Wednesday, 28 July 2021



Named so after Devon city in England where rocks of that period was first discovered. The fossils of Psilopsids, Tycopsids, Sphenopsids, ferns, Liverworts, Horse tails, Pterospermales etc. were discovered from rock sediments of this period. (v) Mississipian Period : This is a period of Palaeozoic era. It is named after the city Mississipi. It lasted for 25 million years. Coal beds were discovered. The fossils of tree-like Lycopods were discovered from rock of that age.

 (vi) Pennsylvanian Period : It is a period of Palaeozoic era. This period was named after the city Pennsylvania. It lasted for 25 years. The Mississipian and Pennsylvanian periods are together called Carboniferous period. The fossils of mosses, cordaitals and Pteridospermales were predominant in the rocks of that age.

 (vii) Permian Period : It is a period of Palaeozoic era. This period lasted for 30 million years. The fossils of Ginkgoales and primitive coniferales were discovered from rock deposits of this period. 5. Mesozoic Era The mesozoic era is the period of 'middle life', It started 225 million years ago and ended 75 million years back.

 Cycads had flourished well in that era, so that it is called 'age of cycads'. The supporters of this view consider that the similarity in the sex organs and life-cycles of bryophytes and pteridophytes is due to their parallel evolution. Campbell (1895), Lignier (1903), Zimmermand (1930, 1938), and Bower (1935), on the other band nostulated that ancestors of pteridophytes were some.

Smaller vascular bundles lie in the middle of mesophyll

A leaf having stomata on both the surfaces is called amphistomatic. Each stoma possesses two dumb-bell shaped guard cells with or without subsidiary cells. Functions : (i) Epidermis provides protection to the leaf interior. (ii) Its thickened cells and cuticle reduce the rate of surface transpiration. (iii) Protection from entry of pathogens. 

(iv) Exchange of gases with the help of stomata. (vi) Folding and unfolding of leaves with help of bulliform cells in response to changes in atmospheric humidity and change availability of water.

 In Mesophyll : chymatous ground tissue of the leaves. Mesophyll of isobilateral monocot leaves is undifferentiated. All the cells are similar. They It constitutes the chloren- are, generally, oval or rounded in outlines.

The enclose small intercellular spaces. intercellular spaces with are connected substomatal cavities, Mesophyll cells contain abundant chloroplasts; therefore, they are the seat of photosynthesis. 3) Vascular Strand : It consists of a number of parallel vascular bundles. Smaller vascular bundles lie in the middle of mesophyll. The Targer vascular bundles and their extensions occupy the whole area between the two leaf surfaces.

 Bundle sheath extensions are sclerenchymatous. Each vascular bundle is covered by a sheath of parenchyma cells having chloroplasts. A single bundle sheath occurs in panicoid grasses while double sheath occurs in festucoid grasses. It has phloem towards abaxial side and xylem towards adaxial side.The Phloem does not show distinction.

   The various components of a monocot leaf are epidermis

It is surrounded by parenchymatous ground tissue. Vascular bundles have conjoint, collateral but closed nature with xylem towards the upper side and phloem towards the lower side. Reasons for Identification : (i) Bifacial flattened structure with stomata nmostly on lower surface. (iii) Vascular bundles with colourless bundle sheath. 

Internall Structure of an Isobilateral Monocot Leaf : (Fig. 6) In monocot leaves the two surfaces are equally green and liable to face the sun. They are, therefore, called isobilateral leaves (Gk. isos equal, bis side).

 The various components of a monocot leaf are epidermis, mesophyll, vascular twice, latris strand and midrib. (1) Epidermis : It is the outer most covering of the lamina on both the surfaces. The cells are parenchymatous rectangular barrel shaped. They are devoid of chloroplasts and are, therefore, transparent.

 Free surface of the epidermal cells possesses cutin and silica thickenings. Separate layer of cuticle is also present. At places the adaxial epidermis of Maize and a number of grasses possess large sized thil.walled colourless protruding cells called bulliform or motor cells. They become flaccid when water is deficient. Consequently, the leaves curl inwardly to minimise exposed surface.

 The leaves become flát again when water is available and bulliform cells become turgid. Bulliform or motor cells, therefore, take part in folding and unfolding of leaves depending upon the moisture content of the atmosphere. Stomata occur on both the surfaces with almost equal frequency. 

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