Above: this pond is covered with flowering plants, including
water-lilies, including the white water-lily, Nymphaea alba,
with its spectacular large flowers (I don't have zoom to get a
good picture of these) and the fringed water-lily,
Nymphoides peltata with its smaller yellow flowers, the petals
of which have fringed margins.
Left: the bulrush (more properly called the Great Reedmace
as 'bulrush' is a name also given to a different plant) or
Cat's-tail, Typha latifolia is one of the few brown flowers in
the British isles and grows in marshes or shallow water
alongside ponds, lakes and slow-moving fresh-water ways.
The Bulrush grows up to 2.5 metres tall. Many tiny flowers
are packed into the cylindrical flower head; the female
flowers form the lower wider and brown section of the flower
head and are arranged in a compact manner; the male
flowers are above these in a looser spike. The flowers are
wind-pollinated and their petals and sepals are modified as
hairs that later form the 'parachutes' that assist seed
dispersal. The male flowers have 1-3 stamens, the female
flowers each a single ovary which develops into a
single-seeded dry fruit and the flower head breaks apart into
a voluminous mass of fluffy seeds that are dispersed by the
wind. The bulrushes develop a mass of rhizomes from which
the shoots sprout each year - reedmaces are perennial.
We have already looked at two amazing ecosystems -
woodlands and meadows, now we look at a third - ponds are
truly amazing places for all manner of bizarre and wonderful
Examining a small piece of weed growing along the pond margin, preferably by floating it in a transparent vessel like a test-tube, some
of the floating weed was seen to contain small bladders, this is the carnivorous plant bladderwort. A small sample of water contained a
tiny cyclops crustacean, complete with egg sacs. The cyclops makes ideal food for the bladderwort, whose hair-trigger activated
bladder traps rapidly suck the hapless creatures inside should they venture close enough to activate the trigger. Also just visible to the
naked eye was a small white nematode roundworm swimming by threshing its rod-like body in a rapid sinusoidal wave, from side to
side, wriggling like an eel. Examination of a drop of water under the microscope reveals all manner of wonders, including beautiful
gliding gastrotrichs and swimming/crawling rotifers. Some of the other micro-organisms found are shown below.
Scenedesmus, a microscopic green alga, consisting of chains typically of 4 cells,
though 2 or 8 cells are also quite frequent, the two end cells each bearing a pair
of long spines that assist flotation and may also deter some predators. At least 4
different species were seen in this sample. There are some 12 species of
Scenedesmus found in fresh water in Britain. In the pictures above there appear
to be three different species: one with small cells forming chains of 2, 4 or 8 cells
and with straight spines. A form with large cells and curved spines; and a
species with large cells and twisted colonies, the cells each having bars across
their middle. In addition there was a species with semilunar shaped cells, forming
colonies of 4 or 8 cells, and no obvious spikes (although the tips of the
semilunar cells were spine-like). Scenedesmus senses the presence of nearby
predators and responds by producing longer colonies of 8 cells, which are larger
and so harder to ingest.
Pediastrum - a colonial green alga that forms
disc-shaped colonies rimmed with spines. In
some species, the disc appear more-or-less
solid, whilst in others it contains spaces and
is net-like, as here.
Development of new colonies of Pediastrum can occur asexually. The protoplast of one of the cells
divides into a mass of biflagellate zoospores which are released through a slit in the parent cell wall as
a packet enclosed by a lens-shaped vesicle. Within this vesicle, the zoospores enlarge as the vesicle
enlarges and move around and rapidly arrange themselves into a disc, one cell thick. The cells on the
rim then produce spines over several minutes, and the vesicle ruptures to release the mature colony.
The vesicle restricts the zoospores as they enlarge within it and experiments have been conducted
with expanding rubber balls in a confined space which suggests that the pores in the Pediastrum
colony are determined by the mechanical restraining forces of the vesicle. Presumably the pores and
spines assist in flotation, and the spines may also protect the colony against predation.
Euglena is a flagellated single-celled green alga
which exhibits massive changes in shape as it moves,
being extremely pleomorphic and fast-moving. The
red stigma is part of the cell's light-sensing apparatus.
In addition, a longer and more rigid form was also
found and a representative (drawn from a different
locale) of this type is shown on the right.
Click here for more information on these organisms.
Crucigenia is a small colonial green alga related to
Scenedesmus and Pediastrum. It is formed by four
cells arranged in a square. Also apparent were pairs
of square colonies and even four square colonies
connected in a sheet by slime in a perfect square!
This species is probably Crucigenia fenestratum,
which has a large open window in the middle of the
These gelatinous spherical colonies of green
algae were common. They are apparently
non-motile and so unlikely to be related to Volvox.
The cells were embedded in a gelatinous matrix.
These could belong to the Coenastrum genus.
Sometimes sheets of similar cells embedded in
gelatinous material were also seen and these
may be related to the numerous spherical
single-celled algae (right).
The following are examples of microscopic planktonic organisms seen in the sample of pond water (apologies for the limited
quality, these photos were taken with an old digital camera simply held over the eyepiece which cannot capture the full resolution
of the microscope).
Aquatic Insects - bizarre and varied inhabitants
Sitting at the pond one day, I was treated to a dazzling spectacle of colour. A mature male broad-bodied chaser (Libellula depressa)
some 2.5 inches in length by my reckoning (the normal quoted length is 6 cm) and with a thick tapering body, about 1 cm in width at its
widest part, was territorialising the pond, using an old cane among the reedmaces as a perch. The males are usually described as pale
blue in colour, but his abdomen looked more silver, brightly reflecting the sunlight, creating a stunning visual display as he flew about. He
attempted to mate with three others of its species, which were likely females, though immature males resemble the females. One of these
females was a dull brown, another had tiger stripes and the third was a spectacular metallic gold in colour, and the latter shimmered as
spectacularly as the male in the bright sun. At least one of the females began laying eggs, rapidly dipping the tip of her abdomen in the
pond, staying on the wing and hovering in an almost vertical posture. They were accompanied by smaller, though very beautiful, blue and
red damselflies. Damselflies look like small dragonflies, but they fold their wings back when at rest, whilst the dragonflies keep their
wings held out sideways like miniature aircraft. The ability of dragonflies to hover in mid-flight has given them the nickname 'helicopters'.
The damselflies mate very different, they pair, the female often being differently coloured, green females accompanying the blue males.
The male keeps a grip on the females back with the tip of his abdomen, as they land to allow the female to curl her abdomen beneath a
leaf to deposit the eggs, whilst the male clasps her in an almost vertical posture. A solitary male munches on an insect he has caught on
a nearby leaf. One female has somehow lost her partner and lays her eggs alone. Dragonflies and damselflies are fierce predators,
eating other insects which they catch on the wing.
Aquatic insects are an extremely fascinating group of organisms which display extraordinary adaptations to their way of life. A large
number of insects have aquatic larvae, including the dragonflies. Dragonfly larvae live in water and will eat other insects, tadpoles and
even small fish. They are fierce predators, respiring through rectal lungs which they irrigate by pumping water in and out through their
anus. The rapid expulsion of water through the anus gives rise to jet propulsion, allowing the larvae to jet forward rapidly as they ambush
prey. Their horrific jaws, born on the end of a hinged 'arm' rapidly shoot out to seize their prey.
This hawker belongs to the genus Aeshna and is possibly the Southern Hawker (Aeshna cyanea).
Hawkers lack the shorter, broader and tapering abdomens of the libellulids and have narrower,
longer and more cylindrical abdomens. This insect was found (freshly dead which is why the
abdomen is sitting on the surface) not far from the river and was about 9 cm long.
When the time comes, the dragonfly larva crawls up a stem, emerging from the water to pupate, the emerging adult spending most of
its life on the wing. Other insects remain on or in the water during their adult lives. Pond Skaters (Water Striders) can be seen rapidly
skimming across the surface water. Their legs are designed to spread the insect's weight so that it can sit on the surface meniscus,
without getting its feet wet (though they do sometimes submerge themselves, for example when the air is cold). The Pond Skater is
able to sense ripples and to recognise those ripples caused by a struggling insect that has fallen onto the water. When they reach their
prey they inject their syringe-like proboscis and suck-out its body fluids. Pond Skaters can also communicate with one-another by
generating ripples. I also saw a lesser Water Boatman, just beneath the surface of the water, rowing itself along with its oar-like limbs. I
also saw an aquatic beetle swimming beneath the surface, but have not identified it (there are quite a number of aquatic beetles).
Some of these other aquatic insects will feed on the pond skaters. Spiders can also be seen walking over the surface of the water to
feast upon crashed and fallen insects struggling on the water and some spiders specialise in this.
Insects have to be able to obtain oxygen if they are to survive when submerged under water. Some insects have what is called a
plastron mechanism - the hairs on their bodies are specially modified to trap a film or bubble of air when they dive (these hairs are
designed to resist collapse under high pressures). In Dytiscus (Great Diving Beetle) and Notonecta (Water Boatman) air is trapped
beneath the elytra (wing covers) and this air communicates directly with the insect's spiracles (openings to its airways or tracheae).
These plastrons and sub-elytral air-spaces can function as what are called 'physical gills' - trapped pockets of air that can actually
absorb more oxygen from the surrounding water. Oxygen diffuses across an air/water interface some three times faster than nitrogen,
so as the insect takes up the oxygen from the trapped air bubble, lowering the partial pressure of oxygen and raising that of nitrogen
within the bubble, oxygen diffuses in (when the partial pressure of oxygen in the water exceeds that in the air space) faster than
nitrogen diffuses out. This maintains the air bubble for longer and in experiments in which the air was replaced by pure oxygen, the
insect actually had to resurface for fresh oxygen sooner than when air was used, since with oxygen the partial pressure of oxygen in
the bubble is always greater than in the surrounding water and although the insect takes more oxygen with it to begin with, it is unable
to extract any from the surrounding water. In Notonecta (the Water Boatman) the hindlegs are used to drive water currents over the
physical gills to irrigate them with fresh oxygenated water. Eventually all the nitrogen in the air space dissolves and then the insect
must resurface to replenish its supply of air. In Dytiscus and Notonecta the trapped air volume can be regulated when under water,
acting to regulate buoyancy.
Structures that have a higher affinity for air than for water, like plastron hairs, are called hydrofuge structures. Hydrofuge hairs also
exist on the siphons of mosquito larvae. Some aquatic insect larvae, like mosquitoes, breath through a siphon - a tube with a spiracle
at its tip that leads straight into the tracheal airways inside the insect. The spiracles are surrounded by hydrofuge hairs which repel
water and prevent it from entering the system and drowning the insect. The water trough in the neighbouring meadow has dozens of
these larvae that wriggle for cover whenever one's shadow passes over them. The hydrofuge hairs cannot repel oil, which is used to
control mosquitoes by applying a film of oil to the water's surface; when the larvae try to breathe the oil enters through the spiracle and
they drown. Eristalis, a type of hoverfly, has aquatic larvae that are called rat-tailed maggots because of the long tail-like extensible
Some aquatic insects have a closed tracheal system which does not open to the outside air via spiracles. These include the
dragonfly nymphs which have 6 double rows of lamellae lining the rectum and forming the branchial basket. Water is pumped over
these tracheal gills, mostly in and out through the anus, and oxygen diffuses across to the trachea which fill the gills. Some aquatic
insects have spiracular gills, like Simulium (black fly) pupae.
Bloodworms are red worm-like aquatic insect larvae that are often found at the bottom of ponds. These are the larvae of midges, like
Chironomus, and certain flies (belonging to the true-flies or Diptera). They are red because they contain a form of haemoglobin
(which contains 2 haem groups per molecule instead of 4 as are found in vertebrate haemoglobin) in their haemolymph ('blood
plasma'). This haemoglobin is used to store oxygen, enough for two days in low oxygen conditions (as might occur in warm stagnant
water). When oxygen is adequate they return to tracheal respiration and replenish their oxygen reserves. The larva of Chironomus has
a closed tracheal system and absorbs oxygen across its 'skin' (cuticle) and also has so-called 'blood gills'. These are regions of the
body wall that are very thin and project from the body surface as blood-filled sacs that are more-or-less devoid of tracheae. However,
these do not seem to have a normal respiratory function, but may assist in recovery from oxygen starvation. The larvae of some
mosquitoes have long anal papillae (projections) filled with tracheae and which are held in a current of water created by mouth
brushes. However, the respiratory role of such tracheal-filled appendages is hard to determine experimentally and a normal respiratory
function is doubted, though they may serve to excrete carbon dioxide.
Flowers by the Pond
The soil around the pond is damp and marshy, conditions which favour some plants more than others.
Left: the Water Mint (Mentha aquatica) or Marsh
Mint found near the pond (and found generally near
freshwater and in marshes) has very fragrant
leaves and distinctive flower heads.
Present in the pond's plankton are
photosynthetic cyanobacteria like this
filament (probably Anabaena). The larger
ellipsoidal cell in the chain is a heterocyst
which is specialised for nitrogen fixation. This
chain of cells is essentially a multicellular
bacterium consisting of 1D tissue. In some
forms, such as Nostoc, filaments may group
together to form large gelatinous capsules 1
cm or more in diameter that may be attached
or free-floating and are found in streams.
Some cyanobacteria, like Oscillatoria, exhibit
gliding motility across solid surfaces
apparently powered by slime jets. Many other
bacteria are present in the pond water,
visible as the bright spots in the photographs
above, some actively swimming by means of
These algae look like a species of Ankistrodesmus with their narrow, pointed
and curved cells. However, in culture Scenedesmus has been observe to also
adopt this type of morphology, so we cannot be certain.
A number of S-shaped cells were
apparent, which could be isolated cells of
Ankistrodesmus or a type like
Monoraphidium contortum - morphology
of algae can vary with life-stage and
conditions and so one has to be cautious
about identifying species on morphology
Below: the yellow iris (yellow flag), Iris pseudacorus, is also present near the pond, and also
along a nearby stream, but flowered in Spring before th reedmaces. These plants are
well-adapted for wet conditions, tolerating a low supply of oxygen to their roots (characteristic of
water-logged soils). The flowers are large and the fruit are large capsules (both about 7 cm or so
in diameter). The plants spread asexually through rhizomes and sexually through
Above: a diagram of a bladder of
bladderwort. These minute bladders
are attached to branching tubular
stolons which make up the leaves of
the plant. (See Carnivorous plants).
Meadowsweet (Filipendula ulmaria) is a perennial herb found
in damp meadows and in this case growing right beside the pond
(on one of its drier edges).
This is an interesting medicinal herb which contains the painkiller
(analgesic) salicylic acid (the fore-runner to aspirin) and whose
roots smell of Oil of Wintergreen when crushed. It is claimed that
this plant has anti-cancer, anti-arthritic, blood-thinning and
anti-microbial properties. The flowers can be infused to make a
tea. Certainly salicylic acid has anti-viral and anti-tumour
properties, which is why it is an effective treatment for warts.
Many plant chemicals are part of the plant's natural defences
against viruses, other parasites and tumours (many tumours are
probably caused by viruses). Meadowsweet was also scattered
about floors in Mediaeval times to freshen the air by hiding bad
smells with its scents. The conversion of oil of wintergreen and
the synthesis of aspirin from salicylic acid by reflux are standard
The yellow flag often grows near to reedmaces, as here, but generally in shallower water.
A variety of Scenedesmus
species all to the same scale.
|In this colony (coenobium)
one of the cells is dividing to
produce a new daughter
Left: Paramecium ciliates. These single-celled
Protozoa swim by means of spiral tracts of tiny
beating hairlike projections called cilia. They feed
on anything organic that is small enough to ingest,
such as bacteria. They are fast-moving and only
rest occasionally, firing out harpoon-like anchors
(trichocysts) when they do so.
Below: At least two more species of Scenedesmus. Below left: this could be Scenedesmus acuminatus.
Below left: this specimen has the morphology of Scenedesmus dimorphus.
These orange-red flasks belong to
Trachelomonas. This flagellate was seen
actively swimming around in the initial
sample, however, by the time these photos
were taken a couple of days later all had
withdrawn into the flask-like loricas or 'shells'
that enclose them and some had formed
thick-walled dormant cysts. This indicates
that conditions changed within the test-tube
and became unfavourable for this spacies,
causing it to enter a dormant phase.
Grasses, Sedges and Rushes
Above Foreground: Pendulous Sedge (Carex pendula) a wetland species growing near to a stream. There are
about 80 species of sedges in the British Isles. Sedges have separate male and female flowers, which may occur
on mixed or single-sexed spikes. In the Pendulous Sedge, the topmost spike or two on each branch contains only
male flowers (the brown spike visible at the top of the image) whilst the other spikes contain female flowers.
Sedges have 3-sided stems that are usually solid, and the leaves are arranged in three rows up the stem. The
flowers are tiny with a single scale-like glume and male or female parts arranged in threes (three anthers or a
tripartite ovary with three stigmas).
Above Background: a yellow flag iris.
Left: a rush growing by the pond (Jointed Rush
(Juncus articulatus)?). Common rush (Juncus
conglomeratus) is also present, which has a very
different flower arrangement. Rushes have the
inflorescence apparently emerging some four-fifths
up the stem; in common rush they appear as a
bunch of flowers forming a semicircular disc on one
side of the stem. Actually, the inflorescence is at the
top of the stem, but one true leaf begins below the
inflorescence and looks like a continuation of the
stem. The base of the stem of the common rush is
clothed in bladeless scale leaves. The leaves may
be curled or fused to form a cylinder with a cavity
that may be pith-filled or traversed by septa (as in
the Jointed Rush). There are about 30 species of
rush in the British isles and they are found in
wetlands and along coasts. Another group of similar
plants, the woodrushes have grass-like leaves with
straggling hairs along one edge. Rushes vary from
annual to perennial. Rushes, unlike grasses and
sedges, have standard recognisable lily-like flowers,
though they are tiny, with 6 papery green or brown
petals. Grasses also occur in wetlands, but unlike
sedges and rushes, some grasses are at home in
dry habitats. Reeds are wetland grasses.
Grasses have hollow cylindrical stems with 2 ranks of leaves. Their flowers have no petals and 2 scale-like lobes (glumes). The Common
reed is the tallest British grass. Grasses also have a ligule - a collar that is either mebranous or a tuft of hairs, at the point where the leaf
meets the stem. Rushes, sedges and grasses are all monocotyledons (plants whose seeds have only one embryonic leaf or cotyledon) and
as such they have narrow leaves with parallel venation and their flower parts arranged in threes or multiples of three. They can be hard to
tell apart. Irises (like the yellow flag), lilies (like the water lily) and orchids are also monocots (as are palms, bamboos and bananas).
Another monocot found near the pond is common water-plantain (Alisma plantago-aquatica) with its small whitish 3-pettled flowers. The
reedmace is another monocot. Monocots have a major role around ponds and streams.
Bryophytes, like these liverworts, are abundant along the banks of the stream. These
non-flowering spore-producing liverworts resemble seaweeds and require moist
places in order to thrive, being poorly adapted to life on dry land, but here they are
very much at home. They require water to enable the motile sperm to swim to the
female eggs. At least two species are apparent here. Also present along the bank
are mosses, ferns and horsetails, which are also 'primitive' spore-producing plants.
Other, dicotyledonous flowering plants are found here. A knapweed (possibly Greater Knapweed, Centaurea nigra) below is busy being
visited by pollinating insects like the honeybee (left) and the hoverfly, Episyrphus balteatus, (right). In knapweed, each 'petal' is actually a
small flower or floret whose petals are fused into a tube. Only the central florets are fertile (the outer ones are there for show, to help attract
pollinators) and when the insect touches the flower, action potentials (electrical spikes) send signals to the stamens which suddenly move
forward, dusting the insect with pollen. Ragged Robin (Lychnis flos-cuculi) - not pictured - was also found growing near to the pond, as it
prefers wet or marshy places.