Plant stems are the backbone of plant structure, supporting leaves, flowers, and fruits. They come in various forms, from soft and flexible herbaceous stems to hard and woody ones, each adapted to different environments and functions.

Stems play crucial roles beyond support. They transport water and nutrients, store food, and even aid in reproduction through specialized structures like rhizomes and stolons. Understanding stem anatomy and function is key to grasping plant biology.

Types of plant stems

• Plant stems are the main structural axes of plants that support leaves, flowers, and fruits
• Stems vary widely in their morphology, anatomy, and growth habits across different plant species
• The types of stems reflect adaptations to different environments and selective pressures

Herbaceous vs woody stems

• Herbaceous stems are soft, flexible, and non-woody (asparagus, basil)
• Woody stems are hard, rigid, and lignified (oak, maple)
  • Woody stems have secondary growth that produces wood and bark
  • Herbaceous stems lack secondary growth and have limited lifespan
• Some plants have semi-woody stems with characteristics of both types (lavender, rosemary)

Climbing, twining and tendrils

• Climbing stems use other structures for support to grow vertically (ivy, grape vines)
  • Stem twiners coil around a support (morning glory, honeysuckle)
  • Leaf twiners use their petioles to coil around a support (clematis)
  • Tendrils are specialized stem or leaf structures that coil around a support (peas, cucumbers)
• Climbing stems allow plants to access more light without investing in thick supportive stems

Rhizomes, tubers and corms

• Rhizomes are horizontal underground stems that store nutrients and produce new shoots (ginger, iris)
• Tubers are swollen underground stem portions that store starch (potato, yam)
  • Tuber eyes are axillary buds that can sprout into new plants
• Corms are short, thick, vertical underground stems that store food (crocus, gladiolus)
  • Corms have distinct nodes and internodes and produce new corms annually

Stolons and runners

• Stolons are horizontal above-ground stems that produce new plants at their nodes (strawberry)
• Runners are specialized stolons that root at their nodes and produce new plants (spider plant)
  • Stolons and runners allow plants to spread vegetatively and colonize new areas
  • They also allow plants to survive adverse conditions and regenerate from damage

External stem anatomy

• The external features of stems reflect their adaptations and functions
• Stems have characteristic structures that can be used for identification and classification

Nodes and internodes

• Nodes are the points on the stem where leaves and buds attach
  • Nodes contain meristematic tissue that can produce new growth
• Internodes are the stem segments between nodes
  • Internode length determines plant height and leaf spacing
• The pattern of nodes and internodes reflects the phyllotaxy (leaf arrangement) of the plant

Buds and leaf scars

• Buds are compact, undeveloped shoots that can grow into branches or flowers
  • Terminal buds are located at the stem tip and control apical dominance
  • Axillary buds are located in the leaf axils and can remain dormant or grow
• Leaf scars are the marks left on the stem after leaves abscise (fall off)
  • Leaf scars have a characteristic shape and arrangement of vascular bundle scars

Lenticels and bark

• Lenticels are small pores in the stem surface that allow gas exchange
  • Lenticels are formed by loosely packed cells that break through the epidermis
• Bark is the protective outer covering of woody stems
  • Bark includes all the tissues outside the vascular cambium (cork, cork cambium, phloem)
  • Bark texture and appearance varies among species and can be used for identification

Stem modifications

• Some stems are modified for specialized functions
  • Thorns are sharp-pointed modified stems that protect the plant from herbivory (hawthorn, honey locust)
  • Cladodes are flattened photosynthetic stems that replace leaves (asparagus, cactus pads)
  • Bulbs are underground storage stems surrounded by fleshy leaves (onion, tulip)

Internal stem anatomy

• The internal anatomy of stems reflects their primary functions of support and transport
• Stems have a characteristic arrangement of tissues that can be observed in cross-section

Epidermis and cuticle

• The epidermis is the outermost cell layer that protects the stem from water loss and damage
  • Epidermal cells are tightly packed and have a thick cuticle (waxy coating)
  • The epidermis may have trichomes (hairs) or other specialized cells
• The cuticle is a waxy layer secreted by the epidermis that prevents water loss
  • The cuticle also protects against UV radiation and pathogen attack

Cortex and pith

• The cortex is the region between the epidermis and the vascular tissue
  • The cortex includes parenchyma cells for storage and collenchyma cells for support
• The pith is the central region of the stem inside the vascular tissue
  • The pith is composed of parenchyma cells that store nutrients
  • The pith may disintegrate in some stems, leaving a hollow center

Vascular bundles

• Vascular bundles are the conducting tissues that transport water and nutrients throughout the plant
  • Vascular bundles include xylem (water-conducting) and phloem (sugar-conducting) tissues
  • Vascular bundles are arranged in a ring in dicot stems and scattered in monocot stems
• The arrangement and number of vascular bundles is characteristic of different plant groups

Primary vs secondary growth

• Primary growth is the elongation of the stem from the apical meristem
  • Primary growth produces primary tissues (epidermis, cortex, pith, primary xylem and phloem)
• Secondary growth is the thickening of the stem from the lateral meristems (vascular cambium and cork cambium)
  • Secondary growth produces secondary tissues (secondary xylem, secondary phloem, cork)
• Herbaceous stems have only primary growth, while woody stems have both primary and secondary growth

Stem tissues and cells

• Stems are composed of several types of tissues and cells with specialized functions
• The major stem tissues include parenchyma, collenchyma, sclerenchyma, and vascular tissues

Parenchyma and collenchyma

• Parenchyma cells are the most common plant cells and have thin primary walls
  • Parenchyma cells perform various functions such as storage, photosynthesis, and wound healing
• Collenchyma cells have unevenly thickened primary walls and provide structural support
  • Collenchyma cells are elongated and grouped in strands or cylinders
  • Collenchyma allows for flexible support in growing regions of the stem

Sclerenchyma and fibers

• Sclerenchyma cells have thick secondary walls strengthened with lignin
  • Sclerenchyma provides rigid support and is common in woody stems
• Fibers are a type of sclerenchyma cell that are long, slender, and tapered
  • Fibers occur in bundles and provide tensile strength to the stem
  • Fibers are commercially important for textiles and paper production (flax, hemp, jute)

Tracheids and vessel elements

• Tracheids and vessel elements are the water-conducting cells of the xylem
  • Tracheids are elongated cells with tapered ends and pits in their walls for water transport
  • Vessel elements are shorter, wider cells that stack end-to-end to form continuous vessels
• Vessel elements have perforated end walls that allow for efficient water transport
  • Vessel elements are more common in angiosperms, while tracheids are more common in gymnosperms

Sieve tube elements and companion cells

• Sieve tube elements and companion cells are the sugar-conducting cells of the phloem
  • Sieve tube elements are elongated cells that stack end-to-end to form sieve tubes
  • Sieve tube elements have perforated end walls (sieve plates) that allow for sugar transport
• Companion cells are specialized parenchyma cells that are associated with sieve tube elements
  • Companion cells have many mitochondria and ribosomes to provide energy and proteins for the sieve tube elements

Apical meristems

• Apical meristems are the growing points of the stem that produce new cells and tissues
• Apical meristems are responsible for primary growth and the formation of lateral organs

Apical dome and leaf primordia

• The apical dome is the rounded tip of the apical meristem that contains the stem cells
  • The stem cells of the apical dome divide to produce new cells for the stem and leaves
• Leaf primordia are the bumps on the sides of the apical dome that develop into leaves
  • Leaf primordia are formed in a specific pattern (phyllotaxy) that optimizes light capture

Primary vs secondary meristems

• Primary meristems are the apical meristems that produce primary growth
  • Primary meristems include the apical dome and the primary meristems of the root and leaf
• Secondary meristems are the lateral meristems that produce secondary growth
  • Secondary meristems include the vascular cambium and cork cambium
  • Secondary meristems are responsible for stem thickening and wood production

Lateral vs adventitious buds

• Lateral buds are the buds that form in the axils of leaves
  • Lateral buds contain apical meristems that can grow into branches or flowers
• Adventitious buds are buds that form in unusual locations, such as on roots or leaves
  • Adventitious buds often develop in response to injury or environmental stress
  • Adventitious buds can be used for vegetative propagation (cuttings, grafting)

Axillary bud development

• Axillary buds develop from meristematic cells in the leaf axil
  • Axillary buds may remain dormant or grow into branches, depending on environmental cues and hormonal signals
• The development of axillary buds is regulated by the hormone auxin
  • Auxin is produced by the apical meristem and inhibits the growth of axillary buds (apical dominance)
  • Removal of the apical meristem (pruning, herbivory) releases the axillary buds from inhibition

Primary stem growth

• Primary growth is the elongation of the stem from cell division and expansion in the apical meristem
• Primary growth produces the primary tissues of the stem (epidermis, cortex, pith, primary xylem and phloem)

Cell division and elongation

• Cell division in the apical meristem produces new cells that elongate and differentiate into primary tissues
  • Cell division occurs in the apical dome and the primary meristems (protoderm, ground meristem, procambium)
• Cell elongation occurs behind the apical meristem and is driven by water uptake and cell wall loosening
  • Cell elongation is regulated by the hormones auxin and gibberellin

Protoderm, ground meristem and procambium

• The protoderm is the outer layer of the apical meristem that gives rise to the epidermis
  • The protoderm also produces the trichomes (hairs) and stomata (pores) of the epidermis
• The ground meristem is the middle layer of the apical meristem that gives rise to the cortex and pith
  • The ground meristem differentiates into parenchyma and collenchyma cells
• The procambium is the inner layer of the apical meristem that gives rise to the primary vascular tissues
  • The procambium differentiates into primary xylem and primary phloem

Primary xylem vs primary phloem

• Primary xylem is the first-formed xylem tissue that conducts water and minerals from the roots to the leaves
  • Primary xylem includes protoxylem (first-formed) and metaxylem (later-formed)
  • Protoxylem has narrow tracheids with annular or spiral thickenings for stretching during growth
• Primary phloem is the first-formed phloem tissue that conducts sugars from the leaves to the rest of the plant
  • Primary phloem includes protophloem (first-formed) and metaphloem (later-formed)
  • Protophloem is crushed and non-functional in mature stems

Vascular bundle arrangements

• Vascular bundles are the conducting tissues of the stem that include primary xylem and primary phloem
  • Vascular bundles are arranged in a ring in dicot stems and scattered in monocot stems
• The arrangement of vascular bundles is determined by the procambium in the apical meristem
  • In dicot stems, the procambium forms a continuous ring that separates the cortex from the pith
  • In monocot stems, the procambium forms scattered strands that are embedded in the ground tissue

Secondary stem growth

• Secondary growth is the thickening of the stem from cell division and expansion in the lateral meristems
• Secondary growth produces the secondary tissues of the stem (secondary xylem, secondary phloem, cork)

Vascular cambium and cork cambium

• The vascular cambium is a lateral meristem that produces secondary xylem and secondary phloem
  • The vascular cambium forms a continuous ring around the stem and is responsible for wood production
  • The vascular cambium consists of fusiform initials (elongated cells) and ray initials (short cells)
• The cork cambium is a lateral meristem that produces the cork (bark) of the stem
  • The cork cambium forms a ring outside the phloem and produces cork cells to the outside and phelloderm to the inside
  • The cork cambium is responsible for the thickening and protection of the stem

Secondary xylem vs secondary phloem

• Secondary xylem is the wood of the stem that is produced by the vascular cambium to the inside
  • Secondary xylem includes tracheids, vessel elements, fibers, and parenchyma cells
  • Secondary xylem provides structural support and water transport for the plant
• Secondary phloem is the inner bark of the stem that is produced by the vascular cambium to the outside
  • Secondary phloem includes sieve tube elements, companion cells, fibers, and parenchyma cells
  • Secondary phloem provides sugar transport and storage for the plant

Annual rings and wood grain

• Annual rings are the concentric rings of secondary xylem that are produced each year in temperate trees
  • Annual rings are formed by the alternation of earlywood (spring wood) and latewood (summer wood)
  • Earlywood has larger, thinner-walled cells for rapid water transport during the growing season
  • Latewood has smaller, thicker-walled cells for structural support during the dormant season
• Wood grain is the pattern of annual rings and wood fibers that is visible in a stem cross-section
  • Wood grain varies among species and can be used for identification and wood quality assessment

Heartwood vs sapwood

• Heartwood is the inner, non-living portion of the secondary xylem that no longer conducts water
  • Heartwood is often darker in color and more durable than sapwood due to the accumulation of resins, tannins, and other compounds
  • Heartwood provides structural support and resistance to decay for the tree
• Sapwood is the outer, living portion of the secondary xylem that actively conducts water and minerals
  • Sapwood is usually lighter in color and more permeable than heartwood
  • Sapwood is the main water-conducting tissue of the stem and is essential for tree survival

Stem functions

• Stems perform several essential functions for the plant, including support, transport, storage, and reproduction
• The functions of stems are related to their anatomy and morphology

Support and orientation

• Stems provide mechanical support for the leaves, flowers, and fruits of the plant
  • Stems keep the leaves oriented towards the sun for optimal photosynthesis
  • Stems keep the flowers and fruits accessible to pollinators and dispersers
• Stems resist bending and compression forces through the production of specialized support tissues
  • Collenchyma and sclerenchyma cells have thickened walls for structural support
  • Wood (secondary xylem) provides rigid support for trees and shrubs

Transport of water and nutrients

• Stems are the main transport pathway for water, minerals, and sugars throughout the plant
  • Xylem tissue conducts water and minerals from the roots to the leaves
  • Phloem tissue conducts sugars and other organic compounds from the leaves to the rest of the plant
• The arrangement and structure of vascular tissues in the stem facilitates long-distance transport
  • The branching network of xylem and phloem allows for efficient distribution of resources
  • The specialized cells of xylem (tracheids, vessel elements) and phloem (sieve tube elements) are adapted for transport

Food storage and perennation

• Stems can store water, carbohydrates, and other nutrients for later use by the plant
  • Succulent stems store water in enlarged parenchyma cells to survive drought (cactus, euphorbia)
  • Tubers, corms, and rhizomes store starch in underground stems for overwintering and regrowth (potato, iris, ginger)
• Stems can also store defensive compounds to deter herbivores and pathogens
  • Resin ducts in conifer stems produce terpenes and other compounds that protect against insects an