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zoid
In botany, a zoid or zoïd is a reproductive cell that possesses one or more flagella, and is capable of independent movement.〔Hoek, C. van den, Mann, D. G. and Jahns, H. M. (1995). ''(Algae : An introduction to phycology )'', Cambridge University Press, UK.〕 Zoid can refer to either an asexually reproductive spore or a sexually reproductive gamete. In sexually reproductive gametes, zoids can be either male or female depending on the species. For example, some brown alga (''Phaeophyceae'') reproduce by producing multi-flagellated male and female gametes that recombine to form the diploid sporangia.〔Heesch, S. and Peters, A.F. (1999). Scanning electron microscope observation of host entry by two brown algae endophytic in ''Laminaria saccharina''. ''Phycological Research''. Japan.〕 Zoids are primarily found in some protists, diatoms,〔 green alga, brown alga,〔Lobban, Christopher S. and Wynne, Michael James. (1981). The biology of seaweeds. University of California Press, US.〕 non-vascular plants,〔Paolillo, Dominick J. Jr. (1981). The swimming sperms of land plants. ''Bioscience''. Volume 31. Issue 5. pp 367-373. University of California Press.〕 and a few vascular plants (ferns,〔 cycads,〔Norstog, Knut J. and Gifford, Ernst M. and Stevenson, Dennis W. (2004). Comparative development of the spermatozoids of ''Cycads'' and ''Ginkgo biloba''. ''The Botanical Review''. Volume 70. pp 5-15. The New York Botanical Garden.〕 and ''Ginko biloba''〔Ogura, Y. (1967). History of discovery of spermatozoids in ''Ginkgo biloba'' and ''Cycas revoluta''. ''Phytomorphology''. Volume 17. pp 109-114. University of Tokyo, Japan.〕). The most common classification group that produces zoids is the heterokonts or stramenopiles. These include green alga, brown alga, oomycetes, and some protists.〔Anderson, Robert A. (2004). Biology and systematics of heterkont and haptophyte algae. ''American Journal of Botany''. Volume 91. Issue 10. pp 1508-1522. St. Louis, MO.〕 The term is generally not used to describe motile, flagellated sperm found in animals. Zoid is also commonly confused for zooid which is one single-celled organism that is part of a colonial animal.〔Zooid. Zooid. Accessed 4/15/12.〕
== Diversity of zoids ==
A zoid contains one or more flagella for motility. In the various species that produce zoids, there is a high level of diversity in the number of flagella produced. The heterokonts generally produce zoids with 2 flagella,〔 while the ''Gingko biloba'' produce zoids with tens of thousands of flagella.〔 The position of the flagella and the arrangement of the microtubules varies among species as well. The following sections will briefly outline general characteristics of the zoids found in each subset as well as provide specific examples.
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