E1 Stems

Where do stems originate from?

Epicotyl area of embryo

Stems

epicotyl + Plumule

Epicotyl

stem above cotyledon attachment

Plumule

tip above embryo that develops into shoot

Roots

hypocotyl + radicle

Hypocotyl

stem below cotyledon attachment

• 1st part to come out

Radicle

tip of embryo that develops into the root

Development of stems can be atributed to:

• apical meristem

• 3 primary meristems

• leaf primordium

• bud primordium

The 3 primary meristems are:

• protoderm

• procambium

• ground meristem

protoderm

gives origin to epidermis

procambium

produces primary xylem and primary phloem

Ground meristem

produces cortex and pith

5 kinds of primary tissues

• epidermis

• primary xylem

• primary phloem

• cortex

• pith

Stem morphology

• stem/branches

• node

• internode

• leaf blade/veins/petiole

• sessile leaves

• apical buds

• axillary buds

Internode

stem region between 2 nodes

Node

point of attachment

petiole

thin region that attaches to the stem

Axillary buds

area between base of leaf and stem

• give rise to axillary shoot

Sessile leaves

attach directly to the plant stem without a supporting stalk, known as a petiole

Apical buds

primary growing points located at the tip of a plant stem, responsible for vertical growth, elongation, and leaf/flower production

Herbaceous stems

primary growth

Woody

secondary growth

annuals

• most monocots and many dicots

  • finish life cycle within a year

The tissues of herbaceous stems are:

• epidermis (cuticle, trichomes, stomata)

• cortex and pith

• vascular bundles

trichomes

hairs on epidermal surface that help to reduce transpiration and allow for gas exchange

Vascular bundles

“steles”

Eustele

• produces bark and wood

• herbaceous dicots have discrete vascular bundles arranged in a cylinder

  • Vascular cambium and cork cambium are likely to be present

Atactostele

• monocots

• vasc. bundles scattered throughout stem and disorderly

• no distinction between cortex and pith

  • produce no wood and bark

Siphonostele

• not solenostele

• vascular tissue is in continuous ring

In the early stages of development, the primary tissues of young herbaceous dicots, woody dicots, and cone-bearing gymnosperms are

all arranged in similar fashion

• looks like eustele

In woody plants, differences begin to appear..?

as soon as vascular cambium and the cork cambium develop

Bark, secondary phloem, and wood (secondary xylem) increase the plants

girth

• making it able to be strudier

Wood

secondary xylem

Secondary tissues

• annual rings

• wood (heartwood and sapwood)

• Bark (Periderm and Lenticel)

• Knots on wood

Annual rings

• contain early and late wood

Early wood

forms in spring

Late wood

summer

Wood

consists of secondary xylem produced by vascular cambium

• heartwood and sapwood

heart wood

• oldest secondary xylem no longer conducts water

  • stores various compounds and appears darker

sapwood

• surrounds heartwood

• lighter in color

• conducting xylem recently made

Bark

consists of all tissues layers external to vascular cambium

• decreases water loss

  • periderm and lenticel

Knots on wood

• where branches grew from the trunk

  • weakest portion of wood

Periderm

outer layer of the bark

Lenticels

raised, localized areas of loosely packed cells

• used for gas exchange

Laticifers

• latex-secreting cells originating from meristems

  • rubber, chewing gum, and some drugs

Girdling

• trees can be killed via girdling

  • vascular cambium is removed

Dendrochronology

study of growth rings and interpreting the history

• increment borer

Can plants such as monocots experience abnormal secondary growth?

yes

• no vascular cambium/vascular bundles

Example of abnormal primary growth

palm trees grow by primary thickening

Modified stems adapt plants for:

different factors in the environment

• Rhizome

• Corn

• Stolon

• Bulb

  • all are below ground

Rhizome

• fleshy, creeping underground stems

• function in propagation

  • ginger

Corms

• short, thickened stem, usually covered with papery leaves

• storage underground

  • gladiolus

Stolons

• slender stems that grow horizontally

Bulbs

• large buds surrounded by numerous fleshy leaves with a small stem at lower end

  • stores food

  • onions, lilies, hyacinths, tulips

Stem: aboveground modifications

• cladophylls

• thorn

• spine

• prickles

• tendril

cladophylls

flattened, leaf-like stems

• greenbriers, some orchids, prickly pear cactus

thorn

• arise from an entire stem

  • modified stems/branches

Spine

• modified leaves

  • cacti have both thorns and spines

Prickles

epidermal outgrowths of the bark

Tendril

slender, sensitive structures of many climbing plants that support the plant

• tendrils are modified stems, leaves, or leaf parts or roots