Living Fertiliser Factory

The moisture-loving insectivorous plants that I grow in my conservatory stand in dishes of water, which inevitably accumulate a layer of ‘green slime’ on the surface. But this is no ordinary green slime. Under the microscope it’s revealed as a cyanobacterium (blue-green alga) called Nostoc, identifiable by those chains of small cells with occasional much larger ones interpolated along the rows. Those large cells are called heterocysts and the contain the enzymes that convert gaseous nitrogen in the atmosphere into soluble nitrogen compounds that plants need for their growth. These cells are living fertiliser factories. They’ve been providing an essential nutrient that plants need for growth, and maintaining natural soil fertility, for millions of years. Nostoc is present in soils everywhere – even in some deserts – but occasionally, during spells of wet weather, it proliferates inside a mucilaginous matrix and forms great convoluted balls of slime (bottom photo) that can cover large areas of bog, wet grass or soil. In that state it’s incredibly slippery: step on it and you’re likely to fall flat on your back.

Zipping up a Feather

There’s a whole lot of moulting going on at the moment in the bird world, as they begin to shed their well-worn breeding plumage. My wife found this distinctive feather on our garden path last week. Under the microscope the underside of the feather (top photo) reveals the beautiful repeated pattern of rows of barbs attached to the central shaft (rachis). At higher magnification (second photo down) you can see the rows of barbules on each barb, each ending in a tiny hook (barbicel). When a bird preens a feather by drawing it through its beak, it’s zipping these rows of hooks on adjacent barbules back together again, to restore the feather’s aerodynamic efficiency. In the bottom two photographs the feather has been flipped over to view the upper surface and reveal a clue to its identity – the blue iridescence in some of the barbules. It belonged to a magpie.

Mitey Fine Claws

This little animal is an acarine mite - a minute but close relative of the more familiar spiders. Both have eight legs but mites are arachnids with a simple globular body, unlike spiders whose bodies are divided into a thorax that bears the legs and an abdomen, separated by a narrow waist. Terrestrial mites are present in vast numbers in damp vegetation at soil level, while aquatic species are in just about every pond. The land living species like those pictured here are equipped with impressive claws (bottom photo), reminiscent of Captain Hook’s hook in Peter Pan, that they use for clambering through the branches of mosses, while aquatic species have hairy fringes on their legs that aid swimming. The aquatic species have complex life cycles, spending their early stages of development as parasites on other pond animals, and there is still much to discover about their way of life – a real research opportunity for amateur naturalists with patience and a microscope at their disposal.
To find out more about mites visit http://www.sel.barc.usda.gov/acari/frames/mites.html

Dune Builder

The photograph at the top of this post shows a cross section of the leaf of marram grass Ammophila arenaria, the grass that’s primarily responsible for trapping wind-blown sand and building the dune systems around our coast that are such important wildlife habitats (bottom photo). Marram grass survives in the arid environment of a sand dune by rolling up its leaves during long periods of drought, so that all the leaves’ breathing pores or stomata (see http://beyondthehumaneye.blogspot.com/search/label/stomata) are inside the rolled leaf, minimising water loss. This cross section of a partially rolled leaf has been stained with fluorescent dyes to light up different cell types within the leaf, with the outside surface of the leaf at the bottom of the picture (smooth, curved surface) and the inner convoluted surface at the top. The outer surface of the leaf at the bottom is composed of a layer of thick walled cells, covered with a thick cuticle to resist wind-blown sand abrasion and this layer also acts like a spring, giving the leaf a natural tendency to roll up under drought conditions. The stomata are hidden on the inner surface of the leaf amongst those stubbly hairs near the bottom of those convolutions – which in the whole leaf are actually ridges and furrows that run along the whole length of the leaf. The clusters of thin-walled blue cells at the base (i.e. in the ‘valleys’) of the furrows of the convolutions are responsible for unrolling the leaf – when it rains and the plant takes up water these thin walled cells inflate like balloons, forcing the leaf to unroll. Other features that you can see in this leaf cross section are the snaking rows of reddish cells which are actually the cells containing most of the chlorophyll, that carry out photosynthesis – in order to make the dyes fluoresce I had to irradiate this leaf section with blue-violet light, which paradoxically makes green chlorophyll fluoresce red. The other distinctive features are the scattered structures that look like ‘smiley faces’ with a pair of large ‘eyes’ with a blue open ‘mouth’ – these are the leaf veins that conduct water and sugars along the leaf – they’re the plant’s internal plumbing system.

Pond Plankton

Mention the word ‘plankton’ and the drifting life in the surface layer of the oceans springs to mind – stuff that blue whales eat - but there’s plenty of plankton in the surface layer of lakes and ponds too. This picture (top) shows the larval stage – known as a nauplius – of a freshwater crustacean called a copepod (bottom photo), possibly Cyclops (that single red eye is a strong clue!). All those long bristles probably improve the nauplius larva's buoyancy, slowing the rate that it sinks and giving it extra leverage when it beats its limbs to stay in the surface layers. Copepod nauplius larvae go through a series of up to six moults before they become a recognisable Cyclops and this in turn goes through a further five moults before it reaches its final adult stage. The whole process takes the best part of a month, during which the nauplius and developing Cyclops are favourite food items on the menu of every passing predator; in this respect a nearly-mature tadpole is probably the blue whale of the pond. Life is hazardous when you’re small and edible!

A smutty tale of exploitation and sex-change

You can easily spot campion flowers that have been infected with anther smut fungus Microbotryum violaceum, because the flower petals are stained with the purplish-brown spores of the fungus (top photograph) that multiply in the plant’s stamens – a symptom that is particularly conspicuous in white campion Silene alba. When the anther smut infects its host it proliferates in the stamens, producing tens of thousands of minute, spiny spores (middle photograph) that are then carried from flower to flower by pollinating insects, like the drone fly that’s visiting the red campion Silene dioica flower in the bottom photograph. Hijacking the plant's stamens and pollinators to produce and spread its spores around would be a remarkable adaptation, but this fungus goes one step further in exploiting its host. Campion plants are either male or female and only males have the stamens that the fungus needs for development of its spores, but when the fungal spores infect a female campion - that wouldn’t normally produce stamens in which the fungal spores proliferate - it induces the female plant to change sex and become male, producing stamens where its spores can multiply.

Tears of a Swan

This strange little protozoan is called Lacrymaria olor, which means ‘tear of a swan’. When contracted it’s about a quarter of a millimetre long and tear-shaped. When it fully extends that long neck it’s about 2mm. long, which is gigantic as single-celled organisms go. The top photograph was taken using polarised light, which generates the attractive colours but the remaining photographs have been taken with interference contrast optics, which produce a less colourful result but more biological information. The next three photos down shows quite nicely why this organism is called ‘tear of a swan’ ......when it arches that long neck it looks - in outline – uncannily like a swan or perhaps, if you’ve a more fertile imagination, like the mythical Loch Ness monster. The final photo shows some structural detail of this remarkable protozoan, including some ingested food particles, contractile vacuoles that it uses to expel excess water and food waste and the lines of cilia that propel it through the water. If you double-click the photos you can see them a little larger. You may also be able to discern the cilia in the two videos, which show how active this organism is. You can read more about it at http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artapr00/rhlac2.html

Sometimes people wonder what motivates anyone to become a scientist. Despite being a practitioner since 1973 I was completely unaware that this amazing organism existed until about a month ago, when I read the Microscopy-UK web site article quoted above. Today was the first time I’d ever found one, in water around the roots of decaying reeds on the edge of a pond. That’s science for you – an unlimited source of new personal discoveries.

Smart Weapons, Dumb Defence

If you take a closer look at some boring houseplants they often reveal inner beauty – sometimes with just a hint of menace. The easy-to-propagate spider plant Chlorophytum (top photograph) must be one of the world’s commonest houseplants. A cross section of one of its leaves shows its inner structure, including the pale yellow water conducting vessels that you can see in the second photo – but also reveals a mass of needle-shaped crystals that fall out of the leaf when it’s cut. These are calcium oxalate crystals, known botanically as raphides, and are probably part of the plant’s defense mechanism, sticking in the tongue and soft throat tissues of any animal that tries to eat it. The raphides, revealed here as brightly coloured needles by polarised light microscopy, are confined inside undamaged cells (third photo) in intact leaves but as soon as the animal takes the first bite they are released from broken surfaces. Some cells contained different-shaped calcium oxalate crystals – often cuboid, octagonal or spiky – that are just the plant’s way of disposing of excess unwanted oxalic acid that would otherwise damage the workings of the cell (fourth photo). Different plants can sometimes be identified by the shape of the druses that they contain and sometimes very small druse crystals are present in large numbers in the cell . When they are, they jiggle around within the cell, due to the random motion of molecules in the cell sap banging into one another – a phenomenon known as Brownian motion. If you take a look at the polarised light video you can see the sparkling, kaleidoscope effect that Brownian motion in druses creates under the microscope. In some plants – such as dumb cane Dieffenbachia – the needle sharp raphides are contained in toughened cells called ideoblasts, like arrows in a quiver (bottom photo), where the cell contents are under pressure, so if the leaf surface is crushed they fly out. Plants like dumb cane can cause loss of voice or even choking in pets and young children that chew the leaves if the plant’s raphides cause the throat to swell.

Tentacled Terror

According to ancient Greek mythology Hydra was the nine-headed, swamp-dwelling beast eventually slain by Heracles. The Hydra in these photographs came from the shallow water at the edge of a swamp too but – disappointingly – only had one head. Pond-swelling Hydra often have several heads because they bud off new individuals from the central column of their body, although I’ve never seen one with more than three heads. These animals are hard to spot because even big ones are only a few millimetres long and they cling to waterweeds, where they’re perfectly camouflaged by their green colouration. That’s due to symbiotic algae that live in their gut wall, and because the algae generate oxygen Hydra can often thrive in stagnant, oxygen-starved conditions – around the swampy edges of shallow ponds for example. The best way to find Hydra is to put a handful of waterweed in a glass container and stand it on a well-lit window ledge – the Hydra will often move to the glass surface and anchor themselves there, waving their tentacles. If you’re lucky you might also witness one of their ways of moving – by somersaulting, tentacle-end over foot-end. Each of those tentacles is equipped with stinging poisonous hairs (nematocysts) that are fired out when an unfortunate pond-creature blunders against their trigger hairs and is paralysed by the toxin – you can find a diagram showing how they work here http://upload.wikimedia.org/wikipedia/commons/b/b0/Nematocyst_discharge.png

The specimen in these photographs shows the amazing way in which Hydra can shape-shift, contracting into a blob crowned with contracted tentacles (top photo) or elongating its column and tentacles when it’s in hunting mode. Some of these pictures were taken with a microscope equipped with polarising optics, which generate the coloured background. You can watch videos of a brown species of Hydra here http://www.arkive.org/brown-hydra/hydra-oligactis/video-09.html

Jaws

Cue sinister music: here’s the animal with the most formidable jaws currently living in my water butt. Strictly speaking it’s an omnivore, dining on algae as well as other animal life, but it has impressive chewing equipment. It’s the larval stage of a chironomid midge – one of those little midges that form dancing swarms at dusk – and it’s the third commonest organism in the water butt at the moment, after the euglenoids and Vorticella (see previous posts). It’s about a millimetre long. The adults do not bite and are unable to feed, so have very short lives - just long enough to mate and lay a gelatinous string of eggs on the surface of still water.

Living Corkscrew

One of the most mesmerising features of the microscopic organisms that live in wet places is the variety of ways in which they get around. Some use rhythmically beating hairs (cilia), for motive power. Others, like amoeba, simply flow over flat surfaces. Euglenoids – minute photosynthetic organisms that are ubiquitous in wet places – blur our concepts of what is an animal or plant (they are neither) and can crawl over wet surfaces and even change shape, but mostly they use a long flagellum, whipping it through the water as the driving force for their movements. One of the most extraordinary euglenoids is this one, called Phacus, which adds an extra twist to the wacky ways of moving by being twisted into a helix. So when it beats its flagellum it corkscrews through the water, as you can see in the video. It's less than a twentieth of a millimetre long.

Living Springs

This unusual single-celled organism, called Vorticella, lives amongst the algae that line the inside my water butt, that collects rainwater from the garage roof. Each individual resembles a bell attached to a patch of algae via a long stalk. It feeds using a ring of beating hairs around the edge of the ‘bell’, creating a water current that sweeps in minute algae. All those green blobs inside are algae that it has trapped. Vorticella reacts to the slightest disturbance in a remarkable way, by coiling its stalk like a spring so that it shrinks back out of sight. The short video clip shows this behaviour quite nicely. Each Vorticella is about a fifth of a millimetre long, at full stretch. You can read more about Vorticella at http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjun03/wdvorticella.html

A Floral Prison

This strange-shaped little flower (top photo), about two centimetres tall, belongs to a houseplant known as String-of-Hearts Ceropegia woodii. You can see the whole plant and find more information about how to grow it at http://www.plantoftheweek.org/week098.shtml
This is a plant that takes midges prisoner, to ensure that its flowers are pollinated. When a flower opens it emits a distinctive scent through that lantern-shaped top that attracts tiny midges. Different species of Ceropegia (and there are several) emit different scents that attract different midge species. The midges that are attracted are always females and they force their way in through the hairs in the ‘lantern’ and enter the chimney of the flower, intent on laying eggs. Once inside they encounter hooked, downward pointing cells in the chimney that make retreat difficult so they begin to head downwards, towards a ring of light at the base created by translucent cells at the bottom of the pot-shaped floral chamber. Soon they reach a band of slippery cells with an oily surface that sends then slithering down, through a stockade of downward pointing hairs (see second and third pictures from top) that make escape impossible, into the floral chamber at the base, There are nectaries at the bottom of this chamber, at the base of those minute, vertical, white petal-like structures, and as the midges drink from these they pick up the sticky pollen-producing stamens (small brown object, bottom picture) which become glued to their proboscis. If the midges were already carrying pollen when they arrived they will by now have pollinated the flower, which then lets them go. Once the flower is pollinated the stockade of hairs withers and the flower bends through 90 degrees on its stalk, so the chimney is horizontal and the midges, carrying pollen, can escape. If the midges didn’t bring pollen to pollinate the flower and trigger this release mechanism, they can wait for up to four days for a suitably equipped midge to join them, pollinate the flower and effect release. Once they are out again, carrying pollen, they seek another flower emitting a similar scent and the whole sequence is repeated. And so the flowers are cross pollinated. Ceropegia woodii comes from South Africa and is a very easy, drought-tolerant plant to grow. The one that is the subject of these pictures is growing in a pot on the top of my bookshelves, sending out dangling strings of paired, heart-shaped leaves and weird flowers, as I write this. With the pot high-up and out of sight, I often forget to water it for a week or two, which seems to suit it fine.

Swallowing a Brick

Microscopic photosynthetic organisms called desmids and diatoms live in vast numbers in the surface plankton of lakes and oceans, where they are responsible for absorbing about 20% of the atmosphere's carbon dioxide and releasing oxygen, all the while acting as the foundation of the food chains for most aquatic animals. The top image here is of a desmid called Euastrum. Desmids are beautifully shaped, bilaterally symmetrical organisms with a distinct waist that divides them into two mirror-image halves. The next two images down show a diatom called Tabellaria. Diatoms are like microscopic pill boxes made of silica, and when the time comes to divide the top half forms a new bottom and the bottom half produces a new top and then – hey presto! – two perfect copies of the original. Tabellaria forms zig-zag chains of cells, joined at their corners. The glassy cases of diatoms are decorated with the most beautifully sculptured patterns, that are best appreciated at high magnification under an electron microscope. You can see some examples and read more about them at
http://www.ucl.ac.uk/GeolSci/micropal/diatom.html

The fourth image down shows another diatom, this time brick-shaped, and the final image shows a minute protozoan that has somehow managed to ingest one of these – you can just about make out the diatom’s outline inside the protozoan cell, which has a fringe of beating cilia like a monk’s tonsure which it uses for propulsion. Ingesting something this size is quite a feat, roughly equivalent to you or I swallowing a brick, and the diatom’s silica case is just about as digestible as a brick, so once the contents have been digested the diatom shell will be expelled. Diatom’s silica shells are virtually indestructible so layers of diatoms that lived in oceans millions of years ago and were digested like this one form fossil deposits, identifiable by their exquisitely preserved surface patterns. These deposits, known as diatomaceous earths, have been mined and used as an abrasive for polishing surfaces and also in filtration systems. The number of diatoms that can live in the ocean’s surface waters is ultimately limited by a shortage of essential iron that they require for growth and development. One proposal for tackling global warming has been to seed the ocean with iron, precipitating vast blooms of diatoms that will remove carbon dioxide from the atmosphere; desperate measures for desperate times, and very controversial too – see http://news.bbc.co.uk/1/hi/sci/tech/7856144.stm

The desmid and diatoms here came from a small boggy pool in Teesdale, in the North Pennines. All of the individual objects in the pictures are less than one tenth of a millimetre in diameter.