AskDefine | Define thallus

Dictionary Definition

thallus n : a plant body without true stems or roots or leaves or vascular system; characteristic of the thallophytes [also: thalli (pl)]

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Noun

thallus
  1. an undifferentiated plant body, as in the thallophytes

Extensive Definition

For the chronologer/historian occasionally mentioned in the works of early Christian writers, see Thallus (historian).
Thallus, from Latinized Greek θαλλος (thallos), meaning a green shoot or twig, is an undifferentiated vegetative tissue of some non-mobile organisms, which were previously known as the thallophytes.
An organism or structure resembling a thallus is called thalloid, thallodal, thalliform, thalline, or thallose.
Even though thalli do not have organized and distinct parts (leaves, roots, and stems) as do the vascular plants, they may have analogous structures that resemble their vascular "equivalents". The analogous structures have similar function or macroscopic structure, but different microscopic structure; for example, no thallus has vascular tissue.
Although a thallus is largely undifferentiated in terms of its anatomy, there can be visible differences and functional differences. A kelp, for example, may have its thallus divided into three regions. The parts of a kelp thallus include the holdfast (anchor), stipe (supports the blades) and the blades (for photosynthesis).
The thallus of a fungus is usually called a mycelium. In seaweed, thallus is sometimes also called frond.
The gametophyte of some non-thallophyte plants -- clubmosses, horsetails, and ferns is termed "prothallus".
thallus in Danish: Thallus
thallus in German: Thallus
thallus in Esperanto: Talo
thallus in French: Thalle
thallus in Italian: Tallo
thallus in Hebrew: יצע
thallus in Georgian: თალუსი
thallus in Hungarian: Teleptest
thallus in Macedonian: Талус
thallus in Dutch: Thallus
thallus in Japanese: 葉状体
thallus in Norwegian: Thallus
thallus in Polish: Plecha
thallus in Portuguese: Talo
thallus in Russian: Таллом
thallus in Serbian: Талус
thallus in Finnish: Sekovarsi
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