Basic Structure Of Secondary Xylem PDF

Basic structure of the secondary xylem Secondary xylem is distinguished into two systems which differ in their orientation and origin. The first is the horizontal system which originates from the ray initials and comprises the xylem rays. The second is the vertical (axial) system which originates...

Basic structure of the secondary xylem Secondary xylem is distinguished into two systems which differ in their orientation and origin. The first is the horizontal system which originates from the ray initials and comprises the xylem rays. The second is the vertical (axial) system which originates from the fusiform initials and comprises the tracheary elements, xylem fibers and axial parenchyma. Xylem parenchyma There are two types of xylem parenchyma. The first is the ray parenchyma, which originates from the cambial ray initials; while the second is the axial parenchyma which originates from the cambial fusiform initials. Xylem parenchyma, store starch, fats, tannins and other substances. a- Ray parenchyma The length of the ray is determined in both cross section and radial longitudinal section (R.L.S.). The width of the ray is determined in tangential longitudinal section (T.L.S.). The height of the ray is determined in both R.L.S. and T.L.S. Sometimes, ray parenchyma extends externally through the phloem and internally through the xylem reaching to the pith; then it is termed as medullary rays. Trans. section Trans. section Trachieds Xylem vessels Xylem ray Axial parenchyma Tang. Long. Sect. (T.L.S.) Tang. Long. Sect. (T.L.S.) Three anatomical planes of a conifer Three anatomical planes of the secondary (Gymnosperm) secondary xylem. xylem of a dicot. Stem (angiosperm) 12  b- Axial parenchyma There are two basic types of the axial parenchyma according to its distribution among the other elements of the secondary xylem. I-Apotracheal In which the axial parenchyma distribution is independent of the vessels. Apotracheal axial parenchyma are subdivided into the following types: 1- Diffuse parenchyma The axial parenchyma is in the form of uniseriate strands or single cells scattered irregularly among the fibers. 2- Banded (metatracheal) parenchyma The axial parenchyma is in the form of concentric bands. 3- Terminal parenchyma Are the parenchyma cells which formed at the end of the growth ring in the form of single cells or continuous layers. 4- Initial parenchyma Are the parenchyma cells which formed at the beginning of the growth ring in the form of single cells or continuous layers. II-Paratracheal In which the axial parenchyma distribution is associated with the vessels. Paratracheal axial parenchyma are subdivided into the following types: 13  1-Scanty paratracheal parenchyma The parenchyma form an incomplete sheath around the vessel. 2-Unilaterally paratracheal parenchyma The parenchyma occurs on one side of the vessels; either external (abaxial) or internal (adaxial). 3-Vasicentric paratracheal parenchyma The parenchyma form an entire sheath (circular or elliptical) around the vessel. 4-Aliform paratracheal parenchyma The parenchyma form an entire sheath, with lateral wing-like extensions, around the vessel. 5-Confluent paratracheal parenchyma Are the aliform parenchyma which form diagonal or tangential bands. Tyloses Xylem parenchyma constitute a significant fraction of the xylem in woody plants. It has many important functions as the storage of materials (as mentioned before) and the radial transfer of water between xylem and phloem through the ray parenchyma. Also, ray and axial parenchyma play a role against fungal pathogens in the secondary xylem. These cells are usually connected with xylem vessels and tracheids via pits and in many plants form outgrowths which penetrate through the pits into the lumen of the vessels or tracheids when the 14 latters become inactive or injured. These outgrowths are termed tyloses (singular: tylosis). The nucleus and part of the cytoplasm of the parenchyma cell, from which the tylosis is formed, migrate to the tylosis. Secondary phloem Secondary phloem elements have a similar arrangement to that of the secondary xylem. Elements of the vertical system originate from the fusiform initials and comprise the sieve elements, phloem parenchyma and phloem fibers. The horizontal system arises from the ray initials and comprise phloem ray parenchyma. Like the secondary xylem; secondary phloem either storied or non-storied. Also, annual growth rings can be observed in some plants. In conifers (type of Gymnosperms), the vertical system comprises sieve cells, phloem parenchyma, albuminous cells and phloem fibers. The phloem rays of conifers are usually uniseriate and usually consists of parenchyma only, but sometimes albuminous cells may also be present. In dicotyledons, the vertical system contains sieve-tube members, companion cells, axial parenchyma and xylem fibers. The horizontal system consists of uni, bi and multiseriate rays which consists of parenchyma cells only. 15