Beginning in the 8th century BCE, however, the Greek alphabet became standard, albeit with some variation among dialects.
Structure[ edit ] Cross section of some xylem cells The most distinctive xylem cells are the long tracheary elements that transport water. Tracheids and vessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are called vessels.
In transitional stages of plants with secondary growththe first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only. The branching pattern exhibited by xylem follows Murray's law. It includes protoxylem and metaxylem.
Metaxylem develops after the protoxylem but before secondary Thesis greek root word. Metaxylem has wider vessels and tracheids than protoxylem. Secondary xylem is formed during secondary growth from 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 Cycadophytathe two main groups in which secondary xylem can be found are: All species have secondary xylem, which is relatively uniform in structure throughout this group.
Many conifers become tall trees: Within this group secondary xylem is rare in the monocots. Main function — upwards water transport[ edit ] The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water conducting channels reaching all parts of the plants.
It transports water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well.
The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees.
Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals. The phloem pressure can rise to several MPa,  far higher than atmospheric pressure.
Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure. Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant.
This causes millions of minute menisci to form in the mesophyll cell wall. The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil. If the water potential of the root cells is more negative than that of the soilusually due to high concentrations of solutewater can move by osmosis into the root from the soil.
This causes a positive pressure that forces sap up the xylem towards the leaves. In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is highest in the morning before the stomata open and allow transpiration to begin.
Different plant species can have different root pressures even in a similar environment; examples include up to kPa in Vitis riparia but around zero in Celastrus orbiculatus. When transpiration removes water at the top, the flow is needed to return to the equilibrium. Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves.
This evaporation causes the surface of the water to recess into the pores of the cell wall. By capillary actionthe water forms concave menisci inside the pores.
The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree 's highest branches.
Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. Cohesion-tension theory[ edit ] The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards against the force of gravity through the xylem of plants.
When two water molecules approach one another, the slightly negatively charged oxygen atom of one forms a hydrogen bond with a slightly positively charged hydrogen atom in the other.Xylem is one of the two types of transport tissue in vascular plants, phloem being the other.
The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports nutrients. The word "xylem" is derived from the Greek word ξύλον (xylon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant.
Greek Evidence on the Authenticity of the Macedonians Hristo Andonovski (Macedonian Review, 1/) The Republic of Macedonia and the Republic of Greece had good diplomatic and neighborly relations in the past, although certain Greek people regarded the Macedonian state a thorn in their flesh and occasionally some extreme .
Chaos. In Greek mythology, Chaos or Khaos is the primeval state of existence from which the first gods appeared. In other words, the dark void of space.
Dr. Bill Bailey's Home Page. Dr. Bailey is known among his friends as a strange but fairly interesting individual; his site is presented for your enjoyment, and to boost his already fairly large ego! Dione. Three goddesses from the Parthenon east pediment, possibly Hestia, Dione, and Aphrodite, c.
BC (British Museum) Dione in Greek mythology was a Greek goddess primarily known as the mother of Aphrodite in Book V of Homer's Iliad. Link: Root word: Meanings: Origin: Examples and Definitions: a/n: not, without: Greek: abyss - without bottom; achromatic - without color; anhydrous - without water.