Apical organization Organization in plants is dependent upon programmed, controlled cell division, followed by growth, further cell division and ultimately,

Apical organizationApical organization

Organization in plants is dependent upon programmed, controlled cell division, followed by growth, further cell division and ultimately, differentiation.

Programmed and controlled cell division occurs within the domain of the vegetative apex.

the apexthe apex

All the tissues within the apex differentiate rapidly. By about 150 µm, cells within the apical region are starting to differentiate. In the pine apex (above), you can see developing leaflets.

The Coleus apex to the right, shows rapidly developing leaflets beneath the apical dome.

Cell divisionCell division

Cell division is responsible for the formation of all cells and tissues in the primary plant body as well as in the secondary plant body.

Cell source

apical and sub apical primary divisionapical and sub apical primary division

apical meristem

apical meristem

provascular tissue

provascular tissue

epidermisepidermis pithpith cortexcortexprimary phloem

primary phloem

primary xylem

primary xylem

ground meristem

ground meristemprotodermprotoderm

undifferentiated

generative source

Secondary celllineage

fascicular cambium

fascicular cambium

primary lineage

the secondary lineagethe secondary lineage

fascicular cambium

fascicular cambium

secondary xylem

secondary xylem

vascular cambium

vascular cambium

secondary phloem

secondary phloem

cork cambium

cork cambium

ASSOCIATED WITH THE VASCULAR BUNDLE ONLY

COMPLETE RING OF CAMBIUM

the secondary protective lineagethe secondary protective lineage

sub-epidermallayers

sub-epidermallayers

phellemphellem

phellogenphellogen

phelloderm

phelloderm

thecork cambium(bark layer)

click the periderm a protective barrier

Development of the peridermDevelopment of the periderm

sub-epidermallayers

sub-epidermallayers

phellemphellem

phellogenphellogen

phelloderm

phelloderm

The first periderm is formed just beneath the epidermis

phellem

phellogenphelloderm

a waterproof, fireproofinsulator

primary organizationprimary organization

groundmeristem

groundmeristem

PITHPITH CORTEXCORTEX

interfascicular

cambium

interfascicular

cambium

phellem/cork

cambium

phellem/cork

cambium

Click for Filling spaces notes

fascicular cambium

fascicular cambium

secondary xylem

secondary xylem

vascular cambium

vascular cambium

secondary phloem

secondary phloem

cork cambium

cork cambium

primary mechanical tissuesprimary mechanical tissues

groundmeristem

groundmeristem

PITHPITHCORTEXCORTEX

collenchymacollenchyma

sclerenchymasclerenchyma sclerenchymasclerenchyma

collenchyma(rare)

collenchyma(rare)

groundmeristem

groundmeristem

PITHPITH CORTEX

CORTEX

interfascicular

cambium

interfascicular

cambium

phellem/cork

cambium

phellem/cork

cambium

development of the vascular cambiumdevelopment of the vascular cambium

fascicular cambium

fascicular cambium

fusiform initials

fusiform initials

ray initials

ray initials

axialxylem

axialxylem

axial phloem

axial phloem

fascicular

cambium

fascicular

cambium

secondary

xylem

secondary

xylem

vascular cambium

vascular cambium

secondary

phloem

secondary

phloem

cork cambium

cork cambium

xylemrays

xylemrays

phloemrays

phloemrays

to cambial derivatives notes pages

cambial divisioncambial division

radial

axia

linitial

phloem

Cell division within the ray and fusiform initials results in the formation of derivative cells that are placed either on the outside of the mother cell, in which case they add to the secondary phloem (green cells), or on the inside endarch to) the mother cell, thus adding to the secondary xylem (blue cells).

xylem

Cell sourceCell source

The apical meristem is the principle source of new cells in the primary as well as within the secondary plant body. All cell division linked to vegetative growth, involves mitosis, and, as a result, the cells that are produced are exact copies of each other. Lineage depends on the position of the initial within the meristem.

the periderm a protective barrierthe periderm a protective barrier

During secondary growth, the diameter of stems and roots increases rapidly, which results in tension and splitting of the existing dermal tissues, which subsequently, will stretch and become disrupted.

The generative layer of the first periderm (phellogen) is initiated within parenchymatous elements in the outer cortex of stems and roots. It offers protection from invasion by insects, pathogens and fungi.

As the stem or root continues to increase in diameter, so successive peridems are formed. These are formed within the secondary phloem.

The periderm is a natural waterproof, fireproof insulator.

Filling spacesFilling spaces

Within all plants the primary packaging tissues are composed of cells that either fill in spaces, or support other areas of the stem, root or leaf. Thus, the parenchymatic elements that are produced (and have lineage back to the apical meristems) are produced from what is termed the ground meristem. In simple terms, the ground meristem is that region of a shoot or root apical meristem that is NOT involved in the production of vascular tissue.

cambial derivativescambial derivatives

The vascular cambium is the source of all need (secondary) differentiation in plants. It contains two systems, the secondary xylem, and the secondary phloem tissue. Each of these tissues is complex, and is developed and has evolved for specific functions – the xylem for th transport of water and water-soluble molecules, and the phloem for the transport of assimilated materials, which consist of sugars and related carbohydrates translocated in water.

Physiologically, the transport xylem is dead at maturity, has secondarily-lignified cell walls, and functions under extreme negative pressure potentials. Transport phloem on the other hand, contains a majority of living cells, with specialized sieve elements, which are geared for rapid, long-distance translocation of the assimilated carbohydrate pool. There transport elements, have thickened walls, are living at maturity and function under a high positive pressure potential.

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transport functionalitytransport functionality

The xylem and phloem conduits form axial tubes. These tubes facilitate rapid, long-distance movement of water and dissolved materials. It follows therefore that the fascicular cambial derivatives that form these transport cells are longer than they are wide, and that the cells will, depending on position form either xylem or phloem.

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the need for lateral communicationthe need for lateral communication

As the secondary plant body enlarges, so the carbohydrate conducting, and water transporting systems become laterally spatially and physiologically further removed from each other. The core of a stem or root, for example, may well contain a number of living cells, that not only require water, but a supply of assimilate and other carbohydrates, in order to maintain their functional state. If this does not happen or if the supply is cut off for some reason, then the core will die.

Lateral communication, and the production of these cells, is due o the activity of specialised cambial cells, called the ray cells. These cells are sort, often cubic in shape and the produce rows (files) of parenchymatous living cells, that interconnect the phloem with the inner xylem core, thereby facilitating exchange of carbohydrate inwards, and water outwards in the living plant.

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