The foliage leaves are probably the most noticeable organs of a flowering plant. Leaves are adapted to perform certain important functions: photosynthesis, respiration and transpiration.
A typical foliage leaf consists of a large, flat leaf blade (lamina), a petiole (leaf stalk) and a leaf base with which the leaf is attached to the stem. The veins are clearly visible on the leaf blade. The way in which the veins are arranged is known as venation of the leaf. The leaf blades of some plants show indentations or clefts. If these indentations reach all the way to the midrib so that the leaf blade is divided into a number of smaller pinnae (leaflets), the leaf is called a compound leaf. If the leaf blade is not divided into leaflets, the leaf is termed a simple leaf. Most monocots have simple leaves, while dicots can have simple or compound leaves.
If a thin transverse section is made through a dorsiventral leaf the different tissues can clearly be observed under a microscope. Three regions are distinguishable, namely the epidermis, the mesophyll and the vascular bundles or veins.
The epidermis covers the lower and upper sides of the leaf. Each side is composed of a single layer of closely-packed rectangular cells. The layers are translucent and are covered with a cuticle on the outer wall. There are many stomata perforating the lower surface. Each stomata are surrounded by a set of guard cells that contain chlorophyll.
Functions of the Epidermis
This tissue forms the bulk of the leaf. It makes up the green tissue of the leaf and consists of thin-walled cells containing chloroplasts. In most dicot leaves the mesophyll is differentiated into palisade parenchyma and spongy parenchyma. The palisade parenchyma consists of thin-walled cells which are usually cylindrical in shape. These cells contain large numbers of chloroplasts. The spongy mesophyll are usually ball-shaped with large intercellular spaces, but usually contains f fewer chloroplasts than the palisade cells. The palisade mesophyll, bundle sheath and spongy mesophyll are known as the ground parenchyma. There is a system of air spaces which communicate with the air chambers behind the stomata. Since the mesophyll cells contain chloroplasts the tissue is also referred to as chlorenchyma .
Functions of the Mesophyll
The vein system of the leaf consists of branched vascular bundles. A vein contains the vascular tissue which consists of xylem and phloem. The lignified xylem cells are situated towards the upper epidermis and the phloem towards the lower epidermis. In the large veins the vascular bundles are usually surrounded by a bundle sheath.
Functions of the Vascular Bundles
Drawing of a transverse section through a dorsiventral leaf to show the different tissues.
A stoma or pore is formed by a pair of bean-shaped guard cells. The guard cells have the ability to open and close the stoma. The inner walls of the guard cells are thick and the outer walls thin. Guard cells differ from the translucent epidermal cells in that they contain chloroplasts. Stomata communicate with the air chambers in the spongy mesophyll. There are more stomata on the lower epidermis of the leaf than the upper epidermis.
The stomata are responsible for the interchange of gases for respiration and photosynthesis.
The stomata allow for the loss of excess water in the form of water vapour, which also allows for cooling.
Stomata with guard cells open and close.
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