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Thallus or Thallos (Greek: Θαλλός), perhaps a Samaritan, [1] was an early historian who wrote in Koine Greek. He wrote a three-volume history of the Mediterranean world from before the Trojan War to the 167th Olympiad , 112–108 BC, or perhaps to the 217th Olympiad (AD 89-93) or 207th Olympiad (AD 49-52).
Thallus (pl.: thalli), from Latinized Greek θαλλός (thallos), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria.
Lichens are grouped by thallus type, since the thallus is usually the most visually prominent part of the lichen. Thallus growth forms typically correspond to a few basic internal structure types. Common names for lichens often come from a growth form or color that is typical of a lichen genus. Common groupings of lichen thallus growth forms are:
The thallus of a crustose lichen has a patchwork or crazy-paving appearance. The patches, or areolae, can be as large as 1 cm in diameter or very small and raised, giving them the appearance of a wart. The surface of the thallus is generally smooth, however it is sometimes broken up by “rimose” cracks.
The primary thallus is composed of small, overlapping scales, while the secondary thallus (which supports the lichen's fruiting structures) is fruticose in appearance. [21] These secondary thalli, which are known as podetia, can be branched, spike-like or cup-shaped. [6]
The thallus has little to no tissue differentiation, being composed of thin, compactly arranged uniform parenchymatous cells. Anthoceros species are host to species of Nostoc , a symbiotic relationship in which Nostoc provides nitrogen to its host through cells known as heterocysts , and which are able to carry out photosynthesis . [ 3 ]
The thallus is perennial with an irregular or disc-shaped holdfast or with haptera. [1] The erect portion of the thallus is dichotomous or subpinnately branched, flattened and with a distinct midrib. Gas-filled pneumatocysts (air-vesicles) are present in pairs in some species, one on either side of the midrib.
Lichen growth forms are used to group lichens by "vegetative" thallus types, and forms of "non-vegetative" reproductive parts. Some lichen thalli have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens) ( illustration, right ), others such as the genus Ramalina adopt shrubby forms (fruticose ...