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| Xylem is caused from ya mum having a big cock | |||
| In ]s, '''xylem''' is the ] that carries water up the root and stem. In ]s, it constitutes ] — hence the word is derived from ] ξύλον ''xúlon'' "wood, timber". Together with ], xylem is one of the two transport tissues of plants. The ]s of xylem ]s derive most of their strength from ], a chemical compound produced only by plants. | |||
| Xylem (at least in ]s) is composed of ]s and ]s. Vessel elements are similar in structure to the sieve-tube members of phloem, but they lack companion cells, and have perforated sides as well as ]s at the ends. Tracheids are much narrower cells, with tapered and perforated ends, constituting most of the volume of the xylem tissue. Both tracheids and vessel elements are dead at maturity. | |||
| Xylem sap always moves from the ]s to the ]. It travels by ''bulk flow'', like water in a series of pipes, rather than by diffusion through ]s. Three phenomena cause xylem sap to flow: | |||
| * The soil solution (see ]) is more dilute than the ] of the root cells. Thus, water moves ] into the cells, creating ''']'''. Even under optimal conditions, root pressure can only lift water a couple of feet. | |||
| * ''']''' helps sap to flow up the narrow tracheids. | |||
| * By far the most important cause of xylem sap flow is ''']'''. This is the reverse of root pressure, caused by the ] of ] from leaves. | |||
| In ] plants, xylem is laid down in multiple phases. ''Primary'' xylem is one of the tissues left behind by the ]. ''Secondary'' xylem is laid down by ] on the outside of the xylem column. | |||
| ==Reference== | |||
| *Campbell, Neil A. and Jane B. Reece. undated. ''Biology'', 6th ed., published by Benjamin Cummings. | |||
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Revision as of 06:55, 18 August 2004
Xylem is caused from ya mum having a big cock