Stems

Chapter 6

STEMS: FORM & FUNCTION

• Function

• External Anatomy

• Internal Anatomy

• Specialized Stems

The Plant Body: Stems

FUNCTION 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.

EXTERNAL ANATOMY

EXTERNAL ANATOMY

STEM APICAL MERISTEM

Apical Dominance

v 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.

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

v 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

v 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.

v On each side of the seat occupied by the presiding officer of the Senate are two Fasces. The Romans had many symbols of Power.

v 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!!!

v Fasces => Fascicles (bundle). 

v An area of Ground Tissue between the Fascicles was called Interfascicular! Remnants of the procambium between the primary phloem and xylem was called Intrafascicular.

v 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

Basswood – 1 & 2 years old

Anatomy of a Woody Stem

• The Baobab tree in Africa is an amazing plant.It is known for its water storage capacity in it trunk.It can store 1000 liters to 120000 liters of water in its swollen trunk.

• Dendrochronology is the science of calculating a tree’s age by its rings.Rings in the stem of a tree can tell the age of a tree.

• Tree rings not only tell us age of the tree but they can also tell us about environmental events, including volcanic eruptions

Modified & Specialized Stems

Food Storage Stems

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 Potato

Taters and Spuds

Rosette

• Rosette - stem with short internodes and leaves attached at nodes

Wild Radish – Rosette & Bolt

Common Mullen – Rosette & Bolt

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

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

Sugar Loading of Phloem and Bulk Flow

Sugar Loading of Phloem and Bulk Flow

Transpiration-Cohesion Hypothesis for Water Movement

Outline

• External Form of a Woody Twig

• Stem Origin and Development

• Stem Tissue Patterns

• Herbaceous Dicotyledonous Stems

• Woody Dicotyledonous Stems

• Monocotyledonous Stems

• Specialized Stems

• Wood and Its Uses

External Form of A Woody Twig

• Woody twig consists of an axis with attached leaves.

– Alternately or Oppositely arranged.

• Leaves attached at a node.

– Stem region between nodes is an internode.

» Leaf has a flattened blade and is usually attached to the twig by a petiole.

External Form of A Woody Twig

• Axil - Angle between a petiole and the stem.

– Axillary Bud located in axil.

• Terminal Bud often found at twig tip.

• Stipules - Paired appendages at the base of a leaf. Often remain throughout leaf life span.

– Deciduous trees and shrubs have dormant axillary buds with leaf scars left after leaves fall.

• Bundle scars mark food and water conducting tissue.

Origin and Development of Stems

• Apical meristem is dormant before the beginning of the growing season.

– Protected by bud scales and by primordia.

• When a bud begins to expand, apical meristem cells undergo mitosis and three primary meristems develop.

Primary Meristems

– Protoderm - Gives rise to epidermis.

– Procambium - Produces primary xylem and primary phloem cells.

– Ground Meristem - Produces tissues composed of parenchyma cells.

• Pith

• Cortex

Origin and Development of Stems

• Narrow band of cells between the primary xylem and primary phloem may become vascular cambium.

– Cells produced by the vascular cambium become components of secondary xylem and secondary phloem.

• In many plants, a second cambium, cork cambium, arises.

– Produces cork cells and phelloderm cells.

Tissue Patterns in Stems

• Steles

– Stele is made up of primary xylem, primary phloem, and pith (if present).

• Protostele - Solid core.

• Sphinosteles - Tubular with pith in center.

• Eusteles - Vascular bundles.

Tissue Patterns in Stems

• Dicotyledons - Flowering plants that develop from seeds having two seed leaves.

• Monocotyledons - Flowering plants that develop from seeds with a single seed leaf.

Herbaceous Dicotyledonous Stems

• In general, annuals are green, herbaceous plants.

– Most monocots are annuals, but many dicots are also annuals.

• Herbaceous dicots have discrete vascular bundles composed of patches of xylem and phloem.

– Procambium produces only primary xylem and phloem, but vascular cambium arises later and adds secondary phloem and xylem to the vascular bundles.

Woody Dicotyledonous Stems

• Vascular cambium of a typical broadleaf tree produces relatively large vessel elements of secondary xylem (spring wood).

– Xylem produced next has smaller or fewer elements, and is referred to as summer wood.

– One year’s growth of xylem is called an annual ring.

Woody Dicotyledonous Stems

• Vascular cambium produces more secondary xylem than phloem, thus bulk of a tree trunk consists of annual rings of wood.

– Examining rings can determine the age of a tree, and provide some indications of climatic conditions.

• Vascular Rays consist of parenchyma cells that function in lateral conductions of nutrients and water.

– Xylem Ray

– Phloem Ray

Woody Dicotyledonous Stems

• Older, darker wood at the center is called heartwood, while the lighter, still-functioning xylem closest to the cambium is called sapwood.

– Formed at roughly the same rate as heartwood.

• Softwood - Xylem consists primarily of tracheids; no fibers of vessel elements.

– Cone-bearing trees.

Woody Dicotyledonous Stems

• Bark - Refers to all the tissues outside the cambium, including the phloem.

– Mature bark may consist of alternating layers of crushed phloem and cork.

Monocotyledonous Stems

• The stems of most monocots have neither a vascular cambium nor a cork cambium and thus produce no secondary vascular tissues or cork.

– Xylem and phloem exist in discrete vascular bundles.

• Secondary meristem produces only parenchyma cells to the outside and secondary vascular bundles to the inside.

Specialized Stems

• Rhizomes - Horizontal stems that grow below-ground.

• Runners - Horizontal stems that generally grow along surface.

• Stolons - Produced beneath the surface of the ground and tend to grow in different directions.

Specialized Stems

• Bulbs - Large buds surrounded by numerous fleshy leaves, with a small stem at the lower end.

• Corms - Resemble bulbs, but composed almost entirely of stem tissue.

• Cladophylls - Flattened,

• leaf-life stems.

• Thorns

• Tendrils

Wood and its Uses

• In a living tree, 50% of the wood weight comes from water content.

– Dry weight is composed of 60-75% cellulose and 15-25% lignin.

• Density and Durability are two of the most important characteristics in commercial wood.

Wood and its Uses

• Sawing

– Radially cut (quartersawed) boards show the annual rings in a side view.

– Tangentially cut (plain-sawed) boards show annual rings as irregular bands of light and dark streaks.

Wood and its Uses

• Knots

– Bases of lost branches covered by new annual rings produced by the cambium of the trunk.

– Found in greater concentration in older parts of the log, towards the center.

Wood and its Uses

• Wood Products

– About half of US and Canadian wood production is used as lumber, primarily for construction.

• Veneer - Thin sheet of desirable wood glued to cheaper lumber.

– Second most extensive use of wood is pulp.

– In developing countries, approximately half of cut timber is used for fuel.

• Less than 10% in US and Canada.

Review

• External Form of a Woody Twig

• Stem Origin and Development

• Stem Tissue Patterns

• Herbaceous Dicotyledonous Stems

• Woody Dicotyledonous Stems

• Monocotyledonous Stems

• Specialized Stems

• Wood and Its Uses

. Seedless Plants like ferns, typically have Apical Cells as their Apical Meristems. This is similar to what we saw with Roots. These apical cells are shaped like an inverted pyramid and divide anticlinally to produce Derivative Cells. These, continue to divide in various planes and produce

. The stems of seed plants do not have apical cells. Some appear to have have discrete Initials similar to those we saw in some root apical meristems. However, shoot apical meristems of gymnosperms and angiosperms are clearly multicellular.

. The apical dome is usually convex or flat and its surface is smooth. However, small protrusions can be seen when the macroscopic leaves are carefully removed. These  small protrusions are Leaf Primordia. The smallest of these is the one most recently produced. The term Node is used to indicate that place on the stem where a leaf is inserted.

. An Internode is that portion of a stem between successive nodes. This terminology will be reviewed below.

The "Meristem Proper" is that part of the shoot apex which is ABOVE the youngest leaf primordium. The term "Shoot Apical Meristem" (SAM) refers to the meristem proper. The "Shoot Tip" would include the youngest leaf primordia and the meristem proper. This is an

. important consideration for plant propagation via "tissue culture". Bacteria and viruses can penetrate the shoot tip but can't do so with the meristem proper. Meristem culture refers to the excision and culture of the pathogen-free meristem proper.  Shoot tips can also be cultured and they are easier to grow than true apical meristems. However, the chances for

. contamination are much greater. The differences in productivity between pathogen-free plants and contaminated plants is staggering, and makes it well worth the extra effort for horticultural crops.

This terminology has basic scientific significance because it allows us to readily and unambiguously discuss these closely-related parts of the shoot system

. A careful examination of the shoot apical meristem (SAM) shows that leaf primordia are produced in precise geometric patters. The term phyllotaxy is used to refer to these geometric patterns. The phyllotaxy is  best seen in buds which have preformed leaf primordia like those in Spruce (Picea). It is possible to trace the progression of primordia that will produce helical files of

. mature leaves once the bud has expanded. These tracings are called contact parastices and can be used to describe the phyllotaxy of the plant. The precise phyllotactic patterns of different plants have been used to design clever experiments on the control of leaf origination and differentiation. The leaves are closely inserted at the apex and there is little internodal

. elongation. However, as older leaves are displaced by new leaves, the former begin to grow rapidly along with the stem. We will say more about leaves later.

. Stems-Primary Growth Dicots

Unlike roots which have a solid, central cylinder of procambium, the procambium in stems occurs in isolated, small cylinders which are distributed in a regular pattern amid the ground tissue. The innermost ground tissue is called the Pith. The external ground tissue is called the Cortex. The stem resembles a column of reinforced concrete.

. The Epidermis would correspond to the outer mold. The vascular bundles would be steel rods, and the ground tissues would be the concrete that fills the empty volume inside the mold. The distribution of vascular bundles is controlled by the developing leaves and even the youngest primordia may have Procambium located immediately beneath it. The Procambium cells stain densely in the images below.

.Long Section of a Shoot Tip showing the distribution of Procambium amid the Ground Tissue of the Pith (Center) and Cortex (Peripheral)

. The Vascular Bundles are called fascicles. This term refers to a Roman symbol which had an axe surrounded by a bundle of sticks, lashed together. The region of ground tissue between the fascicles is called interfascicular. This is best seen in a cross-section of a typical dicot. We will use Coleus to illustrate many aspects of stem anatomy.

Coleus stem X-S stained with Phloroglucinol

. Large Vascular Bundles are located in each corner of the Coleus stem. Smaller bundles occur in-between. Thus, they form a single outline in the stem and they define a central Pith and a peripheral Cortex. The overall distribution of fascicles in Coleus is typical for Dicots.The Vascular Bundles are composed of Xylem and Phloem and are called collateral bundles. Fibers may or may not be associated with the vascular bundle.

.  Large Vascular Bundle from Coleus, stained with Phloroglucinol Note the Tissues Involved - Phloroglucinol stains Lignin

Red.

. Stems-Primary Growth Monocots

Monocots like Sugarcane and Corn have numerous Vascular Bundles in their leaves and consequently in their stems. The distribution of the Vascular Bundles in monocot stems looks disorganized at first glance. However, they are very well organized, but their organization is complex compared to Dicots.

. Typical Monocot Stems

While Asparagus is not the plant usually presented in textbooks, it shows traits which are similar to those in Corn & Sugarcane.

. Large Vascular Bundles are located in each corner of the Coleus stem. Smaller bundles occur in-between. Thus, they form a single outline in the stem and they define a central Pith and a peripheral Cortex. The overall distribution of fascicles in Coleus is typical for Dicots.

. The Vascular Bundles are composed of Xylem and Phloem and are called collateral bundles. Fibers may or may not be associated with the vascular bundle

    Large Vascular Bundle from Coleus, stained with Phloroglucinol

Note the Tissues

Involved – Phloroglucinol

stains Lignin Red.

. Stems Adapted to Desert & Saline Habitats
Leaves are adapted to intercept sunlight. However, the traits that are good for light acquisition are bad for water evaporation. We will review leaf adaptations to dry environments in the next section and will deal with some stem adaptations now.

. One way to deal with the evaporation of water from leaves is to either discard them completely or drop them rapidly when they are stressed.

Most Cacti produce minute photosynthetic leaves which are ephemeral (only last a short time) and contribute nothing to the overall photosynthesis of the plant.

. Cactus spines are highly modified, non-photosynthetic leaves. The cactus stem performs virtually all of the photosynthesis for the plant.

Some cactus Stems

have a Leaf-like shape.

. Shoot Apex of a Prickly Pear Cactus. The photosynthetic leaves are ephemeral while the Spines (Modified Leaves) are persistent.

. A   Flat, Leaf-like Cactus Stem

. Some plants produce two kinds of leaves.

One type is produced during wet periods.

These are shed and more drought resistant leaves are produced at the onset of water stress.

Sagebrush (Artemesia) is an example of the latter

. In some cases, the leaf blade is discarded and a Photosynthetic Petiole assumes

its function.

Plants in the

genus Zygophyllum exhibit this adaptation.

. Retama raetam is a desert shrub. Photosynthesis is performed entirely by the Stems.

Stomata are located in Grooves along the Stem and they are covered by Trichomes.

. This protects the stem from excess water loss when the stomata are open.

Photosynthetic Parenchyma is located just beneath the Epidermis along the inside of the grooves.

Sclerenchyma tissue and Water-storing Parenchyma complete the the Stem Cortex.

. Desert plants often have Succulent Stems that contain Water Storing Parenchyma Cells.

Species of Anabasis grow in dry environments and have succulent stems that may also perform most of the photosynthesis for the plant.

. Plants in the genus Salicornia inhabit dry Saline (salt-rich) sites. The exhibit many adaptations to such environments.

These include the following.

Photosynthesis is performed by the Stem.

The Stems are Segmented. If one part of the plant dies, other parts can survive.

. The stem has an abundance of Water-Storing Parenchyma.

The Vascular Tissue is deeply situated so that it is far removed from the Surface of the Stem where most of the evaporation occurs.

. Cross Sections of Salicornia Stem- the peripheral distribution of the Photosynthetic Chlorenchyma, the broad Cortex of Water-Storage Parenchyma and the central Vascular Tissues.

. Rhipsalis is a Cactus genus that contains Epiphytic species.

Some display various adaptations for survival in dry environments. These include the following.

Leaves are absent and the Stem is the photosynthetic organ.

. The Epidermis has a thick Cuticle.

Stomata are sunken.

The Ground Tissue of the Cortex is thick and contains lots of Water-Storing Parenchyma.

Crystals are abundant and may protect the plants from herbivory.

The Vascular Tissue is deeply seated in the stem to protect it from desiccation.

. Other Anatomical Traits associated with Stems adapted to dry and saline environments include the following.

Multiple Epidermal Layers

Lignified Epidermis

Non-functional or Wax-blocked Stomata

. Photosynthetic Stems are Segmented so that new ones abscise (fall off) like leaves at the end of the wet season.

Stems are split into many parts so that some may survive when others die.

. Stems Adapted to Wet Habitats

Light does not penetrate well through water. Consequently, the Epidermal Cells of  submerged stems have a thin or absent Cuticle.

Epidermal Cells may contain Chloroplasts.

The Epidermis may be the primary site for Photosynthesis in Submerged Stems.

. Stomata may be absent from the Submerged Stem.

Oxygen and Carbon Dioxide are not very soluble in Water. Consequently, most submerged stems have large air spaces (Aerenchyma) in their Cortex. This allows gasses to diffuse throughout the stem.

Parenchyma Diaphragms of  occur periodically.

. This prevents water-logging of the Aerenchyma Passages.

They also provide Structural Support.

Sclerenchyma Fibers are present in the Outer part of the Cortex and provide Structural Support.

Chlorenchyma is found in the Outer Cortex.

. The stems of Makaloa (Cyperus laevigatus) illustrate many of the adaptations of Submerged Stems.

Stomata are absent.

The Photosynthetic Cells are located close to the Surface of the Stem.

Aerenchyma is abundant.

. Aerenchyma (A) in Papyrus Stem

.