Dermal Tissue
Dermal tissue is found in the epidermis and and periderm of a plant. This is the outermost layer. The epidermis produces a noncellular layer called the cuticle on some parts of the shoot system. The cuticle protects against excessive waterloss and infection by microorganisms. The cuticle also restricts gaseous exchange through the surface of the epidermal cells. During secondary growth in roots and stems the epidermis is replaced with the periderm. Some peridermal cells form layers of special cells that die very quickly but leave behind a material that waterproofs the roots and stem which protects the inner tissues from structural damage. Dermal tissue often contains specialized cells such as root hair cells and leaf guard cells. |
Ground Tissue
There are three types of ground tissue. They are parenchyma, collenchyma and sclerenchyma. Perenchyma is a living ground tissue that makes up the bulk of the primary plant body and takes part in several tasks such as photosynthesis, storage and regeneration. It stores nutrients, carbohydrates and water. The parenchyma is capable of healing wounds and regenerating parts of the plant. The spongy tissue used for storage in the center of the roots and stem is called the pith. The ground tissue that surrounds the pith is called the cortex which usually has slightly thicker walls and surrounds the vascular tissue in the roots and stem and is made up of more rigid cells. Collenchyma is living ground tissue that offers flexible support for primary growth. The collenchyma is specialized for supporting the plants primary growth regions and therefore makes the plant stronger. The cell wall are thickened and provide a measure of flexibility which allows the plant to withstand windy conditions. Sclerenchyma is a ground tissue whose mature cells are dead. It's cell walls are composed of cellulose and lignin. Sclerenchyma supports mature plants and protects seeds. This ground tissue has an extra material that provides added strength and rigidity to the cells. These cells may occur as a continuous mass, small clusters or can be individually scattered. |
Vascular Tissue
The xylem is the main tissue for carrying water and minerals throughout the plant. The xylem contains fibers and water conducting cells called tracheids and vessels. Tracheids are xylem cells with tapered, over-lapping ends and pits in their cell walls for conducting water and dissolved materials in the plant. The pits in the walls of the tracheids are unthickened for easy transfer of materials between neighboring cells. Tracheids are longer than vessel elements which make up vessels. Vessels are long tubes of vessel elements that are used to conduct water and dissolved materials in plants. The are long continuous tubes of individual vessel elements that have been joined end to end. Vessel elements have thickened walls and large perforations in the end walls. Tracheids and vessels are both dead at maturity and their lignified cell walls remain. Tracheids and vessels continue to transport water and dissolved substances until they get filled with various deposits. Both tracheids and vessels are found in angiosperms but only tracheids are found in gymnosperms.
The phloem transports sugars and other solutions. The phloem is living tissue unlike the xylem. Sieve tubes are long tubes formed by many sieve elements to allow easy passage for water and dissolved minerals.These provide and obstruction-free pathway for movement of materials. Sieve elements are long thin phloem cells with sieve plates at the end walls. Sieve plates have large pores which allow for easy passage of water and materials. Sieve elements have pits on their side walls. Sieve elements are associated with companion cells which lay next to the sieve elements and direct the activity of sieve tubes and supply needed substances when required. These cells lack a nucleus, ribosomes, golgi bodies, cytoskeleton and vacuoles.