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5 November 2021 4 min read

In 2020, citizen scientist Ruth Palsson was searching for hornworts in the Pilliga, near Narrabri in NSW. She was hoping to find an undescribed species known to grow in the area. Instead, Ruth unearthed two new species of hornworts.

Ruth’s first sighting of the hornworts now named Anthoceros apocynon and A. palssoniae. They are the darker green plants among the daisies in the foreground. Image: Ruth Palsson

Ruth sent her specimens to Dr Chris Cargill at the Australian National Herbarium in Canberra, where she had completed a botanical traineeship in 2015. Together, they wrote a paper describing the tiny plants.

They named the hornworts Ruth found Anthoceros apocynon and A. palssoniae. Chris named the hornwort Ruth had been searching for, which had originally been collected in 2010 by citizen scientist and retired geologist Dr Peter Wellman, A. wellmanii.

All three species seem to be quite rare, perhaps appearing only in years of heavy rain following drought.

"Ruth and I revisited the area again in June this year, but couldn't find any of these species," says Chris.

"However, we did find another undescribed species that for now we are calling Anthoceros 'frilly', due to the many small scale-like lamellae growing perpendicular to the surface of the plant."

A curious life cycle

Hornworts are not common plants. Australia has only 24 described species and there are only between 200 and 250 hornwort species worldwide.

Like mosses and liverworts, hornworts are bryophytes and don’t produce seeds. Instead, they produce spores and, like all land plants, grow in two different phases known as alternation of generations. These phases are much more distinct in hornworts than in many more familiar plants, resulting in what might appear to be two quite different plants.

The newly named hornwort Anthoceros palssoniae (right) and a Phaeoceros hornwort (left). Image: Ruth Palsson

The photo above show hornworts in two different phases of their life cycles. The lettuce leaf-like structure, or thallus, on the left is a gametophyte. The horn-like structures on the right are sporophytes.

Gametophytes produce eggs and sperm. Hornworts tend to grow in wet areas because they require water for the motile sperm to swim to the eggs, either on the same plant or a nearby plant. The fertilised egg, called a zygote, grows on the parent gametophyte, forming a sporophyte. Sporophytes release spores that travel by air or water, or hitch a ride on an animal, before growing into new gametophytes.

"The area near Narrabri is not known for its bryophyte flora, let alone its hornwort flora. People interested in hornworts don’t usually look for them in dry places," says Chris.

"More than likely the spores of these species have been sitting in the soil for years, with the good rains in eastern Australia last year creating the right conditions for these spores to germinate and grow. But we do not really know for sure, because there is nobody monitoring these populations on a seasonal basis, year in and year out."

Recognising new species of hornworts

All hornworts have shared features, the most obvious being the horn-like capsules that produce spores. Hornworts have symbiotic relationships with cyanobacteria, usually Nostoc, to fix nitrogen. The colonies form small black dots in the body of the plant. Hornworts also produce copious amounts of mucilage internally, which helps to protect their sex organs, developing spores and their colonies of cyanobacteria.

"Species of hornworts belonging to the genus Anthoceros also have cavities or holes throughout the plant body filled with mucilage. We don’t know why," says Chris.

"If you find an Anthoceros species in the wild, you might be able to recognise it by its frilly thallus. But knowing exactly which species is which requires comparing the patterns on their microscopic spores.

"Sporophytes produce spores in groups of four, joined in a tetrad at their proximal faces. When the spores mature, the tetrad breaks apart into four individual spores.

"All three new species of Anthoceros share a common feature on the proximal side of their spores. They have a smooth, unornamented strip that forms a Y shape.

"The patterns seen in these new species haven’t previously been found in any species in Australia. But they have been found in species from India and Southeast Asia."

Scanning electron micrographs of A. apocynon spores, showing the different patterns on their proximal and distal sides. The smooth strip forming a Y shape on the proximal side is shared by all three new Anthoceros species. Image: Chris Cargill

A library of plants

Specimens of each of the new species of hornworts are in the cryptogam collection of the Australian National Herbarium. It contains specimens of more than 400 000 ferns, lichens, fungi, algae, mosses, liverworts and hornworts mainly from Australia, Antarctica and the region, but also from across the world. The collection is a resource for biodiversity discovery and conservation.

The Australian National Herbarium is part of the Centre for Australian National Biodiversity Research, a joint venture between Parks Australia’s Australian National Botanic Gardens and the National Research Collections Australia at CSIRO.

Read more about the world of cryptogams.

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