Upper and lower epidermis. Palisade mesophyll, and Vascular bundle - Essay Example

Upper and lower epidermis: The upper and lower epidermis covers the upper and lower surface of the leaf's blade respectively and is transparent in nature in order to permit light into the mesophyll tissues for photosynthesis. Both these structures are one layer thick and consist of flattened rectangular cells. The upper epidermis can be differentiated from the lower epidermis by the number of stomata which are more in the latter. The epidermis protects the leaf against excessive water loss and mechanical injury.

The upper epidermis is covered by a layer of waxy substance called cutin which is secreted by the epidermal cells. This substance further protects the leaf from dehydration. As an additional protection mechanism, epidermal cells in some plants secrete fluids, oils and poisonous substances and might contain soft hair called mullein. Palisade mesophyll: the palisade parenchyma lies just next to and below the upper epidermis and contains oval and elongated cells closely packed together. These cells are thin walled and the primary site for photosynthetic reaction.

They contain small structures called chloroplasts in large numbers that bear chlorophyll- the key ingredient for photosynthesis. Spongy mesophyll: the spongy parenchyma is a layer of loosely packed irregular cells lying between two layers of palisade cells. They also contain chloroplasts although they are fewer in number as compared to the palisade parenchyma. The intracellular spaces between cells allows for efficient exchange of gases. Air spaces: air spaces are actually the intracellular spaces between the spongy parenchyma cells and bear communication with the air chambers behind the stomata.

They facilitate inflow and outflow of gases. Stomata: the se are pores scattered throughout the epidermis (more on the lower epidermis). The stomata allow for the interchange of gases for respiration and photosynthesis. These are pores which are covered by bean shaped cells called guard cells that also contain chloroplasts. These cells open and close in response to heat and light in order to regulate the amount of gases and water vapor loss from the leaf (transpiration) which also allows cooling.

Vascular bundle (Xylem and Phloem): the vascular bundle runs through the centre of the leaf (from the petiole to the tip) and makes up for the vein of the leaf. It is covered by sheath cells called scherenchyma which provide protection and rigidity to the underlying vascular tissue. The two kinds of vascular tissues are Xylem and Phloem. Xylem cells are larger and lie towards the upper epidermis whereas the phloem cells are smaller and lie towards the lower epidermis. The function of xylem is to translocate water and dissolved ions from the soil to the mesophyll tissue.

The phloem in turn conducts organic food (photosynthates) such as glucose from the mesophyll to other parts of the plant for utilization and storage. References: http://www.botany.uwc.ac.za/sci_ed/grade10/anatomy/leaves.htm http://www.pssc.ttu.edu/pss1321/Web%20topics/cpa1B.htm http://encarta.msn.com/text_761558729___5/Leaf.html