Plant Science Notes: Stems

Section 6: Organography of Plants: Stems

Introduction

• The stem is the plant's stalk or primary trunk, facilitating the transport of water, minerals, and nutrients.
• Stems can store food, and green stems can generate food independently.
• In most plants, the stem is the primary vertical shoot, but in some, it's inconspicuous or modified to resemble other plant parts, like underground stems mimicking roots.
• The stem acts as the plant axis, supporting buds and shoots with leaves, and giving rise to roots at its base.

Stem Structure and Development

• Examples: Reed or bamboo stems have easily identifiable nodes and internodes, while peach trees do not.
• The first stem rudiment appears from the seed after the root protrudes.
• The terminal bud at the shoot's apex increases height through continued development.
• Lateral buds and leaves grow from the stem at nodes; internodes are the intervals between nodes.
• Appendages occur only at the nodes.
• The number of leaves per node varies by species.
• Leaf drop causes scars on the stem due to severed vascular bundles.
• Lateral buds produce resembling stems, determining plant branching.
• In trees, lateral shoots form branches, which give rise to branchlets or twigs.
• The axil is where a leaf diverges from a stem.
• An axillary bud forms in the axil of a leaf, produced from stem tissues.
• Vascular bundles within axillary buds connect to those of the stem.

General Characteristics of Stems

• Stems are divided into nodes and internodes.
• Leaves and axillary buds are at the nodes.
• Stems are usually round but can be flattened, lobed, or angular.
• Theoretically, stems have unlimited growth in length due to meristem tissue renewal.
• Growth in length occurs at the growing point, but intercalary growth may also occur.
• Vascular tissue is arranged in vascular bundles, not in a radial pattern like in roots.
• Green stems photosynthesize like leaves; cacti primarily use stems for this.
• Asparagus carries out food-making chiefly by the stem.
• Plants with woody stems are trees and shrubs.
• Shrubs branch from or near the ground, while trees have conspicuous trunks.

Plant Growth Patterns

• Annuals: Complete life cycles in one growing season; the entire plant dies.
• Biennials: Grow from seed in one season, store food over winter, and produce an erect stem in the second season.
• Perennials: May lose leaves or go dormant in winter but continue growing from season to season.

Anatomy of Stems

• Promeristem
  • The apical meristem generates new cells, similar to roots.
  • Initial stem cells are located at the terminus, with outer layers dividing anticlinally, forming the tunic.
  • The corpus divides in all directions, producing the protoderm (epidermis), ground meristem (cortex), and procambium (vascular bundles).
  • The apical meristem also gives rise to leaf primordia.
• Epidermis
  • Derived from the protoderm, the epidermis develops a waxy cuticle and may have trichomes (hairs) and stomata.
  • Trichomes aid in plant identification.
  • Stomata form when meristemoid cells divide to create guard cells, allowing pores to form.
• Ground Tissue
  • Includes the cortex and marrow, both originating from the tunic.
  • Cortex:
    • Hypoderm: Collenchyma cells providing firmness and containing chloroplasts in forbs.
    • Central Cortex: Parenchyma cells with air spaces, storing starch and functioning in assimilation.
    • Starch Sheath: The innermost layer of the cortex, made of starch-filled parenchyma cells.
• Pith
  • Located at the center of dicot stems, composed of parenchyma cells with air spaces in young stems, which lignify with age.
  • Serves as a storage tissue for starch.
• Vascular Tissue
  • Forms from the procambium; in older stems, the procambium becomes interfascicular parenchyma, and vascular rays appear.
  • Primary xylem and phloem differentiate in a collateral arrangement, forming vascular bundles.
  • In dicots, the procambium between the primary xylem and phloem forms the fascicular cambium, creating an open collateral vascular bundle system.

Growth Point of Stems

• The growing point consists of the apical meristem, closely packed nodes, internodes, and young leaves.
• Young leaves undergo hyponastic growth, bending over the apical meristem for protection.
• The apical meristem gives rise to leaf primordia (leaves) and stem primordia (axillary buds).

Primary Growth of Stems

• Three phases:
  • Cell division: Occurs in the apical meristem.
  • Cell enlargement (cell stretching): Occurs directly behind the promeristem; determines node spacing.
  • Cell differentiation: Cells differentiate or specialize into specific cell types.
  • Young leaves unfold around growing points through epinastic growth.

Buds

• A bud is an undeveloped or embryonic shoot, typically in a leaf axil or at the stem tip.
• Once formed, a bud may stay dormant or form a shoot immediately.
• Dormancy: Cessation of observable growth, essential for survival in cold temperatures.
• Buds of woody plants are protected by scales (modified leaves).
• Scales are often covered by a gummy substance for added protection.
• As the bud develops, scales may enlarge or drop off, leaving scars.

Types of Buds

• Useful for plant identification, especially in winter.
• Classified by:
  • Location
  • Status
  • Morphology
  • Function
• Location
  • Terminal: At the tip of a stem (apical).
  • Axillary: In the axil of a leaf (lateral).
  • Adventitious: Elsewhere, like on the trunk or roots.
• Status
  • Accessory: Secondary buds beside a principal bud.
  • Resting: Form at the end of a growth season and lie dormant.
  • Dormant: Delayed growth over a long time.
  • Pseudo-terminal: An axillary bud taking over the function of a terminal bud (sympodial growth).
• Morphology
  • Scaly or covered: Protected by scales.
  • Naked: Not covered by scales.
  • Hairy: Protected by hairs (scaly or naked).
• Function
  • Vegetative bud: Contains embryonic shoots with leaves (foliage bud).
  • Reproductive bud: Contains embryonic flower(s) (floral bud).
  • Mixed buds: Contain both embryonic leaves and flowers.

Branching of Stems

• Growth in length occurs at the terminal bud or growing point.
• Branching occurs at axillary buds, following a set pattern.
• Apical bud dominance: Hormones from the active growing point suppress axillary buds.
• If the growing point dies, axillary buds develop, causing branching.
• The same process repeats in lateral stems.

Monopodial Branching

• Typical of plants with strong apical dominance.
• The main stem's growing point retains function for the plant's life.
• Theoretically, plants have unlimited growth in length.
• Often leads to pyramid-shaped trees (e.g., Eucalyptus spp., Pinus spp., Cypressus spp.).

Sympodial Branching

• The main stem's growing point grows linearly for a short period, then dies or changes.
• An axillary bud near the dead growing point takes over.
• Repeats several times (e.g., Vitis vinifera, Rhoicissus tomentosa).
• The main growing-point changes into a tendril, and an axillary bud continues growth.

Dichotomous Branching

• Frequent in algae and mosses, limited in seed plants (e.g., Aloe dichotoma, Aloe bainesii, Selaginella kraussiana).
• The growing point is involved in branching; axillary buds are excluded.
• The apical cell divides into two cells (growing points).
  • Isotomous dichotomy: Division results in two identical cells.
  • Anisotomous dichotomy: Uneven division results in different-sized growing points.

Functions of Normal Stems

• Support of Leaf Display: Position leaves for optimal photosynthesis.
• Support of Flower Display: Position flowers to attract pollinators and support the weight of perching pollinators.
• Support of Fruit Display: Support the plant's fruit load and position fruits for wind dispersal or attachment to animals.
• Conduct Water and Minerals Upward: Xylem transports water and minerals from the soil to the leaves.
• Conduct Nutrients From Leaves: Phloem transports sugars and amino acids from photosynthesis to the rest of the plant.
• Storage of Water, etc.: Underground stems (potatoes) or fleshy stems (succulents) store water.
• Defense: Spines or thorns protect stems from browsers.

Metamorphic Stems

• Modified to perform special functions beyond normal stem functions.
• Often not recognizable as stems due to modified morphology.

Underground Stems as Storage Organs

• Stem Tuber
  • Forms from thickened rhizomes or stolons.
  • Shoots grow from tops or sides; roots from undersides.
  • Short-lived storage and regenerative organ.
  • Develops from a shoot branching off a mature plant.
  • New tubers attach to a parent tuber or form at the end of a hypogeogenous rhizome.
  • In fall, the plant dies except for new offspring stem tubers.
  • Tubers have one dominant bud, which regrows a new shoot in spring.
  • Some plants form smaller tubers or tubercules that act like seeds.
  • Some stem tubers are long-lived (tuberous Begonia); others survive only until plants are fully leafed out.
  • Generally start as enlargements of the hypocotyl section of a seedling.
  • Vertical orientation with vegetative buds on top and fibrous roots on the bottom.
  • Example: Solanum tuberosum (potato).
  • Underground stems form fleshy, swollen, tuber-like structures.
  • Nodes and internodes are closely packed.
  • Leaves are reduced to scaly cataphylls (“eyes” of the potato).
  • Usually colorless, without chlorophyll, and store organic nutrients like starch.
  • Used for vegetative reproduction.
• Corm (Tunicate Bulb)
  • Short, vertical, swollen underground plant stem for storage.
  • Helps plants survive winter or adverse conditions.
  • Consists of one or more internodes with at least one growing point.
  • Protective leaves modified into skins or tunics.
  • Thin tunic leaves are dry, papery, dead petiole sheaths.
  • Internally made of starch-containing parenchyma cells above a circular basal node.
  • Occurs in plants like Watsonia, Gladiolus, and Sparaxis.
  • Contractile roots keep the corm at a constant depth.
• Rhizome (Rootstock)
  • Stem found underground, sending out roots and shoots from its nodes.
  • Often referred to as creeping rootstalks or rootstocks.
  • Usually fleshy.
  • Internodes are short and compact; nodes are well-developed.
  • Colorless, membrane-like cataphylls on underground parts.
  • Ordinary euphylls surround the growing-point.
  • Contractile roots keep the rhizome at a constant depth.
  • Grows orthotropously (vertically) or diageotropously (horizontally).
  • Functions include storage of reserve nutrients and vegetative reproduction.

Stems as Twining and/or Climbing Organs

• Stolon
  • Stems that grow on the soil surface or just below ground.
  • Form new plants at the ends of the nodes.
  • Often called runners.
  • Horizontal shoot that produces new clones from buds at the tip.
  • Occur in plants such as Fragaria ananassa (strawberry), Cynodon dactylon (quick grass), and Pennisetum clandestinum (kikuyu grass).
  • Internodes can die off, forming daughter plants at each node.
  • Main function is vegetative reproduction.
• Climbing Tendril
  • Occur in climbers for support and attachment.
  • A reduced stem forms a tendril that curls around a support.
  • Enables the plant to grow up a support for better sunlight exposure.
  • Two types of branching:
    • Main growing-point forms a tendril; an axillary bud continues growth (Vitis vinifera).
    • Monopodial branching: Tendrils are formed by axillary buds, and the main growing-point retains its identity (Passiflora edulis).
• Clavicle: Attachment Tendrils
  • Occur in some climbing plants, such as Parthenocissus tricuspidata.
  • The terminal growing point is transformed into a reduced stem system.
  • Each growing-point forms a flat, disc-like structure that adheres to the substrate.
• Runner
  • Thin, limp stems growing parallel to the ground, along the soil surface.
  • Cataphylls are absent, normal euphylls are present.
  • No adventitious roots are formed at the nodes.
  • Occur in plants such as Cucurbita pepo (pumpkin) and Citrullus lanatus (watermelon).
• Voluble Stem
  • Thin, limp stems without adventitious roots at the nodes.
  • Internodes are usually long.
  • Stems oscillate upwards and curl around a support.
  • Plants are often referred to as twining plants or twiners.
  • Example: Phaseolus vulgaris (runner bean).

Stems with Leaf Functions

• Main stem or lateral stems modified to assume the structure of a leaf.
• Contain chlorenchyma and can photosynthesize.
• Euphylls are generally reduced to scale-like cataphylls or thorns.
• Leaf-like stems may be thin, flattened, non-fleshy structures, or thick, fleshy structures.
• The stems may have limited or unlimited growth in length.
• Cladode
  • A lateral stem with the structure and function of a leaf, and theoretically unlimited growth in length.
  • Two types exist:
    • Non-fleshy cladode: Flat, leaf-like structures divided into nodes and internodes; euphylls are reduced to scale-like cataphylls; no thorns are present.
    • Fleshy cladode: Thick and fleshy, storing water and reserve nutrients (Cactaceae, Euphorbiacea). Euphylls are reduced to thorns. Easy to divide into nodes and internodes.
• Cladophyll
  • A lateral stem with the structure and function of a leaf, and having limited growth in length.
  • Length is usually limited to one or two internodes.
    • One internode: Euphylls reduced to cataphylls; the axillary bud grows out to form cladophyll (e.g., Asparagus asparagoides).
    • Two internodes: Euphylls are reduced to cataphylls; the axillary bud grows to form a cladophyll with two internodes; at the node of the cladophyll, there is often a cataphyll with an axillary bud or flower.

Stems with Protective Function

• Stem Thorn
  • Formed by lateral branches with limited growth in length.
  • Formed from an axillary bud.
  • The growing point hardens, and the stem changes into a thorn.
  • Normal euphylls are often present on the nodes of the stem thorn.
  • Adaptations to xerophytic (arid) conditions.
  • Offer some protection against browsers.
  • Examples: Dichrostachys cinerea (sickle bush), Maytenus heterophylla (common spike thorn), and Pyracantha angustifolia (firethorn).

Stems with Reproductive Function

• Bulbils
  • Axillary buds give rise to modified, reduced stems which occur in the axil of the euphylls as stem tubers (Anredera cordifolia) and as bulblets (Lilium tigrinum - tiger lily).
• In some cases, similar tubers form on the euphyll (lamina or petiole).
• These tubers originate from adventitious buds.
• In all cases, the bulbs or tubes are separated from the mother plant and give rise to new plants.
• Stem tuber, corm, and rhizome have already been discussed.