Ferns are a primitive group of

land plants that first appeared in

the fossil record towards the

end of the Devonian, about 360

million years ago. They were an important food source for dinosaurs that

roamed the Earth from 230 to 65 million years before present times,

although many fern families, genera and species did not appear until the

early Cretaceous 150 million years ago.

There are more than 12,000 extant fern species throughout the world,

found on every continent with the exception of Antarctica. They are

most abundant in moist tropical and warm temperate areas although they

can survive surprisingly low temperatures, and grow in all but the most

extreme hot, dry deserts. In Australia, there are almost 400 endemic

fern species.

Pteridophyte is a loose term that encompasses four major groups:

Polypodiophyta (Filicopsida) True Ferns

and three clades that do not produce the conspicuous fronds we associate with

ferns:

Psilophyta Whisk Fern, Skeleton Fork Fern Lycopodiophyta

Lycopodiaceae Clubmosses

Selaginellaceae Spikemoss or Clubmoss

Isoetaceae Quillworts Equisetophyta

Equisetaceae Horsetails

Ferns, characteristically, have well developed

fibrous roots, stems (rhizomes) and green

leaves (fronds) connected by a vascular system that is not present in mosses or

liverworts. This was integral to ferns moving

away from lakes, streams, bogs and rivers to

Elongated sori on margins of fronds of Asplenium bulbiferum

Two sporangia in each sorus of Coral Fern

drier areas. Roots provide access to

underground water and nutrients, as well

as anchoring the plant to the ground,

while phloem transports sugars from

photosynthesis throughout the plant.

Development of a waxy layer (cuticle) on

the leaf surface limited water loss, and

stomates (leaf pores) enabled gaseous

exchange. As for shape and form, the

elegantly curled heads of emerging fern

fronds are a model for leaf development:

they are called fiddleheads, or croziers.

Like bryophytes, ‘true ferns’ reproduce by

spores which are mounted on the

undersurface of leaves in sporangia.

These, in turn, are usually aggregated into sori. The position of sori on

the undersurface of fronds and their characteristic shape are features

that are used by botanists to identify ferns.

When spores released from the sori germinate on suitable, and usually

moist, substrates, they grow into what is known as a prothallus (a haploid

gametophyte); this is a small, delicate, usually heart-shaped structure

just a few mm across. Gametes (sperm and eggs) are produced from the

gametophyte, and, as long as there is a film of water across the

prothallus, mobile, flagellate sperm swim across the surface to fertilise

female egg cells, ultimately resulting in the development of the diploid

Hymenophyllum dilatatum, a filmy fern

Tree ferns can be many metres high

sporophyte. This will then grow much larger to become a readily

recognisable fern. Many species can also reproduce asexually from

sections of rhizome, and from plantlets that occur

on leaf margins.

Ferns can grow in a

great diversity of

habitats; aquatic,

terrestrial, epiphytic

(growing on trees or

tree branches), epilithic

(growing on rock). They

also have an

extraordinarily diverse

range of growth forms,

from tiny free floating

aquatic plants, to leafy,

green herbaceous plants

to arborescent tree ferns that can grow many

metres tall. Filmy ferns which have soft,

delicate leaves, often only a few cells thick, are often mistaken for

mosses or liverworts. Ferns, like mosses, don’t grow in marine

environments.

Ferns may be very decorative and are much sought after as house plants

but contribute much less to landscapes and global cycles than bryophytes

(mosses). One exception is the floating fern Azolla which hosts nitrogen-

fixing cyanobacteria, an

important, sustainable

biological fertilizer in Asian

rice fields. And we should not

forget that ancestors of

modern ferns were a

significant component of

Carboniferous and Permian

coal-forming ecosystems.

Australian National Herbarium, Canberra. https://www.anbg.gov.au/fern/taxa/pteridophyte.html Australian Plants Society, Central Coast Group: http://www.australianplants.org/fsgseven.htm Plantnet: http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=cl&name=FILICOPSIDA Rainforest Australia: http://rainforest-australia.com/Rainforest_ferns.htm Wikipedia: https://en.wikipedia.org/wiki/Fern

Alison Downing, Brian Atwell, Kevin Downing

Department of Biological Sciences