Hsua 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 extinct vascular plant

Vascular plant

Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...

s, known from the Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

. its name of the genus honours the Chinese palaeobotanist Jen Hsu.

Features

The main stems (axes) of Hsua robusta are about an inch thick, with circinate, pseudo-monopodial side branches emerging from the sides.
The small side branches emerge immediately above a dichotomous branching point of the main axis. The lateral branches are in a plane (planar). In the centre of the axes is a protostele with an eliptical cross-section. The protoxylem was probably inside the xylem

Xylem

Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

, maturing from the inside out. The tracheid

Tracheid

Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...

s have annular secondary wall thickening.

Round- to kidney-shaped sporangia lie on dichotomous branches at the end of the lateral branches, and open with symmetrical valves.

The trilete spores are 18 to 36 µm in size.

The 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....

 is unknown.

In Hsua deflexa, the main axis was creeping with the lateral axes at right angles from it. The axes had thorny spikes.

Preservation

Hsua robusta is very well preserved in chert

Chert

Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...

s and as compression fossil

Compression fossil

A compression fossil is a fossil preserved in sedimentary rock that has undergone physical compression. While it is uncommon to find animals preserved as good compression fossils, it is very common to find plants preserved this way...

s in the Xujiachong Formation, Yunnan

Yunnan

Yunnan is a province of the People's Republic of China, located in the far southwest of the country spanning approximately and with a population of 45.7 million . The capital of the province is Kunming. The province borders Burma, Laos, and Vietnam.Yunnan is situated in a mountainous area, with...

, China

China

Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

. Hsua deflexa comes from the same formation, which is Early Devonian (~Pragian to Emsian, around ).

Systematics

The terminal sporangia of Hsua robusta placed it in the traditional family Cooksoniaceae (order Rhyniales). Kenrick and Crane, based on a cladistic analysis, placed it in its own family, Hsuaceae, in the order Sawdoniales

Sawdoniales

The Sawdoniales were among the first vascular plants in the fossil record. They are zosterophylls, descending from the same ancestors that modern lycophytes evolved from. They bore lateral, reniform sporangia, branched dichotomously, and grew at the ends by unrolling...

, based on spore characteristics.