Shoot System-Stem-

Prepared by:

Group III

BS Bio 1-C

StemsThe main body of the

portion above the ground of the tree, shrub, herb, or other plant; the ascending axis, whether above or below the ground of a plant, in contradiction to the descending axis or root.

MAJOR FUNCTIONS

OF STEMS

-Stems support

-Stems Conduct

-Stem produce new ling tissue

STEMS SUPPORT Provides mechanical support and raise leaves into the

air, thus facilitating photosynthesis. Flowers and fruits are also produced in position, for facilitating pollination and seed dispersal.

STEMS CONDUCTo Provides a pathway for movement of water and mineral

nutrients from roots to leaves and for transfer of foods , hormones and to other metabolites from one part to another.

STEMS PRODUCE NEW LIVING TISSUEo Provide new living tissue for normal metabolism of

plant.

EXTERNAL PLANT

MORPHOLOGY

A stem is an organ consisting of

An alternating system of nodes, the points at

which leaves are attached

Internodes, the stem segments between

nodes

An axillary bud is a structure that has the potential to form a lateral shoot, or branch

An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot

Apical dominance helps to maintain dormancy in most non-apical buds

Lenticels are structure that permit the passage of gas inward and outward.

Leaf scar are characteristic scar on stem axis made by leaf abscission.

Bud scales are small modified leaves for protection from desiccation.

Dormant shoot apex with its protective scales is a BUD.

Bud Scars are the scars left from the removal of bud.

Leaf primordium is an immature leaf of the shoot.

Intercalary meristem the portion of the internodes above the node . Made up of actively dividing cells responsible for the elongation of the monocot stem.

Fig. 35-12

Apical bud

This year’s growth(one year old)

Bud scale

Axillary buds

Leafscar

Budscar

Node

Internode

One-year-old sidebranch formedfrom axillary budnear shoot tip

Last year’s growth(two years old) Leaf scar

Stem

Bud scar left by apicalbud scales of previouswinters

Leaf scar

Growth of twoyears ago(three years old)

Shoot Apex

Organization

The outer group consisting of one or more peripheral cell layer is known as the TUNICA. These cells divide anticlinally(perpendicular to the surface of the shoot apex)

The CORPUS lies below the tunica and initially has a single layer of cells. Corpus cells divide anticlinallyand periclinally(parallel to the surface of the shoot apex.)

Shoot Apex organization

A shoot apical meristem is a dome-

shaped mass of dividing cells at the shoot

tip

Leaves develop from leaf primordia

along the sides of the apical meristem

Axillary buds develop from

meristematic cells left at the bases of leaf

primordia

Fig. 35-16

Shoot apical meristem Leaf primordia

Youngleaf

Developingvascularstrand

Axillary budmeristems

0.25 mm

Primary Meristems

Protoderm- the outermost layer of cells.

It develops into epidermis--- the special

primary tissue that covers and protects all

underlying primary tissues. The epidermis

prevents excessive water loss and yet

allows for exchange of gases necessary for

respiration and photosynthesis.

Primary Meristems

Ground meristem- Comprises the

greater portion of meristematic tissue of

the shoot tip. Primary tissues forming

from the ground meristem are:

a) Pith- in the very center of stem

b) Cortex- in a cylinder just beneath the

epidermis and surrounding the vascular

tissues. Sometimes pith and cortex are

connected by pith rays.

Primary Meritsems

Procambium cells give rise to

primary vascular tissues

namely;

a) Primary phloem

b) Primary xylem

STEM ANATOMY,

PRIMARY

STRUCTURE

• Meristems are perpetually embryonic tissue and allow for indeterminate growth

• Apical meristems are located at the tips of roots and shoots and at the axillary buds of shoots

• Apical meristems elongate shoots and roots, a process called primary growth

Stems undergo primary growth which results in the formation of primary tissues. These include the

Epidermis

Ground tissue

primary vascular tissues (primary xylem and primary phloem)

The young dicot stem

Summary of Primary Development

Protoderm Epidermis

Ground meristem Cortex

Apical Meristem Pith and pith rays

Procambium Phloem

Vascular Cambium

Xylem

Primary Growth development The term stele is applied to the part of the stem that includes

primary vascular tissues, pith, and pith rays. The primary plant body is composed of the above primary tissues.

The main functions of these primary tissues may be summarized as shown below.

Epidermis: Protects underlying tissues.

Vascular tissues

Phloem: Conducts Food

Vascular Cambium: produces secondary phloem and secondary xylem

Xylem: conducts water and mineral salts , and gives strength to stem.

Cortex: Stores food and in young stems, manufactures food, strengthens and protects.

Pith: Stores food

Pith rays: Store food, and conduct water, mineral salts, and food radically.

The young dicot stemThe stellar type exhibited by a dicot

stem is a EUSTELE.

The type of xylem maturation is known as Endarch.

Secondary growth is present.

THE MONOCOT STEM

Fig. 35-17b

Groundtissue

Epidermis

Keyto labels

Cross section of stem with scattered vascular bundles(typical of monocots)

Dermal

Ground

Vascular (b)

Vascularbundles

1 mm

The monocot stem

The vascular bundles are scattered throughout the ground tissue. The type of stele exhibit is ATACTOSTELE.

In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring.

They do not have secondary growth.

Fig. 35-17

Phloem Xylem

Sclerenchyma(fiber cells)

Ground tissueconnectingpith to cortex

Pith

Cortex

1 mm

Epidermis

Vascularbundle

Cross section of stem with vascular bundles forminga ring (typical of eudicots)

(a)

Keyto labels

Dermal

Ground

Vascular

Cross section of stem with scattered vascular bundles(typical of monocots)

(b)

1 mm

Epidermis

Vascularbundles

Groundtissue

Eustele vs. Atactostle

STEM

ANATOMY, SECONDA

RY STRUCTURE

• Secondary growth occurs in stems and

roots of woody plants but rarely in leaves

• The secondary plant body consists of the

tissues produced by the vascular cambium

and cork cambium

• Secondary growth is characteristic of

gymnosperms and many eudicots, but not

monocots

Woody dicot (Tillia sp.)

Stem anatomy, secondary structure

These tissue layers form the Periderm.

The outermost layer is the phellem,

consisting of cork cells.

Immediately inner to it is the phellogen, or

the cork cambium, consisting of flattened

dividing cells.

The third layer is the pheloderm, few cell

layers in thickness.

Fig. 35-19a1

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growth

in a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondary

xylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Fig. 35-19a2

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growth

in a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondary

xylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growth

in a two-year-old stem

(a)

Periderm (mainly

cork cambia

and cork)

Secondary phloem

Secondary

xylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Cork

Bark

Most recent corkcambium

Layers ofperiderm

Fig. 35-19b

Secondary phloem

Vascular cambium

Secondary xylem

Bark

Early wood

Late wood Cork

cambium

Cork

Periderm

0.5

mm

Vascular ray Growth ring

Cross section of a three-year-

old Tilia (linden) stem (LM)

(b)

0.5 mm

The Vascular Cambium and Secondary

Vascular Tissue

The vascular cambium is a cylinder of meristematic

cells one cell layer thick

It develops from undifferentiated parenchyma cells

In cross section, the vascular cambium appears

as a ring of initials

The initials increase the vascular cambium’s

circumference and add secondary xylem to the

inside and secondary phloem to the outside

Secondary xylem accumulates as wood, and consists of tracheids, vessel elements (only in angiosperms), and fibers

Early wood, formed in the spring, has thin cell walls to maximize water delivery

Late wood, formed in late summer, has thick-walled cells and contributes more to stem support

In temperate regions, the vascular cambium of perennials is dormant through the winter

Tree rings are visible where late and early

wood meet, and can be used to estimate a

tree’s age

Dendrochronology is the analysis of tree ring

growth patterns, and can be used to study

past climate change

As a tree or woody shrub ages, the older

layers of secondary xylem, the heartwood,

no longer transport water and minerals

The outer layers, known as sapwood, still

transport materials through the xylem

Older secondary phloem sloughs off and

does not accumulate

Fig. 35-22

Growth

ring

Vascular

ray

Secondary

xylem

Heartwood

Sapwood

Bark

Vascular cambium

Secondary phloem

Layers of periderm

The Cork Cambium and the

Production of Periderm

The cork cambium gives rise to the secondary plant body’s protective covering, or periderm

Periderm consists of the cork cambium plus the layers of cork cells it produces

Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm

Lenticels in the periderm allow for gas exchange between living stem or root cells and the outside air

A plant can grow throughout its life; this is

called indeterminate growth

Some plant organs cease to grow at a certain

size; this is called determinate growth

Annuals complete their life cycle in a year

or less

Biennials require two growing seasons

Perennials live for many years

Sequoia sempervirens

Pinus aristata

Monocot vs.Dicot

Parameter Monocot Dicot

Extent of cortex No distinct cortex Cortex found at the

outer part of ground

tissue

Presence or absence of

pith

Absent Present

Type of stele Atactostele Eustele

Presence or absence of

vascular cambium

Absent Present

Modified StemModification of the stem would depend on

the need of the plant to survive…

… like the animals it learns how to

adapt.

Bulb – consist of small amount of vertical stem and a massive quantity of thick, fleshy storage leaves.

- most of them consist of concentric rings of scales attached to a basal plate.

.

Other bulbous plants

Daffodil Reticulate iris

Stolon / runner = with long

internodes just below the

surface of the ground that

typically terminating in a new

plant

= use for propagation

Fig. 35-5c

Stolons

Stolon

Corm - formed from a swollen bases

of stems.

- A corm consists of one or

more internodes with at least one

growing point

Examples of Corm

Crocuses Gladioli

Rhizome= the stem is

horizontal and underground

with short internodes and

bearing with scale-like leaves.

Other rhizome plants

Johnson grass

Sorghum halepenseCogon grass

Imperata cylindrica

Tuber = a thick under ground

storage stem, usually not upright

= bearing outer buds

= lacking protective scales

Fig. 35-5d

Tubers

Aerial MODIFICATIONS OF STEM

•TENDRILSIN grapes

Axillary bud is modified into tendrils.

•CLADOPHYLL /

PHYLLOCLADE

The entire shoot is flattend & leaf like.

References

• Campbell, N.A., J.B Reece and L.G. Mitchell. 1999. Biology. 5th ed. USA: The Benjamin/Cummings Publishing Co. Inc.

• Weier, E.T., R.C Stocking., M. G Barbour and Rost T. L.1982. Botany an Introduction to Plant Biology. 6th ed. USA: John Willey and Sons Inc.

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