Question

development and composition of periderm.​

Answer 1

Answer:

In roots and stems having secondary growth, the epidermis is replaced by a protective layer of secondary origin known as periderm. It generally develops in gymnosperms and dicotyledonous axis and is rarely produced in leaves or monocotyledons. The periderm is also formed along surfaces exposed after abscission of plant parts, such as leaves or branches. It also evolves as protective layer near injured parts (wound periderm).

The term periderm is more distinct than bark. The latter designates all tissues outside the vascular cambium. In secondary state, it consists of secondary phloem and all tissues outside it. It can be distinguished into outer non-living and inner living parts. The functional phloem is the innermost part of the living bark.

Explanation:

The periderm consists of the phellogen or cork cambium, the meristem that produces the periderm; the cork or phellem, the protective tissue produced outside by the phellogen, and the inner cortex or phelloderm, the living parenchyma, formed inside by the phellogen. Because of the formation of cork, the tissues outside it usually die out.

The phellogen is simple in structure and it has only one kind cells. They appear as continuous tangential layer (lateral meristem) of rectangular, radially flattened cells in cross section. In longitudinal section they are rectangular or polygonal in outline.

The phellem or cork cells are often prismatic in shape and can be elongated vertically, radially or tangentially to form irregularly shaped structures. These are compactly arranged and absent inter-cellular spaces. These are dead cells at maturity. The cell walls are suberized.

The suberin, a fatty substance, generally occurs as a distinct lamella that covers the original primary cellulose wall. The thickness of cells walls of cork is variable. In thick-walled cells lignified cellulose layer exists on the inner side of suberin lamella. The walls of cork cells are brow or yellow in colour, while coloured resinous or tanniferous material can be filled in the lumina.

The commercial cork generally has thin walls and air-filled lumina. It is highly impervious to water and resistant to oil. It has thermal insulating quality and is light in weight. The mature cork of type is also a resilient and compressible tissue. These properties also make it useful to protect the plant parts.

In many species viz., Rhododendron maximum, the phellem includes non-suberized cells, called phelloid cells together with cork. These may also be thin or thick walled. When thick walled these are known as sclereids.

The phelloderm is a typical parenchyma which may be distinguished from other parenchyma by being present in the same radial files as the phellem cells.

Answer 2

PERIDERM

• in roots and stems of various gymnosperm and dicotyledons, the epidermis is replaced by a layer of secondary origin called periderm.

Composition -

• The periderm is composed of phellogen, phelloderm, and phellem
• Phallogen is meristematic tissue responsible for the formation of the periderm.
• Phellem(cork) -Cells that grow outward of phellogen are termed as phellem or cork
• Phalloderm- cells that grow inward to cork cambium are termed phelloderm. it is parenchymal in origin.

Development Of Periderm-

• The first periderm develops during the first year of growth of stems originating from the sub-epidermal layer.
• Periderm originates in the pericycle in the roots.
• In some cases where the roots cortex also stores food, the periderm can develop near the surface.
• The first phallogen is usually initiated around the circumference of the axis uniformly.
• Phallogen arises from meristematic cells which give later give rise to phelloderm and phellem.
• Due to phallogen activity, the plant axis increases in girth.
• During development as soon as the initial layer develops, it divides tangentially(give rise to phellem) and to some extent radially(give rise to phelloderm )
• In the manner of function, two kinds of barks are formed - scale bark and ring bark.
• Scale bark occurs when subsequent periderm exist in restricted overlapping strata.
• Ring bark - Formation of periderm successively around the axis.

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