leaf, or even a flower|
|Herbaceous||a type of stem that is green and will generally bend; most are annuals|
|Woody||stems are covered by bark and often will not bend|
|Meristems||generate new tissue on an annual basis|
|Apical meristem||cells elongate the plant|
|Lateral meristem||cells widen|
|Parenchyma||cells form the bulk of non-woody plants, like the inside part of an apple|
|Collenchyma||cells support the growing parts of plants; have thicker walls for supporting the weight of the plant|
|Sclerenchyma||cells support the non-growing parts of plants|
|Lignin||cells are tough and elastic|
|Dermal||tissue covers the outside of the plant and acts as a raincoat, security, and controls gas exchange|
|Ground||tissue makes up the majority of the plant body and contains all three cell types|
|Pith||the center of the stem and is made up of parenchyma cells|
|Vascular||tissue forms longitudinal vascular bundles, which run the length of the stem and form veins in leaves|
|Apical||meristem is located at the tips of stems and increases in length, responsible for formation of bud scales, leaves, and flowers or cones|
|Intercalary||meristem is located between the tip and the base of stems and leaves and increases length between nodes|
|Lateral||meristem is located on the sides of stems and increases the diameter of a plant|
A successful study experience should culminate with your ability to:
- Define stems
- Indicate knowledge of their functions
- Illustrate the structure of different stems
- Point out shoots on plants
- Recall the types of meristems