Plant Stem Structure and Function

Stem Functions

• Stems support leaves and reproductive structures.
• Stems conduct water, dissolved minerals, and carbohydrates.
• Stems produce new living tissues at apical meristems for primary growth and at lateral meristems for secondary growth.

Stem Variations

• Stems can be flexible and bendable, like morning glories, for trailing and twining.
• Stems can be wide and prominent, like baobab trees, to store water and starch and adapt to the environment.

Plant Stem Types

• Herbaceous stems are flexible, exhibit only primary growth, and are found in both dicots and monocots; monocots typically lack secondary growth.
• Woody stems have sturdy trunks and wood, display both primary and secondary growth, and secondary growth is prevalent in most seed plants, though the pattern differs; only dicots have this type of stem.

Primary Growth

• Primary growth occurs at apical meristems.

Tissues in Herbaceous Stems

• Epidermis, a protective outer layer covered by a water-conserving cuticle.
• Vascular tissues, with xylem conducting water and dissolved minerals, and phloem conducting dissolved sugar.
• Storage tissues, including the cortex and pith, which are ground tissue.

Herbaceous Dicot stems

• Have vascular bundles arranged in a circle with evenly distributed xylem and phloem.
• They also posses a distinct cortex and pith.

Herbaceous Monocot Stems

• Have scattered vascular bundles, unevenly distributed xylem and phloem, and ground tissue instead of distinct cortex and pith.

Stem and Root Differences

• Stems have nodes, internodes, leaves, and buds, but lack a pericycle and rarely have an epidermis.
• Roots have caps and root hairs; herbaceous roots possess an endodermis and pericycle.
• A node is an area on a stem where one or more leaves are attached, and an Internode is the stem area between two successive nodes.
• A Bud is an undeveloped shoot containing an embryonic meristem, which may be terminal (at the tip) or axillary (on the stem's side).

General Differences Between Herbaceous Eudicot Roots and Stems

• Roots lack nodes or internodes, leaves, and buds, are non-photosynthetic, and lack pith, while stems posses all of these characteristics
• Roots lack a cuticle, have a root cap, and root hairs, while stems have a cuticle, no root cap, and trichomes.
• Roots have a pericycle and endodermis, while stems lack a pericycle and rarely have an endodermis.
• Roots form branches internally from the pericycle, while stems form branches externally from lateral buds.

Secondary Growth Characteristics

• Occurs in woody eudicots and confiers; it emerges from the vascular cambium, between primary xylem and pholem
• Vascular cambium is a lateral meristem that creates secondary xylem (wood) to the inside and secondary pholem to the outside
• It is not initially a solid cylinder of cells, but becomes continuous as secondary cells are produced.
• Certain parenchyma cells divide as the cambium expands. This connects cells and creates a complete ring of vascular cambium

Cork Cambium

• A lateral meristem generates cork parenchyma inside and cork cells outside, forming the outer bark of woody plants.
• It arises near the stem's surface.
• Can be a continuous cylinder of dividing cells or overlapping arcs of meristematic cells that form parenchyma cells in successively deeper layers of the cortex, eventually secondary phloem.

Lenticles

• Pores facilitate gas exchange of oxygen, carbon dioxide, and water vapor.

Tree-Ring Dating

• Determines a tree's age by counting annual rings, with each ring representing a year.

Wood

• Secondary xylem is present, along with everything on the interior side of the vascular cambium.

Bark

• Consists of everything exterior to the vascular cambium, including the secondary phloem and the periderm (cork cambium + cork).

Bark Variation

• Bark adapts to the environment for survival and can be deeply fissured, shaggy/rough, scaly, or peeling.

Stem Modifications

• Rhizomes are horizontal underground stems that serve as storage organs and a means of sexual reproduction.
• Tubers are thickened ends of rhizomes that are fleshy, enlarged, and used for food storage.
• Bulbs are round, fleshy underground buds with short stems and fleshy leaves.
• Corms are short, thickened underground stems specialized for food storage.
• Stolons are aerial horizontal stems with long internodes that often form buds that develop into separate plants, a means of asexual reproduction.
• Tendrils are slender, threadlike appendages of climbing plants that often grow in a spiral and stretch out, twining around any suitable support.
• Cladophylls are shoot systems where leaves do not develop; the stem becomes flattened and assumes photosynthetic functions.

Economic Importance of Stems

• Important source of food, medicine, paper, resin, fuel, and lumber.

Leaf Function

• Involved in photosynethesis, gaseous exchange, takes in O2 and releases CO2 during respiration
• Takes in CO2 and releases O2 during photosynthesis.
• Transpiration occurs, through which water vapor cab be lost from the surface of the leaf - this reduces water loss and the stomata closes to retain liquid
• Photosynthesis is a biological process where leaves capture light energy and converts it to chemical energy

Plant Adaptations - Leafs

• Desert plants (xerophytes) have thick and reduced leaves with stomata that open only at night and a thick waxy layer.
• Floating leaves have stomates located on the upper epidermis only.
• Submerged leaves have no stomates found.
• Conifers have waxy needle leaves, are evergreen, and possess a thick, waxy cuticle and sunken stomata.

Leaf Morphology

• Simple leaves have a single blade per petiole, while compound leaves are divided into smaller leaflets originating from a single axillary leave.
• Pinnately compound leaves have leaflets in pairs along an extension of the petiole, while palmately compound leaves have all leaflets attached at some point at the end of the petiole.

Leaf Attachments

• Connate perfoliate, perfoliate, clasing, sessile, petiolate and sheathing are all forms of leaf attachment

Venation

• Parallel venation has veins running parallel with one another (for monocots).
• Netted venation includes pinnately veined leaves (one primary vein within an enlarged midrib) and palmately veined leaves (several primary veins fan out from the blade's base).

Phyllotaxy

• Alternate (leaves attached alternately to a spiral along a stem with one leaf per node).
• Opposite (two leaves arise from the opposite node).
• Whorled (three or more leaves occur at the node).

Leaf Shape/Outline

• Linear (thin). -Lanceolate (spear shaped). -Ovate (oval shaped). -Cordate (heart shaped).

Leaf Base

• Cuneate. -Rounded. -Truncate. -Cordate.

Leaf Margin

• Entire. -Undulate. -Finely serrate. -Coarsely serrate. -Double serrate. -Crenate.
• Lobed.

Leaf Apex

• Acuminate. -Cuteapiculate. -Aristate. -aristate. -caudate.

Leaf Anatomy - Dicots

• Cuticle covers the epidermis of aerial parts, enabling plants to survive in dry conditions by minimizing water loss and pathogen entry, made of lipid and hydrocarbon polymers, hydrophobic, waterproof.
• The transparent epidermis allows light penetration into the mesophyll.
• Ordinary epidermal cells vary in shape, often covered by a cuticle.
• Trichomes/hair cells reduce water loss, protect from predators, and vary in shape and structure.
• Guard cells control the opening and closing of stomata.
• Palisade mesophyll, vein/vascular bundle, bundle sheath, xylem and pholem.
• Spongy mesophyll with air space is needed for CO2 flow and photosynthesis.
• Lower epidermis: stomata control water movement; can be found in the upper epidermis; passageway for atmospheric gases.
• Guard cells are always next to stomata.

Factors Affecting Opening/Closing of Stomata

• Light makes stomata open, darkness makes them close.
• Stomata close to reduce water loss if there is low water/moisture availability
• Higher temperatures makes them close.

CO2 and Stomatal Density

• High CO2 results in low density, and low CO2 results in high density.

Monocot Leaf Characteristics

• Usually narrow.
• Wraps around the stem in a sheath.
• exhibits parallel venation.
• Ligule.
• Auricle (collar extension).
• Appendage surrounds the stem at the junction of the blade and sheath.
• The mesophyll layer is not well differentiated into palisade and spongy layers.

Eudicot Leaf Characteristics

• Usually broad and flattened blade.
• Netted venation.

Bulliform Cells

• Large, thin-walled cells located on the upper epidermis and both sides of the midvein.
• Reduces water loss and May help leaf roll or fold inward during drought .

Modified Leaves

• Cotyledons/seed leaves and first true leaves
• Colored bracts are petal-like and attract pollinators.
• Insect-trapping Leaves that live under nutrient-poor conditions they digest insect bodies to obtain nitrogen and other essential nutrients
• Tendrils allow plants to cling to objects and have reduced leaves.
• Spines are reduced leaves that discourage herbivory and reduce water loss, serving as spine, Stems serves as the primary organ of photosynthesis
• Storage Leaves that retain water in large vacuoles
• Absorptive Leaves capture aquatic gasses
• Reproductive Leaves
• Window Leaves are buried in soil with the transparent part exposed to light to reduces loss of water in arid environments
• Flower pot Leaves: Catch water and debris for nutrient collection

Leaf Abscission

• Most woody plants with broad leaves shed them in fall in temperate climates, helping them survive in low temperatures that involves physiological and anatomical changes.

Abscission Process

• As autumn approaches, plants reabsorb sugar, and essential minerals are transported out of leaves.
• Chlorophyll is broken down.
• Red water soluble pigments are synthesized and stored in vacuoles of leaf cells, turning leaves red/orange.
• A protective cork layer develops on the stem side of the abscission zone, the area where the leaf petiole detaches from the stem, composed of tin-walled parenchyma cells.
• Enzymes dissolve the middle lamella in the abscission zone, this "cement" holds primary cell walls of adjacent cells together.
• After leaf detaches, a protective layer of cork seals off the area, forming a leaf scar.

Economic Importance of Leaves

• Food, spices, and drinks, such as cabbage, lettuce, spinach, and celery.
• Dyes.
• Fibers, such as abaca and pineapple.
• Fuel is derived from flammable resin.
• Drugs.