Apical 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 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 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 division apical meristem provascular tissue epidermis pith cortex primary phloem primary xylem ground meristem protoderm undifferentiated generative source Secondary cell lineage fascicular cambium primary lineage

the secondary lineage fascicular cambium secondary xylem secondary xylem vascular cambium secondary phloem secondary phloem cork cambium cork cambium ASSOCIATED WITH THE VASCULAR BUNDLE ONLY COMPLETE RING OF CAMBIUM

the secondary protective lineage sub- epidermal layers sub- epidermal layers phellem phellogen phelloderm the cork cambium (bark layer) click the periderm a protective barrier

Development of the periderm sub- epidermal layers sub- epidermal layers phellem phellogen phelloder m The first periderm is formed just beneath the epidermis phellem phellogen phelloderm a waterproof, fireproof insulator

primary organization ground meristem ground meristem PITH CORTEX interfascicular cambium interfascicular cambium phellem/cork cambium Click for Filling spaces notes fascicular cambium secondary xylem secondary xylem vascular cambium secondary phloem secondary phloem cork cambium cork cambium

primary mechanical tissues ground meristem ground meristem PITH CORTEX collenchyma sclerenchyma collenchyma (rare) collenchyma (rare) ground meristem ground meristem PITH CORTEX interfascicular cambium interfascicular cambium phellem/cor k cambium

development of the vascular cambium fascicular cambium fusiform initials ray initials ray initials axial xylem axial xylem axial phloem fascicular cambium secondary xylem secondary xylem vascular cambium secondary phloem secondary phloem cork cambium cork cambium xylem rays xylem rays phloem rays phloem rays to cambial derivatives notes pagescambial derivatives

cambial division radial axial initial 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 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 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 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 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. click here for the next page

transport 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. click here for cambial derivatives click the need for lateral communication back

the 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. Click to go back to the presentation