PLANT TISSUE

DERMAL TISSUE

1. Epidermis- The epidermis is a covering usual one cell layer thick covering most of the plant. the epidermis is replaced by the periderm know as the the bark on the trunk. the epidermis covers stems, roots, flowers, leaves and seeds, but root tips are not covered. these cells are living and have very thick cell walls. they are tightly interlocked with change patters. its main purpuse is to protect the underlayers of ceels form water loss. its secondary funtion to to provide pyshical protection defences against pathogents. epidermal cells secrete a waxy substance called cutin over their exterior suface to form a protective covering called the cuticle which prevents water loss. epidermal cells to dont contian cholorpasts. to prevent water loss.
   
   
2. Guard cells- Gaurd cells are specialized epidermal cells. two guard cells suround a pore called a stoma. the stoma is an opening that alows gases to pass through. these gases are usaly oxygen, carbondioxide and watervapor. the guard cells regukate the size of the opening. increacing turgur pressure in the guard cells wident the stoma.
   
   
3. Trichomes- Trichomes or commonly called as hairs are growths form epidermal cells. hairs funtion in absorbtion , secreation and protection. the majority of plnat hairs are found in the roots. they providde more surface area for the abortion of water a nutients. coton is a good exapmle of plnat hiars.

GROUND TISSUE

1. Parenchyma- The cells of parenchyma are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other. They are usually stuffed with plastids(pigment containing vacoules). In areas not exposed to light, colorless plastids predominate and food storage is the main function. The cells of the white potato are parenchyma cells. In areas with light, e.g. leaves they predominate photosynthesis. There are several different types of parenchyma cells, categorized by function. For example, chlorenchyma cells contain chloroplasts and are specialized for photosynthesis. Aerenchyma cells contain large intracellular air spaces and function in gas exchange.
   
   
2. Collenchyma- They usually occur as strands or as continuous cylinders beneath the epidermis. Collenchyma cells are relatively long, with the ability to stretch. They are distinguished by their unevenly thickened cell walls. The function of collenchyma is to provide support for the primary plant body. They are most often found in areas that are growing rapidly and need to be strengthened. The petiole or "stalk" of leaves is usually reinforced with collenchyma.
   
   
3. Sclerenchyma-The walls of these cells are very thick and built up in a uniform layer around the entire cell. Often, they die after the cell wall is fully formed. Sclerenchyma cells are usually found associated with other cells types and give them mechanical support. They provide strength and support in parts that have ceased elongating. Sclerenchyma is found in stems and in leaf veins. They may be found in all parts of a plant. There are two type of sclerenchyma cells fibers and sclereids. Fibers are long slender cells which occur in strands or bundles e.g. hemp and flax. Fibers have very thin. Fibers provide support for the plant. Sclereids vary in shape, often branched, may occur singly or in groups in ground tissues throughout the plant. They make up the seed coat of seeds, shells of nuts, and give the pear its gritty texture. Their function is primarily for protection.

VASCUKAR TISSUE

1. Xylem-Xylem is a complex tissue and one of the major components of the vascular tissue system. It is the principle water conducting tissue in vascular plants. it carries water and dissoved matirial up from the roots to the leaves. Cell types found in xylem are parenchyma, sclerenchyma fibers and tracheary elements. There are two types of tracheary elements Tracheids and Vessel elements.
   • Tracheids- These are individual cells tapered at each end so the tapered end of one cell overlaps that of the adjacent cell. thier cell walls are thick and aligne lineraly. at maturiry the cytoplasm dies off. Their walls are perforated so that water can flow from one tracheid to the next.
   • Vessel elements - elongated shape but not as long or thin as tracheids. They have perforated open end walls. Vessel members are joined together end-to-end in long tubes called vessels. Vessels are generally wider than tracheids and therefore permit faster flow through the xylem.
2. The main components of phloem are
   • sieve elements
   • companion cells.
   Sieve elements are so-named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells. The result is a sieve tube that conducts the products of photosynthesis sugars and amino acids from the place where they are manufactured,leaves, to the places where they are consumed or stored. Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions. Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions.
   
   Companion cells move sugars and amino acids into and out of the sieve elements. In "source" tissue, such as a leaf, the companion cells use transmembrane proteins to take up - by active transport - sugars and amino acids from the cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve elements by diffusion through plasmodesmata. The pressure created by osmosis drives the flow of materials through the sieve tubes. In "sink" tissue, the sugars and amino acids leave the sieve tubes by diffusion through plasmodesmata connecting the sieve elements to the cells of their destination.