Notothylas
Encyclopedia
Notothylas is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of hornwort
Hornwort
Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...

s in the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Notothyladaceae
Notothyladaceae
The Notothyladaceae is the only family of hornworts in the order Notothyladales....

. The genus is found globally, but is usually overlooked. It is the smallest of all the hornworts, with a yellow-green gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....

 thallus
Thallus (tissue)
Thallus, from Latinized Greek θαλλός , meaning a green shoot or twig, is the undifferentiated vegetative tissue of some organisms in diverse groups such as algae, fungus, some liverworts, lichens, and the Myxogastria. Many of these organisms were previously known as the thallophytes, a polyphyletic...

 that is seldom more than a centimeter in diameter, and usually much smaller.

The genus Notothylas is also unusual among hornworts in that the sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...

 is bullet-shaped and does not grow very large (less than two millimeters). The sporophytes grow outwards rather than upwards, and like Megaceros
Megaceros
Megaceros is a genus of hornworts in the family Dendrocerotaceae. The genus is found in the Old World tropics of east Asia and Australia. Its name means 'big horn', and refers both to the exceptionally large size of the gametophyte thallus and to the large, horn-shaped sporophyte that the plants...

, there are no stoma
Stoma
In botany, a stoma is a pore, found in the leaf and stem epidermis that is used forgas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening...

ta on the surface of the sporophyte. The elater
Elater
An elater is a cell that is hygroscopic, and therefore will change shape in response to changes in moisture in the environment. Elaters come in a variety of forms, but are always associated with plant spores...

 cells do not grow helical thickenings.

A number of classification systems place Notothylas in its own order Notothyladales (frequently misspelled Notothylales in the literature). This classification is based on the assumption that the unique physical characteristics of the genus reflect an early divergence from other hornworts. However, this assumption is not supported by either phylogenetic analysis or fossil evidence. More recent classifications expand the definition of the family Notothyladaceae
Notothyladaceae
The Notothyladaceae is the only family of hornworts in the order Notothyladales....

to include four other genera.
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