30.2 Stems

Stem Structure and Function

Functions and structure of stems

Stems do three major jobs: they physically support the above-ground parts of the plant, they serve as the transport highway between roots and leaves, and they store food and water for later use.

• Support: Stems hold up leaves, flowers, and fruits, positioning them for light capture, pollination, and seed dispersal.
• Transport: Vascular tissues run through the stem. Xylem carries water and dissolved minerals upward from roots. Phloem carries sugars and other organic compounds from leaves to the rest of the plant.
• Storage: Parenchyma cells in the stem can stockpile carbohydrates (mainly starch) and water.

The basic architecture of a stem is built around nodes and internodes. Nodes are the points where leaves attach and where lateral buds form. Internodes are the stretches of stem between nodes, and their elongation is what makes the stem grow taller.

Three tissue systems make up the stem:

• Dermal tissue: The outer covering. In young stems, this is the epidermis; in woody plants, it's replaced by the periderm.
• Vascular tissue: Xylem and phloem, bundled together to handle long-distance transport.
• Ground tissue: Everything that isn't dermal or vascular. Includes the pith (central region), the cortex (between the epidermis and vascular tissue), both of which function in storage and support.

Tissue types in stems

Dermal tissue protects the plant and regulates gas exchange.

• The epidermis is a single cell layer coated with a waxy cuticle that reduces water loss. Tiny pores called stomata allow gas exchange ($CO_2$ in, $O_2$ out) and release water vapor through transpiration.
• In woody plants, the periderm replaces the epidermis. It includes cork, made of dead cells impregnated with suberin, a waxy substance that blocks water loss and protects against pathogens and physical damage.

Vascular tissue handles long-distance transport.

• Xylem is made of tracheids and vessel elements, both elongated, hollow cells that conduct water and minerals from roots to leaves.
• Phloem is made of sieve tube elements and companion cells. Sieve tube elements form a continuous pipeline for sugars; companion cells provide metabolic support to keep the sieve tubes functioning.
• Xylem and phloem are grouped into vascular bundles within the stem.

Ground tissue fills the remaining space and provides storage and support.

• Pith: Parenchyma cells in the center of the stem that store starch and provide some structural support.
• Cortex: Parenchyma cells between the epidermis and vascular bundles that also store starch and contribute to support.

Primary vs secondary stem growth

Primary growth increases the length of the stem. It happens at apical meristems, the clusters of dividing cells at the tips of shoots and roots. Primary growth produces all three tissue systems (dermal, vascular, and ground) and is the only type of growth in most herbaceous plants.

Secondary growth increases the diameter of the stem. It occurs in lateral meristems and is characteristic of woody plants like trees and shrubs.

Two lateral meristems drive secondary growth:

1. Vascular cambium sits between the xylem and phloem. It produces secondary xylem (wood) toward the inside of the stem and secondary phloem toward the outside.
2. Cork cambium (phellogen) produces the periderm, replacing the epidermis as the stem thickens.

Secondary xylem accumulates year after year, forming annual rings. Each ring has two visible zones:

• Earlywood (spring wood): Large-diameter, thin-walled cells produced in spring when water is plentiful. Appears lighter in color.
• Latewood (summer wood): Smaller, thicker-walled cells produced in summer and fall when water is scarcer. Appears darker.

One earlywood-latewood pair equals one year of growth. Counting rings gives the tree's age, and ring width patterns can reveal past climate conditions (a field called dendrochronology).

Modifications of stems

Not all stems look like typical upright stalks. Many plants have modified stems specialized for storage, reproduction, climbing, or defense.

• Rhizomes: Horizontal underground stems that store nutrients and enable asexual reproduction. They help plants survive drought or cold. Examples: ginger, iris, ferns.
• Tubers: Enlarged, fleshy underground stems packed with starch. A potato is a tuber; the "eyes" on a potato are actually buds that can sprout new plants.
• Bulbs: Short, vertical underground stems surrounded by fleshy modified leaf bases that store nutrients. Examples: onion, garlic, tulip. These help plants survive dormant periods.
• Corms: Resemble bulbs but the storage tissue is the swollen stem itself, not modified leaves. Examples: gladiolus, crocus.
• Stolons (runners): Horizontal stems that grow along the soil surface and produce new plants at their nodes. Strawberries spread this way.
• Tendrils: Thin, coiling stems that wrap around objects for support, allowing the plant to climb toward light. Examples: grapevines, passionflowers.
• Thorns: Hard, pointed modified stems that deter herbivores. Examples: honey locust, hawthorn. (Note: thorns are modified stems, while spines are modified leaves and prickles are outgrowths of the epidermis.)

Stem structure and plant function

Stem anatomy directly shapes how a plant grows and where it can survive.

Herbaceous vs. woody plants: Herbaceous plants have soft, flexible stems with little or no secondary growth. Woody plants develop hard, rigid stems through extensive secondary growth, allowing them to grow much taller and live much longer.

Monocot vs. dicot vascular arrangement: This is a classic comparison for exams.

• Monocots (grasses, palms) have vascular bundles scattered throughout the ground tissue. They generally lack vascular cambium, so secondary growth is limited and stems tend to stay slender.
• Dicots (oaks, maples, sunflowers) have vascular bundles arranged in a ring. The vascular cambium between xylem and phloem allows extensive secondary growth and thicker stems.

How modifications enhance survival:

• Underground storage stems (rhizomes, tubers, bulbs, corms) let plants survive drought, cold, or fire by keeping energy reserves safely below ground.
• Vegetative reproduction through stolons, rhizomes, and tubers allows rapid colonization without relying on seeds.
• Tendrils let plants compete for light in dense environments without investing in thick, self-supporting stems.
• Thorns reduce herbivory, improving survival in areas with heavy grazing pressure.

Meristems and stem growth

Meristems are localized regions of undifferentiated, actively dividing cells. They're the source of all new growth in a plant.

Three types of meristems are found in stems:

• Apical meristems: Located at the tips of shoots. They drive primary growth (lengthening).
• Lateral meristems: The vascular cambium and cork cambium. They drive secondary growth (thickening). Only present in plants that undergo secondary growth.
• Intercalary meristems: Found at the base of internodes in monocots (especially grasses). These allow the stem to regrow quickly from the base after being cut or grazed, which is why your lawn keeps growing back after you mow it.