Ferns
Above: ferns, ivy and other scrambling plants growing in the field-layer. This one looks like
Dryopteris felix-mas, the Male Fern.

The 'field-layer' is defined differently by different people. Sometimes it refers to the 'herb layer'
where herbaceous plants grow, ranging in height from a few inches to a couple of metres. This is
the definition used here. Others take it to mean the 'ground level' where mosses, microscopic
algae and lichens grow on the soil surface.

Filicales

Above the frond of a fern. A frond is a large leaf, also called a megaphyll. Ferns are a type of
non-flowering plant, and a remarkably ancient lineage that resemble living fossils. However, far
from this they are still remarkably successful despite the later appearance of the seed-bearing
and flowering plants. Indeed,
bracken, Pteridium aquilinum, is possibly the most populous
vascular plant on Earth!

The frond shown above, resembling a branch, is actually a single leaf. It is a
compound leaf
made up of smaller
leaflets. Typical of many ferns, this frond is pinnate, or feather-like,
meaning that a series of leaflets or
pinnae (singular: pinna) arise on either side of the main-axis
(
rachis). The leaf stalk or petiole of a frond is called the stipe and this continues as the main
axis, or rachis, of the frond, throughout its length. The part of the leaf bearing pinnae is called
the
lamina (plural: laminae) or blade. In the example above the pinnae are alternate, but in some
forms they may be directly opposite one-another on the rachis, in pairs. The pinnae may be
simple lobes, but in many ferns, including the one above, the pinnae are themselves pinnate or
feathery and divided into secondary pinnae or
pinnules. Each pinna has a side-branch of the
rachis, called the costal vein, travelling along its length and the pinnules are borne on this. Such
a frond, in which the pinnae are themselves pinnate, is called a
bipinnate or twice-pinnate frond.

The fern above belongs to the order Filicales, the largest order of ferns and the one we consider
first. There are about 10 000 fern species recognised as living today. About 300 genera, and
most of these 10 000 species, belong to the Filicales.
The fronds of many ferns, like bracken, are tough and rough in texture. These fronds have a
typical leaf structure consisting of palisade and spongy mesophyll (see
leaves). Typically there is
a thick waterproofing cuticle on top of the leaf and stomata underneath. Some ferns, however,
have very thin laminae consisting of a single layer of cells and so resembling the fronds of certain
algae in this respect. These so-called filmy-ferns require moist conditions.

The fronds of a typical fern develop as vertical spirals which uncoil and straighten as the frond
develops. This folding/coiling is called
circinnate vernation.

The Life-Cycle of Ferns

Ferns undergo alternation of generations. The main plants that we see are the sporophyte
generation. The sporophyte is
diploid (that is each of its cells contains two copies of every
chromosome, 2n, one from each parent). (Homosporous) ferns can have more chromosomes
than any other organism on Earth, with a typical species having about 100 (n = 50) and up to
1260 occuring (n = 1130)! This is thought to have arisen chiefly by
chromosome fission - the
fragmentation of several large chromosomes into more smaller ones, rather than by polyploidy
(the duplication of chromosome pairs).

This alternates with a much smaller and less conspicuous
gametophyte generation which is
haploid (having only one copy of every chromsome, n).

The fronds of the sporophyte may be sterile, but when ripe they may become fertile fronds. The
pinnae or pinnules of such a frond develop sac-like structures, each up to a few millimetres long,
on their undersurface. These are the
sori (singular sorus). Each sorus is actually a shield-like
structure, called the
indusium, covering a cluster of spore-containing vessels or sporangia
(singular: sporangium). Each sporangium is a tiny capsule borne on a stalk. The stalk may be
one or several cells thick. Each sporangium has groups of hardened cells, with thickened walls,
often forming a trip or ring called the
annulus, and some sort of terminal pore or slit called the
stomium. The annulus encircles the sporangium either tranversely or vertically or at an oblique
angle and is interrupted by the stomium.

When the sporangium dries, the thickened cells lose water and the thickenings of their cell walls
are such that uneven stresses are set-up in the drying cells. These stresses try to straighten the
annulus, and eventually the sporangium splits across the stomium and the upper half of the
sporangium folds back like a lid opening on a hinge. Initially the lid moves back slowly, then at a
critical point, the water in the thickened annular cells evaporates and gas bubbles develop within
these cells and the lid rapidly flicks backwards, dispersing the spores.
Above: sori on the underside of a pinna on a ripe fertile frond of Dryopteris felix-mas. If you look
at the under-surface of a frond tip, then if it is fertile the sori are usually quite visible. In
Dryopteris, for example the sori are large and reniform (kidney-shaped). In some ferns sori are
not produced, but many individual sporangi instead form a felt-like covering to the pinna
under-surface. The sporangia in a sorus may be all of the same age, but in some species they
develop in a sequence along on an elongating axis called the receptacle (a so-called
gradate
sorus
, in which older sporangia are toward the base of the receptacle). In some the sporangia
are of different ages but all mixed together, a
mixed sorus, as in Dryopteris and most common
temperate Filicales.

Ferns may be homosoprous or heterosporous. In the homosporous frens the spores are all of
the same size and type and about 40 to 80 micrometres in diameter. The spore mother cells
develop inside the sporangium enclosed in two layers of the sporangium wall, called the
tapetum, which disintegrates to nourish and complete the development of the developing
spores. The outermost part of the spore wall, the
exine, is formed largely of materials deposited
from the disintegrated tapetum. The exine has a distinctive pattern of bars and ridges which can
aid taxonomic identification. The spores can usually remain viable for two to three years (though
in some species of fern they contain chlorophyll and are short-lived). The spores are produced
from the spore mother cells by
meiosis, a reduction division in which the number of
chromsomes are halved so that only one copy of each is present - the spores are haploid.

The Gametophyte Generation

The sporophyte is the dominant generation and is diploid and produces haploid spores.
Germination of the spore gives rise to a chain of cells, or filament, resembling certain
microscopic algae. It is photosynthetic, independent of the sporophyte, and grows by cell
division at the tip. This is the developing gametophyte and in some forms it remain filamentous,
but in most, including
Dryopteris, the apical cell begins to divide in other planes and a
heart-shaped (
cordate) gametophyte results, anchored by small root-like rhizoids. Spore
germination requires moisture and light. The gametophyte usually requires continually moist
conditions to grow and resembles a liverwort bryophyte in form. Those of some species can
survive occasionaly drying out. Most are also sensitive to sub-zero temperatures, requiring a
period of warmth. In some forms the gametophyte is subterranean, growing beneath the soil
surface. Development of the gametophyte depends on gradients of the phytohormone
auxin
which is secreted by the apical cell (the apical meristem or growth zone). Since the spores were
haploid and the gametophyte is produced by copy cell divisions or mitosis, the
gametophyte is
halpoid
.

The gametophyte is the sexual generation. The small body of the gametophyte is called the
prothallus. Typically, the gametophyte develops male organs first, developing female organs
only when the male organs are withering. The sexual organs (gametangia) are typically formed
on the moister undersurface. The tiny male structures are the
antheridia (singular
antheridium). Each antheridium is a tiny vessel filled with developing
antherozoids (male
spores/gametes, spermatozoids or sperm cells). The antherozoids escape when the antheridia
are ripe and when a film of surface moisture is present. They are rather bizarre multiflagellate
coiled cells (see
cell motility). They swim toward the female organs, the archegonia (singular:
archegonium). Since each gametophyte tends to produce antheridia and archegonia at different
times, cross-fertilisation is strongly favoured.

The production of male gametes is coordinated by pheromones called antheridogens (a
derivative of the plant hormone gibberellic acid). Mature (heart-shaped) gametophytes secrete
antheridogen which stimulates immature gametophytes to mature and produce antheridia.
When the antheridia have been released, a gametophyte becomes insensitive to antheridogen
and produces archegonia whilst synthesising and secreting its own antheridogen. This
sequence in which production of male gametes occurs first is called protandry and such plants
are said to be protandrous. This favours cross-pollination. However, ferns can sometimes
self-fertilise and this becomes more common in species that are sparsely distributed, such as
those inhabiting patchy, dry environments.

Each archegonium is a tiny flask-shaped or bottle-shaped structure, partially embedded in the
prothallus, and containing a single egg cell. When it is ripe, the contents of the neck above the
egg cell turns into mucilage which releases chemicals (such as
malic acid) which attract the
antherozoids, by
positive chemotaxis (direct movement towards a source of a chemical
attractant). One antherozoid only will succeed in fertilising the egg. Since the antherozoid and
egg are both haploid, the fertilised egg cell, or
zygote, is diploid.

About 48 hours after fertilisation the zygote divides by mitosis and begins to form the
embryo.
The embryo is anchored in the gametophyte prothallus by a foot, through which it derives
nourishment from the gametophyte. The developing structures are covered by a cap or
calyptra, formed from the neck cells of the archegonium by cell division. The mechanical
pressure exerted by the calyptra on the embryo is essential for its normal development.
Eventually, the developing shoot apex, root apex and first leaf break through the calyptra. The
developing sporophyte remains dependnet on the gametophyte for nourishment until it develops
enough chlorophyll to make its own food by photosynthesis. Eventually, the sporophyte, rooted
and photosynthesising becomes independent and the gametophyte withers away.
Heterosporous ferns

In these ferns, the sporangia are of two different types - one type produces a single large
megaspore, the other up to 64 microspores. This compares to the one million to one hundred
million spores produced in the sporangium of a homosporous fern. The megaspore germinates
to produce a reduced female gametophyte, consisting of a single archegonium with a few body
cells. Each microsopre gives rise to a single male antheridium containing 16 antherozoids.

Internal Structure of the Fern Sporophyte

Ferns are an ancient lineage of vascular plants. They are largely well-adapted to life on dry
land, but the gametophyte and the antherozoids still require moisture for the life-cycle to
complete. They are more terrestrial than many bryophytes, and are somewhere between
bryophytes and gymnosperms (such as conifers like pine trees, cypresses, sequoias and yew
trees) in evolutionary terms.

As we have already said, the leaves of ferns are either filmy or with developed palisade and
spongy parenchyma, waterproofing cuticle and stomata.

Like conifers, the vascular tissue of
xylem tracheids and phloem. However, in some species
some of the tracheids may have their end-walls broken-down to form
narrow xylem vessels.
(See
transport in plants). The arrangement of the vascular tissue is highly variable. In many
forms the only stem is the subterranean rhizome, from which fronds emerge directly, others may
have a definite stem, which is typically surrounded by leaf bases or stipes fused to its outer
surface. The stem or rhizome vascular cylinder (
stele) may consist of one of the following
arrangements of xylem and phloem (bounded by endodermis), depending on species:

1.
Protostele - a solid core of xylem surrounded by phloem, the simplest condition.
2.
Siphonostele - a core of xylem with a central pith of parenchyma, surrounded by phloem on
the outside;
leaf gaps (breaks in the vascular cylinder) occur where some vessels arch away to
enter a leaf stipe as a
leaf trace.
3.
Solenostele - like a siphonostele, with leaf gaps, but with phloem on both the inside and
outside of the xylem, so that the xylem is ensheathed by phloem.
4.
Dictyostele - xylem surrounded or sheathed in phloem, but many overlapping leaf gaps
results in a stele which is a network of vessels, and so appears as distinctly separate vascular
bundles (
meristeles) in cross-section (similar to that in flowering plants) but the vessels
anastomoze or fuse with one-another at intervals. Sometimes more than one such vascular
cylinder may be present. several vascular bundles enter each leaf as leaf traces.
Above: ferns and mosses growing in the damp environs of a waterfall.
Above: a diagrammatic cross-section through a fern petiole (stipe). Click image for full size. This
fern has a dictyostele. Each meristele is bounded by a cylinder of cells forming the endodermis.
Between the endodermis and the conducting vascular tissue (xylem and phloem) is a thin layer
of tissue called the pericycle. The adaxial surface of the stipe is that side nearest the axis of the
stem or rhizome, the abaxial that side furthest from it. Compare this to the structure of the
stem
in flowering plants.

There is little or no
secondary growth in ferns, though there is early and late primary xylem
(protoxylem and metaxylem) as in flowering plants. In the absence of wood, ferns rely largely on
tough
sclerenchyma to support their bodies, often forming a cylinder of cells with thickened walls
just beneath the epidermis. This sclerenchyma makes ferns tough but flexible and has been
used for construction in earthquake-prone zones where it fairs better than heavier and more
brittle wood.

In Filicales, the stem grows from a single apical cell, which is tetrahedral in shape and gives rise
to the other cells by mitosis. This contrasts with the muticellular apical meristems of flowering
plants.

The roots grow from a few cells at the root apex, by cell division, and the roots typically contain
two vascular strands (they are diarch). The tip of each root is protected by a root cap, allowing it
to penetrate the soil more easily. Many produce root hairs. The roots of at least some ferns are
known to form mutually beneficial symbisoses with fungi, forming
mycorhizae.

The gametophyte is structurally much simpler, and less differentiated with less specialised tissue
cells and more like a bryophyte in this respect.
fern antherozoid
Left: a model of a typical fern antherozoid
(spermatozoid). Fern antherozoids are
typically helical or pear-shaped cells bearing
a helical arrangement of
flagella. Click image
to view full size.

I would like to see a comprehensive
mechanical analysis of their mode of
locomotion!






Below: a typical life-cycle of a homosporous
fern, click image to view full size.
sori on Dryopteris
Above: a thicket of woodland ferns.
fern life-cycle
Above: types of vascular cylinder in ferns as seen in cross-sections (transverse sections) of the
stem. The sections are at a point where a leaf branch is starting to emerge from the stem on the
right-hand side. In all except the protostele, the vascular bundle entering the leaf (leaf trace)
leaves a gap in the xylem and phloem, called a leaf gap. A leaf gap begins at the point the leaf
trace branches away from the main stele to enter the leaf stalk, and continues for a short
distance further up/along the stem before closing off. In the dictyostele condition, there are
many overlapping leaf gaps, turning the vascular cylinder into a series of anastomosing
vascular strands, called meristeles, which form a (reticulate) network-like lattice of vessels.
Recall that in many ferns, the stem proper is a horizontal subterranean rhizome and the 'stems'
are actually leaf stipes. In the rhizome there may be a marked (dorsiventral) asymmetry in the
vascular cylinder as the rhizome has a definite upper (dorsal) and lower (ventral) surface. The
vascular cylinders and bundles are generally bounded by a layer of cells called the endodermis,
and between this and the vascular tissue there may be a layer or two of cells called the
pericycle. The dictyostele is seen as the more advanced condition, the protostele as the least
evolved.

Note, that there has been a tendency in evolution for more evolved steles to concentrate the
vascular tissue towards the outside of the stem. This is because the vascular tissue has a
mechanical supporting role (in addition to the sclerenchyma) and locating this tissue towards
the outside increases resistance to buckling for a given amount of tissue. This is an adaptation
to life on land.
Other types of ferns

In addition to the Filicales, there are two other extant orders of ferns. The Ophioglossales or
tongue-ferns are a small order of ferns (sometimes classed as psilopsids rather than
ferns)comprising three genera: the moonworts,
Botrychum, adder's-tongue ferns, Ophioglossum,
and
Helminthostachys. Helminthostachys is restricted to the Polynesian Islands and parts of the
Asian Tropics.  They are characterised by a separation of the fertile part of the frond from the
sterile part. In
Ophioglossum vulgatum, the sterile part consists of a single leaf-like structure, with
a smooth margin and a network arrangement of veins. The central axis continues past this leaf
to the fertile portion of the frond, which is a spike-like structure (the 'adder's tongue' part). In
Botrychium lunaria, the sterile part is a pinnate leaf borne on a side-branch from the main axis.
Again the main axis continues and terminates in the fertile region, which is a finely-branched
structure bearing sporangia. There is no uncoiling circinnate vernation, as in the Filicales, in the
Ophioglossales the fronds instead grow out from the margins of bud-like structure, the
primordium.

The
Marattiales is the third order of extant ferns and is confined to the tropics. These have
short upright stems or trunks bearing large, fleshy and usually with pinnate/bipinnate or palm-like
fronds, which may be up to 5 metres long (15 feet). Filmy forms, and forms with simple
non-compound fronds also occur. Unlike other ferns, which grow from an apical cell, the
Marattiales have a multicellular apical meristem, more like that found in flowering plants. Many
mucilage canals occur within the plant body, but no or little sclerenchyma.

Tree Ferns

Not all ferns are confined to the field-layer. In the tropics tree ferns can be found, forming part of
the main canopy or understorey. These tree ferns may have been among the dominant trees in
ancient forests. The tree ferns, often classified in the order Cyatheales, may form stems or
trunks up to 20 metres tall. A few other types of ferns are also tree ferns. Examples are
Dicksonia and Cyathea. New species are still being discovered, though many have apparently
gone extinct over the past century due to habitat destruction.
Cyathea has a polycyclic stele,
that is the vascular cylinder consists of several concentric cylinders of vascular tissue, which
anastomose with one-another and all contributing to the leaf traces. The trunk consists of the
vascular cylinders, contained within the stem proper, surrounded by the fused leaf bases with
their tough vascular bundles, and sometimes also surrounded by a mantle of adventitious roots,
which provide much of the support. Fossil tree ferns include types that show another
'experiment' with tree trunks, in which the trunk consisted of a mass of intertwined branching
stems and roots.
unfurling fern
unfurling fern
Above and below: unfurling fern fronds.
Field-Layer: Ferns and Herbs