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My textbook "CGP AS-Level Biology Exam Board: Edexcel Complete Revision & Practice" says xylem is a tissue.

Then I read from this website that "[Xylem's] major components include xylem parenchyma, xylem fibers, tracheids, and xylem vessels." Therefore, from my understanding, xylem is made up of multiple tissues including xylem parenchyma tissues, xylem vessel tissues and etc to carry out the functions of transporting water and nutrients in plants? Therefore that would make xylem an organ instead of a tissue?

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In plants, there are three types of tissue: meristematic, simple, and complex.

Meristematic (embryonic and totipotent)

Simple (composed of only one type of cell), such as

  • parenchyma
  • collenchyma
  • sclerenchyma

Complex (composed of more than one type of cell), such as

  • dermal (protective covering)
  • vascular (conducting tissue)
  • xylem (conducts water and dissolved minerals)
  • phloem (conducts water and dissolved organics)
  • ground (bulk of the body; primarily parenchyma, collenchyma & sclerenchyma)

With this information in mind, let's take a look at what the source you provided says that makes up Xylem tissue.

"[Xylem's] major components include xylem parenchyma, xylem fibers, tracheids, and xylem vessels

Here are the roles of said cells in Xylem:

  • Xylem parenchyma are cells specific to storage and short distance transport.

  • Sclerenchyma Cells (xylem fibers) are specific to support and protection.

  • Vessel Members (which are conducting cells) are specific to water, mineral transport, and support mostly in advanced angiosperms.

  • Tracheids (which are also conducting cells) are specific to water, mineral transport, and support mostly in gymnosperms and lower angiosperms.

I hope this information helped you understand why Xylem is not an organ, but rather a complex tissue of plant.

References:

http://facweb.furman.edu/~lthompson/bgy34/plantanatomy/plant_cells.htm

http://www.bio.miami.edu/dana/226/226F09_5.html

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  • $\begingroup$ Thank you very much for the detailed answer. May I then ask what is the fine line that distinguishes complex tissue and organ? (i.e. at what point can we say something is an organ and not a complex tissue?) $\endgroup$
    – Bøbby Leung
    Feb 18, 2019 at 10:58
  • $\begingroup$ Ok, so just disregard the accessory word 'complex' for a moment and think about just tissue. A tissue is the combination of cells. Xylem is made up of a collective of specialized cells in order to perform its tasks. An organ is made up of a collective of tissues. The reason xylem is a complex tissue is because different cells make it up, whereas in a simple tissue is made up of the same cells. Hope this helped! $\endgroup$
    – Sartoaster11
    Feb 20, 2019 at 22:53
  • $\begingroup$ Thanks again. But I'm still not sure at what point can we say something is an organ and not a tissue. In the case of xylem, your argument is that it is a tissue since it is made up of different cells to perform its tasks. But can't I also say, xylem is made up of different tissues to perform its task e.g. xylem parenchyma tissue, sclerenchyma tissue, vessel members tissue and tracheids tissue - since tissue is simply a combination of cells. Thereby, making xylem an organ? $\endgroup$
    – Bøbby Leung
    Feb 24, 2019 at 5:01

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