STEMS: FORM & FUNCTION

• Function • External Anatomy • Internal Anatomy

• Specialized Stems

The Plant Body: StemsFUNCTION OF STEMS• Stems support leaves and

branches. • Stems transport water and solutes

between roots and leaves. • Stems in some plants are

photosynthetic.• Stems may 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.

Asclepias - milkweed

Cercis canadensis - redbudThe 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

EXTERNAL ANATOMY

EXTERNAL ANATOMY

STEM APICAL MERISTEM

Apical Dominance Apical dominance refers to the

suppression of growth by hormones produced in the apical meristem.  The Christmas tree pattern of pines indicates strong apical dominance.  Bushy plants have weak apical dominance.  If apical meristem is eaten or destroyed, plants may become bushy. 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.

PRIMARY & SECONDARY GROWTH

Monocotyledonous & Dicotyledonous Flowering Plants

Monocot Stem – cross section

INTERNAL STEM ANATOMY

Typical Stem Cross Section (Dicot Stem)Helianthus annuus-sun flower annual

Pith

A ring of vascular bundles

Epidermis

Cortex

Epidermis - window, reduce

water lossCortex 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° phloemXylem

-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: supportFunctional Phloem: conduct CH2O away from

leafVascular Cambium:

add 2° Xylem and 2° PhloemXylem:

conduct minerals up from soil

outsi

de

to

center

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

If you have ever been to Washington DC you will see how the early architects and artists for governmental buildings were impressed with Greco-Roman architecture and symbols.

On each side of the seat occupied by the presiding officer of the Senate are two Fasces. The Romans had many symbols of Power. One of them was a bundle of sticks lashed together

in a cylinder with a long axe in the center. This is a Fasces! Early Botanists noted that the vascular tissue in stems appeared in discrete bundles which they called Fascicles!!!

Fasces => Fascicles (bundle).  An area of Ground Tissue between the Fascicles was

called Interfascicular! Remnants of the procambium between the primary phloem and xylem was called Intrafascicular.

Information obtained from: http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT311/PrimSec/primarysecondary4.htm

and http://en.wikipedia.org/wiki/Fasces

Fasces• Fasces (from the Latin word fascis, meaning bundle) symbolise summary power and jurisdiction, and/or "strength through unity.“

• The traditional Roman fasces consisted of a bundle of birch rods tied together with a red ribbon as a cylinder around an axe.

• One interpretation of the symbolism suggests that despite the fragility of each independent single rod, as a bundle they exhibit strength.

• See: http://en.wikipedia.org/wiki/Fasces

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 Phloemcamb

ium

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 yearspring of the next year

This tree is Pinus aristata (bristlecone pine).One individual of this species shows more than 5000 growth rings! Inner wood, harvested by boring, was used to validate carbon-14 dating. Imagine the stories that this California tree could tell…perhaps something of migration of Asian peoples down the western coast of North America! They were contemporaries of Pharaohs!

Modified & Specialized Stems

Food Storage StemsPrickly Pear Cactus

Bamboo Shoots Kohlrabi

Food Storage Stems - Sugarcane

Food Storage Stems - Asparagus

Rhizomes

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

• Examples of plants that can produce rhizomes are irises, ferns, and grasses.

Stolons

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

• Examples of plants that can produce stolons are strawberry and airplane plants

Tuber

• Tubers - accumulation of food at the tips of underground stolons

• The "eyes" of a potato are the nodes of a starch-ladened stem

History of the PotatoPotato first domesticated in region of modern day Bolivia and Peru

Failure of the potato crop in 1845-49 led to the Irish Potato Famine

Taters and Spuds

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

Bulb

• Bulbs - large buds with a small stem at the lower end surrounded by numerous fleshy leaves, adventitious roots at base

• Examples include onion, tulip, and lily

Corm• 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

• Examples include crocus and gladiolus.

Cladophylls

•Cladophylls - leaf-like stems; examples include butcher's broom, asparagus

Photosynthetic Stems

• Cacti - stout fleshy stems that are modified for food and water storage and photosynthesis.

Thorns

• Honey locust (modified stem)

• Black Locust (modified leaf stipules)

Tendrils

Grape Tendrils

Sugar Loading of Phloem and Bulk Flow

Sugar Loading of Phloem and Bulk Flow

Transpiration-Cohesion Hypothesis for Water Movement