The ground meristem produces parenchyma cells of the cortex and pith region. These parenchyma cells store food, and also manufacture food in the presence of chloroplasts.

Three primary meristems namely, protoderm, procambium and ground meristem originate from the apical meristem found at the tip of stems.

The procambium produces the vascular bundles, while the protoderm forms the epidermis

ANATOMY OF STEM

During the formation of primary tissues, the leaf primordia produce mature leaves and the bud primordia produce buds.

A layer of meristematic cells found between the primary xylem and primary phloem forms the vascular cambium.

Branches of the stem xylem and phloem tissue enter the leaves and buds forming leaf traces, while the leaf gaps are filled with parenchyma cells.

Bud gap

Bud

Leaf gap

Leaf scar

Leaf gap

Leaf

Vascular

A portion of a young stem showing leaf gaps and bud gaps in the cylinder of vascular tissue

The vascular cambium produces tracheid, vessel elements, and parenchyma cells (secondary xylem) internally, and sieve elements, companion cells, fibre and parenchyma (secondary phloem) externally.

In woody species, a second cambium (cork cambium or phellogen) is formed either from the cortex, epidermis or phloem tissue.

The cork cambium produces cork cells or phellem externally and parenchyma cells (phelloderm) internally.

The cork cells have suberin deposition on their cell wall making the cells water proof.

The cork tissue reduces water loss from the stem and protects the stem from mechanical damage.

During the formation of cork tissue, parenchyma cells that are below the stomata form the lenticels (for gaseous exchange)

Lentisel

Herbaceous Dicot Stem

Most herbaceous dicot stems contain primary tissues,

even though the cambium may produce some secondary

tissues

Possess individual vascular bundles, arranged in a

cylinder, that separates the cortex from the pith.

In some species, the xylem and phloem tissues form a

continuous cylinder, and a vascular cambium forms later

in between the two primary tissues. The vascular

cambium produces secondary xylem and phloem.

A cross section of a typical herbaceous dicot stem

Helianthus stem cross section

Woody Dicot Stem

The formation of primary tissues in both herbaceous and

woody stems are similar.

During secondary growth, the parenchyma cells form

the xylem rays and phloem rays.

.

However, the difference occurred when secondary xylem

or wood is formed in woody species, and a greater

portion of the stem consists of wood.

Secondary phloem

Primary phloem

Secondary xylem

Pith

Primary xylem

Narrow xylem rays

Broad xylem ray

Annual ring of xylem

Vascular cambium

Narrow phloem ray

Broad phloem ray

Primary phloemCortex

Phelloderm

CorkCork cambium

A cross section of a woody Tilia stem

Transverse surface

Bark

Vessel

Ray

Tracheid

Vessel

Ray

Fiber

Sieve tube memberCambium

Xylem

Phloem

Radial surface

A three-dimensional, magnified view of a block of a woody dicot

A cross section of a woody dicot stem

In older trees, a large portion of the protoplast of

parenchyma cells that surround vessels and tracheids

grows through the pits of the conductive cells enlarging

into a balloon that almost fills the tracheary elements.

Such outgrowth is termed tylose. It restricts the flow of

water and nutrients through the tracheary elements. As a

result, gums, tannins and stains begin to accumulate and

darkens the wood tissue.

Small resin canals in Yellow PineLarge resin canals in Sugar Pine

Resin canals in a portion of a pine (Pinus)

Tylose-forminglayer

Tylose

Xylem: Variations in Wood Structure

Primary wall

Secondary wall

Pit withplasmodesmata

VesselRay cell

Pine trees have xylem tissue consisting of only tracheids,

without fibre or vessel elements, termed softwood.

The xylem tissue which is close to the cambium, lighter in

colour, and still functional as a conductive tissue is termed

sapwood.

The old, dark coloured wood found in the centre of a

woody dicot stem is termed heartwood. Heartwood does

not function anymore as a conductive tissue, instead it

provides strength and support to the tree.

The bark refers to all tissues outside the cambium, and

including the phloem.

The wood of a dicot tree is termed hardwood.

The inner bark consists of the primary and secondary

phloem. The outer bark is the periderm.

Hardwood (18X)

Softwood (18X)

Hardwood (18X)

Bark (Kulit kayu )

Monocot Stem

Most monocot species are herbaceous, their stems

are without a vascular cambium or a cork cambium.

Xylem and phloem tissues are found inside the

vascular bundles that are scattered inside the stem.

In most grass species, intercalary meristems are

present for stem elongation.

A typical monocot stem with scattered vascular bundles

Bundle sheath cell

Sieve tube member

Companion cell

Vessel element

Air space

Ground tissue(parenchyma)

Phloem

Xylem

A portion of a cross section of a Monocot (corn-Zea mays) stem.

A single vascular bundle of corn (Zea mays)

The stem of Dracaena and Sansevieria possess

secondary meristems that are different from the vascular

cambium found in dicot plants and conifers. Their

secondary meristems produce only parenchyma cells on

the outside and secondary vascular bundles internally.

Palm trees differ from other monocots due to their large

size. Even though without a cambium, their parenchyma

cells continue to divide and enlarge to increase the tree

size.

Bicollateral vascular bundle

Vascular bundles

Concentric - amphicribral Concentric - amphivasal