Plant Stems and Shoots
Stems and the leaves, buds, flowers, and fruits they support are referred to as shoots. Leafy shoots are made of stems and leaves. Flowering shoots have stems and flowers.
As a shoot’s apical meristem cells divide and elongate, it extends the length of the stem. Periodically during this growth cycle, the stem’s apical meristem will produce nodes where one or multiple leaves and buds grow laterally from the stem. The sections of the stem between the nodes are the internodes.
The initial growth of all stems produces primary tissue. These stems grow out of the epicotyl or embryonic shoot of a seed, from the apical meristem of an existing stem, or from the apical meristem of a bud. Stems of herbaceous (non-woody) plants consist only of this primary tissue. However, woody plants develop secondary tissues consisting mostly of xylem, which become wood and bark. This secondary growth moves water and nutrients from the center of the stem to the outer portions of the stem while also providing girth and support.
The main stem of a woody plant is called the trunk. After one year, stems develop an outer bark or cork and become twigs. When stems are two or more years old, they become branches.
Stems have two primary functions: transport and support. Stems support leaves and position the leaves where they can receive light. They transport needed water and nutrients to the leaves. Leaves then use the light for photosynthesis to produce food for the plant. The stems then transport the substances produced through photosynthesis to parts of the plant that are still developing as well as to tissues where the plant stores food.
Modified Stem Formations
There are other stem formations that differ quite a bit from the typical stem morphology discussed previously. These are referred to as modified stem structures. Modified stems can occur both above the ground and below the ground. Crowns, stolons, and spurs are above-ground modified stems. Bulbs, tubers, rhizomes, and corms are below-the ground modified stems.
A crown is an arrangement of leaves and flowers on short internodes that appear as if they occur in a rosette. Examples of this include strawberries, African violets, and ferns.
A stolon is often referred to as a runner. It is a fleshy or semi-woody stem that grows horizontally across the surface of the soil. These runners have nodes and buds or leaves. Roots can emerge at the nodes of the runners creating new plantlets. Strawberries are great examples of plants with stolons or runners.
Spurs are short stubby stems that grow laterally off branches of certain trees. Spurs occur in various fruit trees like pears, apples, cherries, plums, etc. as well as in ginkgo trees. The spurs on fruit trees are where leaves, flowers, and eventually fruit will develop. In the case of ginkgo trees, they are where naked ovules occur, which develop into seeds after fertilization.
A bulb is a compressed stem surrounded by fleshy storage leaves. These storage leaves surrounding the stem are called scales. The scales also surround a central bud that appears at the top of the stem. In the spring, the stem extends from the ground and produces leaves and flowers. The food produced by the leaves through photosynthesis is then stored in the bulb for next year. Onions, tulips, and daffodils are examples of plants that produce bulbs.
A corm is a type of below-ground modified stem that is shaped like a bulb, but it is not surrounded by fleshy scales. Gladiolus and crocus produce corms.
Rhizomes are very similar to stolons in that they are stems that grow horizontally, though rhizomes grow at or just below the ground. Bermuda grass, bearded iris, and canna all produce rhizomes.
A tuber is a swollen underground terminal portion of the stem that stores sugars and starches. The above ground portion of the stem produces leaves and flowers. These underground stems have nodes with buds and internodes unlike tuberous roots. Irish potatoes are an example of tubers, while sweet potatoes are examples of a tuberous roots.
Herbaceous (non-woody) plants have a thin layer of protective tissue around the outside called the epidermis. The epidermis is covered with a layer of cutin, a waxy substance that reduces water loss. The cortex layer is along the outside of the stem just inside the epidermis. The large region in the center of the stem is called the pith.
Two main types of vascular tissues are located inside the stem in straw-like bundles called vascular bundles and are used for transport: xylem and phloem. The portion of the vascular bundle closest to the center of the stem consists of xylem and is responsible for transporting water and minerals from the roots to the stems and leaves. The outer portion of the vascular bundle consists of phloem and is responsible for transporting the photosynthates (sugars or food created primarily by the leaves through photosynthesis) and other compounds from the leaves to other parts of the plant. These vascular bundles form a network to the tip of every root, shoot, and leaf and provide support making the stems strong. There is a thin layer of meristematic tissue called the vascular cambium that separates the xylem from the phloem and produces secondary growth.
The structure of herbaceous stems is different for monocot herbaceous plants and dicot herbaceous plants. In herbaceous monocots the vascular bundles are scattered throughout the soft tissue that forms most of the stem. In herbaceous dicots the vascular bundles form an outer ring between the cortex and pith.
Hardwood (Angiosperm) and Softwood (Conifer) Stems
Still more differences exist between herbaceous dicot stems and those of woody dicots. As mentioned previously, vascular bundles in herbaceous dicots have a single row of meristematic cells between the xylem and phloem called the vascular cambium. These cells divide laterally and longitudinally to increase the diameter of the stems and roots. But woody dicots have two types of meristems that produce lateral growth: the vascular cambium and the cork cambium. Like herbaceous dicots, the vascular cambium of woody dicots increases the diameter of the stem or trunk. The cork cambium produces cork, the outer protective tissue of bark.
The inner side of the vascular cambium produces new xylem cells increasing the girth of the stem or trunk. These secondary xylem cells (wood) have thick, lignified walls. The newer wood near the vascular cambium that makes up the perimeter of the tree is called active xylem or sapwood. This active xylem transports water and nutrients. Heartwood makes up the inner portion of the tree and consists of inactive xylem. While the inactive xylem is incapable of transporting water and minerals, it makes the structure of the tree strong and can be used to store various types of debris. As new layers of xylem are produced, most woody dicots produce growth rings that represent one year of growth.
Some woody dicots produce stems called canes with relatively large amounts soft tissue in the middle. The soft tissue is pith, and its primary function is food storage. Plants like roses, grapes, and blackberries are examples of woody dicots that produce canes.
Lateral cell division also occurs on the outside of the vascular cambium producing secondary phloem. This causes the existing phloem to be pushed outward and compress. The vascular cambium cells also divide side-by-side to increase the circumference of the vascular cambium ring itself. All tissues outside the vascular cambium are referred to collectively as bark. The cambium is most active during the spring and early summer months.
On woody dicots, the cork cambium produces cork, which replaces the epidermis on the outside stem. The meristematic cells in the cork cambium primarily produce cork cells on the outside which produce a waxy waterproofing substance called suberin. Though the cork cells eventually die, they continue to provide the tree with protection from water loss, extreme weather, insects, and disease.