Prepared by:Pooja KhanparaAPIP, Jamnagar

STEMS

STEMS• Origin• Functions • External Anatomy • Internal Anatomy • Specialized

Stems

Origin

First stem of a plant develops from part of a seed embryo called epicotyl, w/c is a continuation of the hypocotyl

The Plant Body: StemsFUNCTIONS OF STEMS• Produces & support appendages

of plant (leaves, flowers, fruits) • transport water and solutes

between roots and leaves. • Stems in some plants are

photosynthetic.• Produce & store materials

necessary for life (e.g., water, starch, sugar).

• In some plants, stems have become adapted for specialized functions.

Stems support a display of leaves.

Stems orient the leaves toward the light with minimal overlap among the leaves.

The stem supports a display of flowers

The stem does photosynthesis…

and stores water.

Opuntia-prickly pear

This stem does photosynthesis, stores water, but also produces a defense chemical: mescaline…a hallucinogen.

Lophophora williamsii - peyote

Two Types of Aerial Stems

Herbaceous Stems Woody Stems

Soft & greenLittle growth in diameterTissues chiefly primaryChiefly annualCovered by epidermisBuds mostly naked

Tough & not greenConsiderable growth in

diameterTissues chiefly secondaryChiefly perennialCovered by corky barkBuds chiefly covered by

scales

Herbaceous Stem woody stem

EXTERNAL ANATOMY

STEM APICAL MERISTEM

PRIMARY & SECONDARY GROWTH

Apical DominanceApical dominance refers

to the suppression of growth by hormones produced in the apical meristem.

Lateral branch growth are inhibited near the shoot apex, but less so farther from the tip.

Apical dominance is disrupted in some plants by removing the shoot tip, causing the plant to become bushy(thick).

INTERNAL STEM ANATOMY

Monocotyledonous & Dicotyledonous Flowering Plants

Monocot Stem – cross section

Typical Stem Cross Section (Dicot Stem)

Helianthus annuus-sun flower annual

Pith

A ring of vascular bundles

Epidermis

Cortex

Epidermis - window, reduce water loss

Cortex Collenchyma- extensible support

Cortex Parenchyma- photosynthesis, etc.

Fibers- rigid support

Functional Phloem- conduct sugars etc. away

from leaf to rest of plantVascular Cambium- adds 2° xylem and 2° phloem

Xylem-conduct water and minerals

up from soilPith

-water storage, defense?

VIP Stem: Provide both name and function labels:

outside

to center

Epidermis: reduce evaporation, gas exchange

Cortex: photosynthesis, collenchyma support

Vascular Bundles: conduction

Pith: water storage? defense? disintegrate?

Vascular Bundle:

Phloem Fibers: support

Functional Phloem: conduct CH2O away from leaf

Vascular Cambium: add 2° Xylem and 2° Phloem

Xylem: conduct minerals up from soil

outs

ide

to c

ente

r

Vitis vinifera - grape

Vitis vinifera - grape

Notice how the vascular cambia of adjacent vascular bundles line up side by side.

Notice that cambium tissue differentiates between the bundles, connecting the cambia together.

Remnants of the procambium:Intrafasicular cambium Interfasicular cambium

Vitis vinifera - grapeThe vascular cambium makes 2° tissues:

2° phloem2° xylem

Basswood – 1 & 2 years old

Three years of Secondary Growth

Tilia - basswood

SecondaryXylem

Secondary Phloemcambium

A cork cambium differentiates and produces a periderm.

Epidermis

Cork Cambium

Phelloderm

cutin

suberin

Cork Cells

Over time, the epidermis dies.

The cork cells build up to for a thick layer for the bark of a tree. We use this to make stoppers for wine bottles and so on.

When suberin is fully developed, the cortex cells will eventually be in the dark. So these chloroplasts will lose their function!

Bark =

epidermis + periderm +

cortex + phloem + vascular

cambium

Wood =

secondary xylem only!

Pith =

a small percentage of tree diameter

at maturity

Anatomy of a Woody Stem

Sequoia sempervirens - giant sequoia

The trees pictured below have long lost their epidermis on the woody portion of the stem

The study of the growth rings in wood: Dendrochronology

Each year the cambium produces a layer of secondary xylem and a layer of secondary phloem.

This photo shows secondary xylem from parts of three years in Pinus strobus (white pine).

mid-summer of one year

fall of that yearwinter of that year

spring of the next year

StemsStems—the axes of plants—consist of nodes (where leaves and axillary buds are produced) separated by internodes.

Node - region of the stem where the leaf and bud are borne.

Internode - the part of the stem between two adjacent nodes.

Herbaceous - not woody; dying down at the end of the growing season.

Woody - hard in texture, containing secondary xylem, and persisting more than one growing season.

Acaulescent - having an inconspicuous stem.

Caulescent - having a distinct stem.

Modified & Specialized Stems

Bulbs• Bulbs - large buds

with a small stem at the lower end surrounded by numerous fleshy leaves that store nutrients; adventitious roots at base

• Eg. onion, tulip, hyacinth, daffodil and lily

Corms• Corms - resemble

bulbs but composed entirely of stem tissue surrounded by a few papery scale like leaves, food storage organs with adventitious roots at the base of corms

• Eg. crocus and gladiolus.

Rhizomes

• Rhizomes - horizontal stems that grow below the ground with adventitious roots

• Eg. irises, ferns, and grasses.

Cladophylls

•Cladophylls or cladodes - leaf-like stems modified for photosynthesis

•Eg. butcher's broom, asparagus, orchids (eg. Epidendrum)

Succulent Stems

• Succulent stems - stout fleshy stems that are modified for water and food storage

Eg. cacti

Thorns- for protection from grazing animals

• Bougainvillea

• Honey locust (modified stem)

• Black Locust (modified leaf stipules)

Food Storage StemsPrickly Pear Cactus

Bamboo Shoots Kohlrabi

Food Storage Stems - Sugarcane

Tubers

• Tubers – swollen regions of stems that store food for subsequent growth

• The "eyes" of a potato (irish potatoes Solanum tuberosum) are the nodes of a starch-ladened stem

Stolons

• Stolons or runners - horizontal stem that grow above the ground with long internodes

Eg. Bermuda grass

(Cynodon dactylon)

Spider plant (Chlorophytum)

Fern (Nephrolepis)

Bermuda grass (Cynodon dactylon).

Rosette• Rosette - stem with short internodes and leaves

attached at nodes

Wild Radish – Rosette & BoltWild Radish – Rosette & Bolt

YEAR ONEYEAR ONE YEAR ONEYEAR ONE

A FLOWERING ANNUALA FLOWERING ANNUAL

Common Mullen – Rosette & BoltCommon Mullen – Rosette & Bolt

YEAR ONEYEAR ONE

YEAR TWOYEAR TWO

A FLOWERING BIENNIALA FLOWERING BIENNIAL

Buds

Buds are short embryonic stems. In angiosperms they are found at the nodes, in the leaf axil (the angle formed by the stem and the petiole of the leaf).

Axillary bud - a bud located in the leaf axil.Terminal bud - a bud at the apex of a stem.

PHYSIOLOGY OF STEMS

• Conduction of Materials by Xylem1) Root pressure – powered by transpiration of water from the leaves

2) Transpiration pull and water cohesion – water is pulled up from the roots due to adhesion of water to the xylem walls & tension generated by the water-potential gradient bet. leaves & xylem

Other contributing factors:3) Atmospheric pressure

4) Action of Living cells 5) Imbibition in cell walls of xylem 6) Capillary attraction

PHYSIOLOGY OF STEMS

• Conduction of Materials by Phloem - nutrient-rich fluid in the phloem moves from areas of high

solute concentration & water pressure to areas of low solute concentration & water pressure

• Hypotheses of phloem function are:1) cytoplasmic streaming2) movement through interface3) pressure flow or mass flow

Transpiration-Cohesion Hypothesis for Water Movement

Sugar Loading of Phloem and Bulk Flow

Sugar Loading of Phloem and Bulk Flow