L. 7: The Stem

main structural axis of the shoot system

stem?

• supports aerial part of plant

• transports water & minerals

stem main functions?

• produce carbohydrates

• asexual reproduction

• storage

stem specialized functions?

runners/stolon

• thin, horizontal stem that grows above the ground

• primary job is asexual reproduction (cloning w/out seeds)

cladodes and cladophylls

modified stem or branch that becomes flattened or leaf-like and performs photosynthesis because it contains chlorenchyma

succulent stem

a stem specialized for storing large amounts of water

thorns

• linked to primary transport network of the vascular tissues

• appear at the leaf axil where a typical branch would emerge

• protects the plant from herbivores

prickles

• exhibit random growth along the stem, petiole, or surface of leaves

• can easily be snapped off with lateral pressure from a thumb

culm

• the main above-ground stem of grasses and grass-like plants, usually characterized by distinct nodes and internodes.

• supports leaves and reproductive structures and provides pathways for transport.

tendrils

• slender, coiling structure that helps a plant climb by attaching to or wrapping around nearby support.

• anchorage

thigmotropism

what do tendrils exhibit?

bulb

• short underground stem surrounded by thick, fleshy storage leaves. The fleshy leaves store carbohydrates, especially starches and sugars

• The short stem (basal plate) mainly serves as the central structure from which the roots and leaves develop

basal plate

short stem of bulb that serves as the central structure from which the roots and leaves develop

rhizome

horizontal underground stem that can store nutrients and carbohydrates

corm

• short, upright, swollen underground stem that stores carbohydrates and grows vertically. It can also reproduce asexually.

• Unlike a bulb, the main storage tissue of a corm is stem tissue.

tuber

• enlarged ends of rhizomes or stolons specialized for storing food, especially starch

• for storage and can also contribute to vegetative reproduction

node

has meristematic tissue and is the points at which leaves are attached

phyllotaxy

the arrangement of leaves which helps maximize exposure to sunlight and reduce overlapping or shading of leaves

distichous

arranged in exactly two vertical rows on opposite sides of the stem

spiral

positioned singly at each node, winding around the stem in a continuous ascending spiral pattern (1 leaf per node). most common phyllotaxy since no leaves are covered

opposite

two leaves per node. Leaves grow opposite each other

decussate

each pair of leaves grows at a right angle (90°) to the pair directly above or below it, forming a cross shape when viewed from the top

whorled

three or more leaves per node. Leaves form a circle

internode

the stem segments between nodes. Its length determines how far apart the leaves are arranged along the stem

gibberellin

plant hormones that regulate growth processes like stem elongation, seed germination, and flowering

intercalary meristem

meristematic region located at the base of internodes that promote elongation and growth

leaf axil

• the upper angle formed between a leaf stalk (petiole) and the stem from which it grows. It is essentially the "armpit" of the plant.

• Often has axillary buds which can develop into a new branch or flower

axillary bud

a bud that develops in the axil of a leaf and has the potential to grow into a lateral branch or other shoot structure. Some stay dormant because of apical dominance

apical dominance

the phenomenon in which the auxin produced by the shoot apex in the terminal/apical bud suppresses the growth of axillary buds below it since the terminal bud has a lot of auxin. Lateral branches from axillary buds are prevented from being formed by the shoot apex. To stop this occurrence, cut off the terminal bud

terminal/apical bud

• is the primary growing point located at the very tip of a plant stem or twig. It dictates the plant's upward growth and overall shape.

• Most of the growth of a young shoot is concentrated near the apical bud.

• The terminal bud produces auxin, which moves downward through the stem

auxin

indirectly suppresses the growth of axillary buds, so the lateral branches do not grow as strongly while the main shoot continues growing upward

cytokinin

antagonize auxin as it promotes axillary bud growth, while auxin from the terminal bud suppresses axillary bud growth, so their opposing effects help regulate apical dominance

leaf scar

the mark left on a plant's stem after a leaf detaches and falls off. Leaf scars can help identify plant species and determine where leaves were previously attached

lenticels

Small regions in the bark/periderm that allow gas exchange between internal tissues and the atmosphere. They are especially noticeable on woody stems

shoot apical meristem

A region of actively dividing cells at the shoot tip that produces new cells for primary shoot growth

epidermis

The outermost tissue layer of the primary stem. Usually: One cell layer thick and covered by a cuticle

cuticle

a waxy, hydrophobic layer covering the epidermis of aerial plant organs that reduces water loss and pathogen entry

stomata

• microscopic pores in the epidermis that regulate gas exchange. Each stoma is controlled by guard cell

• Called stomata if the plant has not undergone secondary growth yet

trichomes

• Hairlike or other specialized epidermal outgrowths.

• For: Protection from herbivores, Reduction of water loss, Reflection of excessive radiation, and Secretion

ground tissue

primarily composed of parenchyma, but other cell types can occur.

cortex

The ground tissue region located between the epidermis and the vascular tissues in many stems, particularly evident in typical eudicot stems

pith

the central ground-tissue region inside the vascular tissue of many eudicot stems. Usually consists primarily of parenchyma

endodermis

the innermost layer of the cortex that forms a boundary between the cortex and the vascular tissue, helping regulate the movement of water and dissolved substances into the vascular cylinder

pericycle

the layer of cells just inside the endodermis that surrounds the vascular tissue and can give rise to lateral roots

dicot stem

• Distinct cortex and pith

• Vascular bundles arranged in a ring

• Vascular cambium may be present

• Secondary growth commonly occurs in woody eudicots

monocot stem

• Ground tissue is generally not clearly separated into cortex and pith.

• Vascular bundles are scattered throughout the ground tissue.

vascular bundles

A discrete strand of vascular tissue containing xylem and phloem

phloem fiber cap

 A region of fibers associated with the phloem, providing mechanical support. Fibers are sclerenchyma cells with thick, lignified walls.

bundle sheath

A layer or group of cells surrounding a vascular bundle for Mechanical support and Regulation/protection of the vascular bundle. Absent in dicot

air space

a space between cells that allows the movement and exchange of gases within the vascular bundle. Used to be the protoxylem but it broke down because of stress

stele

the central cylinder of vascular and associated tissues within the stem, containing the vascular system and surrounding ground tissues

protostele

vascular tissue forms a solid central core with no central pith. Xylem occupies the center. Phloem surrounds or occurs between portions of the xylem. Considered the simplest stele organization

actinostele

central xylem is star-shaped or has radiating arms. Phloem is located between the xylem arms

atactostele

observed in monocot stems in which the vascular bundles are scattered throughout the ground tissue rather than arranged in a distinct ring

siphinostele

vascular tissue surrounds a central pith. Vascular tissue forms a cylinder around the pith

eustele

observed in dicot stems in which vascular tissue occurs as separate vascular bundles arranged in a ring around a central pith. It is characteristic of dicot/eudicot and gymnosperm stems

vascular cambium

A lateral meristem responsible for secondary growth, which increases the thickness or diameter of the stem

fascicular cambium

The vascular cambium located within the vascular bundles. It produces secondary xylem inward and secondary phloem outward. Differentiates from procambium in roots and stems

interfascicular cambium

Cambium that develops between adjacent vascular bundles and connects with the fascicular cambium to form a continuous cambium ring. In the stem, it differentiates from cortical cells separating vascular bundles

ray initials

Vascular cambium cells that produce radial vascular rays (xylem and phloem rays), which extend inward and outward through the stem. Vascular rays extend horizontally (radially) from the vascular cambium as it transports and stores substances laterally across the stem

fusiform initials

long, tapered, and elongated vascular cambium cells that produce the cells of the axial system, which is oriented mainly vertically/longitudinally along the stem. Includes secondary xylem and secondary phloem

secondary xylem

xylem produced by the vascular cambium toward the inside of the stem. It provides support and conducts water and minerals

softwood

Wood generally produced by gymnosperms because it has no vessels. It is mainly composed of tracheids

hardwood

Wood generally produced by angiosperms as it contains vessels in addition to other xylem cells

sapwood

The younger, outer portion of secondary xylem that is still active in conducting water and minerals

heartwood

The older, inner secondary xylem that is generally no longer involved in water conduction because its conducting pathways become clogged by tyloses (outgrowths from parenchyma cells), tannins, resins, and gums. It primarily provides structural support and is often darker than sapwood because of the clogging materials. Tannins, resins, and gums confer the darker color and deter insects from eating wood because it is bitter. It also has antimicrobial properties

growth rings

Concentric layers of secondary xylem produced during different periods of growth

spring wood

Secondary xylem produced during the period of rapid growth. It generally has larger cells and thinner walls

summer wood

Secondary xylem produced later in the growing period. It generally has smaller cells and thicker walls

annual ring

A complete ring of secondary xylem produced during one growing season, consisting of spring wood and summer wood

dendrochronology

the study of growth rings. Ring patterns can indicate climate changes

secondary phloem

phloem produced by the vascular cambium toward the outside of the stem. It conducts sugars and other organic substances

cork cambium

A lateral meristem that produces the protective tissues of the periderm during secondary growth. Leads to the formation of the outermost protective layer of bark, preventing water loss and protecting the plant from injuries and pathogens

periderm

• The protective tissue that replaces the epidermis as the stem increases in diameter

• Protects the stem from water loss, physical damage, and pathogens

phellem

• periderm layer

• The cork itself; cork cells produced outward by the phellogen. Contains cork cells as they are generally dead at maturity and help protect the stem and reduce water loss with the help of suberin and lenticels. Insulates and protects the internal living tissues of woody stems and roots from environmental damage and water loss

phellogen

• periderm layer

• he cork cambium itself; a lateral meristem that produces phellem outward and phelloderm inward. Leads to procambium.

phelloderm

• periderm layer

•  Living cells and has large intercellular spaces produced inward by the phellogen

bark

The tissues located outside the vascular cambium, including the secondary phloem and periderm