510 words free sample essay on Organography

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Organography means description of various organs of plants and ani­mals. Anatomically root is character­ised by unicellular hairs, lack of hy­podermis, presence of a broad cortex, radial exarch closed vascular bun­dles. Stems are characterised by mul­ticellular stem hairs, hypodermis, conjoint vascular bundles which are endarch. Dicot stems possess collenchymatous hypodermis, endarch, conjoint collateral or bicollateral open vascular bundles arranged in rings. The ground tissue is distinctly layered.

In monocot stems the ground tis­sue is not distinguished into layers and they possess sclerenchymatous hypodermis, closed conjoint collateral vascular bundles with bundle sheath scattered in the ground tissue. Endodermis in either stem or root contains barrel shaped cells with ra­dial thickening called Casparian thickening and also contains few thin walled cells called passage cells.

Monocot roots possess more than 6 number of vascular bundles in its stelar region with a broad pith, whereas dicot roots possess maximum of 6 vascular bundles in a narrow pith.

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Two types of leaves are distin­guished based on the internal struc­ture: dorsiventral leaf and isobilateral leaf. Dorsiventral leaves are commonly found in dicotyledonous plants and exhibit two dissimilar epidermal layers with lower epidermis having stomata. The mesophyll tissue is distinguished into upper palisade paenchyma and lower spongy parenchyma, which open through its intercellular spaces and stomata to out side. In isobilateral leaves, both the epider­mal layers are similar and contain stomata. Bulliform cells are also seen in epidermal layers. Mesophyll tissue contains only spongy paren­chyma.

Secondary growth occurs in stems and roots of dicotyledonous plants to meet the need of conduction and me­chanical strength. In stems, second­ary growth occurs by the activity of the vascular cambium which pro­duces secondary xylem to the inner side and secondary phloem to the outer side. Due to variation in sea­sons, the cambium activity varies and it produces annual rings consisting of broad and narrow secondary xylem tissues.

The wood formed during spring season is called spring wood (or early wood) and is made of vessels with bigger lumen. The wood formed during autumn season is called autumn wood (or late wood) and is narrow. In a transverse section of a log of wood, a central dark region is marked which is called heart wood (or duramen) and it is surrounded by a narrow zone of light coloured wood called sap wood (or alburnum). The wood may be porous or non-porous depending on the presence or absence of vessels. Gymnospermic woods are non-porous and soft whereas angiospermic woods are porous and hard.

Extra-stelar secondary growth oc­curs after stelar secondary growth starts. It occurs by the activity of cork cambium (or phellogen) which forms an outer phellem (or cork layer) and inner phelloderm (or secondary cor­tex). Phellem, phellogen and phelloderm make a layer called peri­derm which is a protective layer and shed scale bark or ring bark depend­ing on the activity of phellogen. Roots of dicotyledonous plants also exhibit secondary growth due to development of a secondary cambium from the pericycle and pith cells. However, annual ring formation is not distinct in root.

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