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Xylem Part -1.pptx
- 1. Complex Permanent Tissue Vascular tissue / conducting tissue Human body – circulatory system Not all plant Xylem and Phloem
- 2. Vascular tissue is a complex conducting tissue, formed of more than one cell type The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant. The cells in vascular tissue are typically long and slender. Since the xylem and phloem function in the conduction of water, minerals, and nutrients throughout the plant, it is not surprising that their form should be similar to pipes. The individual cells of phloem are connected end-to-end, just as the sections of a pipe. The new tissue is aligned with existing vascular tissue, maintaining its connection throughout the plant. The vascular tissue in plants is arranged in long, discrete strands called vascular bundles. These bundles include xylem and phloem, as well as supporting and protective cells
- 3. • In the plant kingdom, the earliest examples of an organized vascular system are encountered among the brown algae, which possess phloem with no xylem. • The water-conducting system evolved much later; its adaptation and refinement became mandatory for the development of plants on land, because the lignified conducting and supporting cells in the xylem provide the long-distance water conduction and the mechanical support needed in a terrestrial habitat.
- 4. Location of xylem and phloem In stems and roots, the xylem typically lies closer to the interior of the stem with phloem towards the exterior of the stem. In the stems of some Asteridae dicots, there may be phloem located inwardly from the xylem. In leaves, the vascular bundles are located among the spongy mesophyll. The xylem is oriented toward the adaxial surface of the leaf (usually the upper side), and phloem is oriented toward the abaxial surface of the leaf (usually the underside). The main reason behind aphids are typically found on the underside of the leaves rather than on the upper, is the phloem transports sugars manufactured by the leaves , transport of food in the form of sugar through phloem and they are closer to the lower surface
- 5. Location of phloem towards the exterior of stem or root and xylem towards interior of stem Location of phloem towards the abaxial surface of leaf and xylem towards adaxial surface of leaf
- 6. Primary and Secondary Xylem • Primary xylem is the xylem that formed during primary growth of procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem. • Secondary xylem is the xylem that is formed during secondary growth of vascular cambium. Although secondary xylem is also found in members of the "Gymnosperm" groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the two main groups in which secondary xylem can be found are:
- 7. Conifers (Coniferae): there are some six hundred species of conifers. All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is used and marketed as soft wood. Angiosperms: there are some quarters of a million to four hundred thousand species of Angiosperms. Within this group secondary xylem is rare in the monocots. Many monocot Angiosperms become trees, and the secondary xylem of these is used and marketed as hard wood.
- 8. Protoxylem and metaxylem • As a young vascular plant grows, one or more strands of primary xylem form in its stems and roots. • The first xylem to develop is called 'protoxylem'. In appearance protoxylem is usually distinguished by narrower vessels formed of smaller cells. • Some of these cells have walls which contain thickenings in the form of rings or helices.
- 9. • Functionally, protoxylem can extend: the cells are able to grow in size and develop while a stem or root is elongating. Later, 'metaxylem' develops in the strands of xylem. • Metaxylem vessels and cells are usually larger; the cells have thickenings which are typically either in the form of ladder like transverse bars (scalariform) or continuous sheets except for holes or pits (pitted). Functionally, metaxylem completes its development after elongation ceases when the cells no longer need to grow in size.
- 10. Patterns of protoxylem and metaxylem There are four main patterns to the arrangement of protoxylem and metaxylem in stems and roots Centrarch refers to the case in which the primary xylem forms a single cylinder in the center of the stem and develops from the center outwards. The protoxylem is thus found in the central core and the metaxylem in a cylinder around it. This pattern was common in early land plants, such as "Rhyniophytes", but is not present in any living plants.
- 11. Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside towards the center, i.e., centripetally. The metaxylem is thus closest to the center of the stem or root and the protoxylem closest to the periphery. The roots of vascular plants are normally considered to have exarch development. Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside towards the periphery, i.e., centrifugally. The protoxylem is thus closest to the center of the stem or root and the metaxylem closest to the periphery. The stems of seed plants typically have endarch development.
- 13. Mesarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the middle of a strand in both directions. The metaxylem is thus on both the peripheral and central sides of the strand with the protoxylem between the metaxylem (possibly surrounded by it). The leaves and stems of many ferns have mesarch development.