Beyond the Human Eye

Fatal Attraction

noreply@blogger.com (Phil) — Sun, 17 Mar 2013 10:56:00 +0000

Fungus gnats that emerge in swarms from soil in plant pots have become the bane of many gardeners' lives. If you grow plants in commercial potting composts on your house window ledge or in a greenhouse or conservatory, it's inevitable that you'll encounter these irritating pests because it seems that all currently available bags of potting compost are infested with them.

These little insects are scientifically known as Bradysia paupera and belong to a group known as sciarid flies. Each female can lay around 200 eggs which hatch into a worm-like, transparent larva that feeds on organic matter in the soil and also on young plant roots. A heavy infestation is capable of killing seedlings. They breed all-year-round, with overlapping generations that take less than a month to progress from egg to adult, so combating them is a constant challenge, but fortunately they have a fatal weakness - the colour yellow. They are attracted to these sticky yellow sheets of plastic that you can buy in garden centres and are glued to them as soon as their feet touch the surface.

Yellow strips of sticky plastic plastered with dead flies are unsightly but there is a more aesthetically attractive alternative - the carnivorous butterwort, Pinguicula sp., whose sticky leaves are like natural flypaper and which produces attractive flowers throughout the year. To see how effective this is, scroll down to the bottom of this post, and to see how it works, click here.

The little club-shaped structures on either side of the insect are halteres - balancing organs which smooth its flight path as its wings beat up and down.

Long-legged sciarid flies spend much of their time running around over the surface of the soil, where they lay their eggs.

Whiteflies caught on the sticky hairs of a butterwort leaf

Sciarid flies trapped on the sticky surface of a butterwort leaf

Scale insects ....

noreply@blogger.com (Phil) — Sun, 06 Jan 2013 19:06:00 +0000

I recently discovered that the orchid on my desk is infested with these tiny scale insects Coccus hesperidium. Each one looks like a miniature tortoise, about 3-4mm. long and tightly attached to leaves.

The tell-tale symptom was the sticky secretion on my desk under the leaves - scale insects suck sap, like aphids, and excrete honeydew.

When I took a close look it was easy to see how the infestation had built up so rapidly. Under the heat of the microscope lamp these infants, known as 'crawlers', emerged from under their mother. The females reproduce parthenogenetically, producing about 1000 nymphs during their three month life span and sheltering the young under their shield.

When you flip a scale insect over you can see the hollow cavity which acts as a nursery for the nymphs. Each is less than 0.25mm long

Nymphs are flattened and are very active, but as they begin to develop their broad shield they settle in one spot to feed.

Here's an older individual, where you can see the shield beginning to grow outwards around the insect. The waxy shield makes these insects impervious to most insecticide sprays - so they are difficult to control. Suffocating them under horticultural oil sprays or picking them off laboriously with a paintbrush dipped in alcohol are really the only effective treatments for infected plants.

This nymph, which is about 3mm. long, has settled to feed. It's probably about a month old.

After two months they reach maturity and begin to reproduce, and then their shield grows darker as they age.

Scale insects infest a very wide range of plant hosts. You can read more about them by clicking here.

Mosquito larvae...

noreply@blogger.com (Phil) — Wed, 24 Oct 2012 19:52:00 +0000

Our unusually wet summer has provided plenty of opportunities for breeding mosquitoes, with water butts and containers retaining pools of water all through the year. The water butt attached to our greenhouse has been swarming with culicine mosquito larvae.

The larvae hang from the surface film, breathing through a siphon tube, which you can see in this image on the tail of the larva; you can see a small bubble of air attached to the siphon tip. The tiniest disturbance of the surface film sends the larvae wriggling down into the depths.

For images of an adult culicine mosquito, click here and here.

How to Recruit an Army

noreply@blogger.com (Phil) — Thu, 30 Aug 2012 08:51:00 +0000

Plants primarily secrete nectar as an energy source to tempt pollinators to visit their flowers, but the secretion of this substance appears to have evolved long before flowering plants appeared. Many plants, including some ferns, secrete nectar from extrafloral nectaries - i.e. nectaries in other positions on the surface of the plant.

Legumes, like the common vetch Vicia sativa in the image above, have extrafloral nectaries on their stipules (the small, leaf-like projections on either side of the base of a leaf stalk). The extrafloral nectary is the black spot on the image above and a closer look ....

... reveals what its function might be. Ants are famous for their attraction to sweet substances and regularly visit the plant for the sugar that leaks out of these locations. This might deliver two kinds of selective advantage to the plant that would outweigh the cost of using some of its assimilated sucrose in this way. In some plants it might deflect ants, which are usually very inefficient pollinators, away from the larger source of nectar that's there to service more efficient pollinators, like bees. In other plants it may be a way of recruiting a defensive army of ants because they become aggressive towards herbivorous insects that might try to plunder their food supply; in Acacia trees for example, the defensive benefits of hosting ants are well documented.

Extrafloral nectaries are found in a wide variety of plants and are often located on leaf petioles and mid-ribs. This is a vertical section through an extrafloral nectary on the underside of the mid-rib of a cotton plant (Gossypium sp.), stained with fluorescent dyes. The bright yellow cells at the top are xylem vessels, conducting water to the leaf blade. The very small, brick shaped blue cells below are dividing cambial cells and also phloem sieve elements that are conducting assimilated sucrose away from the leaf blade. Below that are some larger, blue-stained parenchymatous cells and then, at the very bottom, there are thin-walled finger-shaped cells which constitute the extrafloral nectary tissue, on the lower surface of the leaf mid-rib.

The blue staining is due to cellulose in the cell walls binding to a dye called calcofluor, which then fluorescence blue in UV light. You can see from this image that there's a very thin cellulose cell wall in those finger-shaped extrafloral nectary cells, because they barely fluoresce. So they easily leak sucrose that accumulates in them. The other interesting feature of this section is the orange staining in the small cells immediately above those extra-floral nectary cells. This is the endoplasmic reticulum/ Golgi complex inside the cells - the membranes and secretory vesicles that manufacture substances and transport them between cells via channels in the cell walls called plasmodesmata; these brightly-fluorescing cells seem to be highly metabolically active, so maybe the nectary cells are secreting something else, as well as sucrose.

There are some scientific papers on cotton extrafloral nectaries, their role and how they might be exploited in biological control programmes in this crop here, here and here.

Millions of Mites

noreply@blogger.com (Phil) — Wed, 08 Aug 2012 17:27:00 +0000

By the time that summer arrives the foliage of most trees shows signs of insect attack, but these little eruptions on the surface of an alder leaf are caused by eriophyid mites, which are not insects but are related to spiders. I think the mite species that has produced these is Eriophyes laevis inangularis.

Each of these little domes is a chamber that's formed when the mites feed on cells on the undersurface of the leaf, leading to uneven growth that results in the formation of a pouch where the mites can feed and breed.

This is the underside of the leaf, with the little yellow, sausage-shaped mites crawling around the entrances to the chambers, which are lined with nutritive cells that provide sustenance for the mites.

Here they are at higher magnification .........

............ and at still higher magnification, when the elongated body with four legs at the head end is visible in the mite in the top, left-hand corner. Each chamber is home to a brood of mites and a tree with a severe infestation could be covered with hundreds of thousands of them. Eriophyid mites also commonly infest sycamore and field maple leaves, producing large numbers of red pouches on the leaf surface.

These are three of the mites, each being about one fifth of a millimetre long, with only four legs.

The outer cuticle of the animal has a distinct pattern that differs between species, although the easiest way to identify species is via the symptoms that they cause on the host plant.

Here is the head, legs and cuticle patterning at higher magnification.

In addition to infesting sycamore, field maple and alder leaves eriophyid mites also attack many other plants, including goosegrass (aka cleavers) Galium aparine, whose growth is distorted by Eriophyes galii.

Typically, infested leaves curve inwards at the edges and become spoon-shaped, like the bottom, second-from-the-left leaf in this picture.

Here's the goosegrass eriophyid - the dark, globular structure top left is an air bubble on the microscope slide.

In this view you can see some of the surface patterning and an internal structure - perhaps an egg?-

... and in this plane of focus the surface pattern of the cuticle is apparent.

Aphids in a Savage Landscape

noreply@blogger.com (Phil) — Mon, 16 Jul 2012 21:53:00 +0000

When aphids infest plants they tend to find a good spot to feed and then stay in one place, where they'll insert their stylets into the plant's phloem, tap its sugary sap and then settle down to reproduce.

When you take a close look at plant surfaces you can sometimes see why these pests are more or less sedentary. Many plants, like this goosegrass Galium aparine, are covered with epidermal hairs (trichomes) that make it difficult to tiny aphids to move around.

In the case of goosegrass the hooked hairs are primarily for attaching their weak stems to supports as they grow, but those curved spines are also awkward obstacles for minute aphids to negotiate.

Aphids

noreply@blogger.com (Phil) — Sat, 09 Jun 2012 21:49:00 +0000

Aphids, also known as greenfly, are extremely successful sap sucking insects with a phenomenal rate of reproduction, which makes them major agricultural and horticultural pests. Within a month or so the little family group of 15 individuals in the photo below have the capacity to leave many hundreds of descendants, thanks to their ability to reproduce without sex. They give birth to live parthenogenetic young, which are clones of their parent and can themselves begin to reproduce within a few days of birth.

This individual has given birth to one offspring which is already feeding on the host plant, while a second is just about to be born. These already have the developing embryos of the next generation developing inside them.

This aphid parent is giving birth while still feeding - you can see its sylets, like a hypodermic syringe, inserted into the vein of the leaf. Winged aphids like this disperse widely between crops.

Many aphids only undergo sexual reproduction as winter approaches, leaving genetically variable eggs that will include some that are better adapted to endure the rigours of winter. These well adapted survivors will hatch and clone themselves in spring.

Past its Use-by Date .....

noreply@blogger.com (Phil) — Mon, 14 May 2012 15:39:00 +0000

If you have ever gone away on holiday and forgot that you left some cheese in the cheese dish, then ....

..... this will be a familiar sight when you return home. This slab of Cheddar has become .........

...... a battleground for fungal colonies ............

....... that jostle for supremacy when the colonies collide, and in doing so create a rather attractive, furry abstract design.

Once the mould has smothered the cheese surface it's time to reproduce ....

..... via stalked sporangia ......

.... that resemble little white pom-poms...

.... each of which releases .......

... vast numbers of these minute conidiospores, each just a few thousands of a millimetre in diameter.

Spud-You-Won't-Like......

noreply@blogger.com (Phil) — Wed, 09 May 2012 20:41:00 +0000

Eating green potatoes that have been stored for too long in bright light is a big mistake. They can make you very ill - not because of the green pigment which is chlorophyll and is no more harmful than eating green lettuce, but because potatoes that are exposed to light produce a toxic glycoalkaloid called solanine below their skin. It's most likely a natural defence mechanism, to protect the plant from insect pests and fungal pathogens.

This is a section taken perpendicular to the potato surface, through those green cell layers. It's been stained with a fluorescent dye that has a particular affinity to the toxin, which fluoresces brightly in its presence, so you can see glowing crystals of solanine inside these cell layers. The rounder, translucent greenish objects are starch grains.

Wild potatoes have a much higher solanine content than cultivated varieties. Part of the domestication process of many of our crops has been selective breeding to remove natural toxins that protect the plants from pests and diseases - but also poison people. That's why we have to use applied pesticides on crops, to replace their natural equivalents that have been bred out of the plants, whose defences have been weakened in order to make them edible: it's a vicious circle!

Plant Harpoons

noreply@blogger.com (Phil) — Tue, 01 May 2012 20:34:00 +0000

This lethal-looking weapon, just a couple of millimetres long, is the defensive weaponry deployed by a prickly pear cactus called Opuntia rufida. Most prickly pear species are armed with formidable spines that are several centimetres long and capable of drawing blood but this species has a surface ....

... covered with these small areoles - dense clusters of tiny, rigid hairs called glochids that are only loosely attached to the plant and ....

...... are easily dislodged by the slightest touch - or even by the wind. Those in this picture were gently brushed and you can see how they've broken loose.

Each glochid is tipped with a sharp point (here magnified x100) that easily penetrates soft flesh like the lips and eyes of an animal attempting to eat the plant .....

..... and backward pointing barbs make it very difficult to remove. These microscopic harpoons are intensely irritating and potentially dangerous if they end up in your eyes, mouth or throat. The easiest way to remove them from skin is to use sticky tape to pull them out but if they end up in more vulnerable areas you may need hospital treatment. You can find medical advice here.

Opuntia rufida grows in arid parts of Texas in the United States. For more on prickly pears, click here.

Pond Population Explosion

noreply@blogger.com (Phil) — Thu, 05 Apr 2012 20:22:00 +0000

A combination of unusually warm spring weather and spawning frogs that stirred up the mud and so released a lot of of nutrients into the water recently led to an algal bloom in our garden pond. Large patches of algae floated on the surface in mucilaginous mats that trapped bubbles of oxygen.

Under the microscope, at x40 magnification, the algal cells were round, highly motile and present in vast numbers. This group represents the population in about 2% of a single drop of water.

The same, but at x100 .... and at ....

.... x400. I am not certain what species this is but I think it may be Chlamydomonas. The paired flagella of each cell are not resolvable with this microscope at this magnification.

The most striking aspect of this algal bloom, apart from the sheer numbers of cells, is the hyperactivity of the algae. The bubbles of scum on the pond surface may seem static, but at this magnification they represent a surface film of frantic activity.

Within Every Grass Leaf There Are Hidden Smiley Faces .....

noreply@blogger.com (Phil) — Fri, 30 Mar 2012 22:01:00 +0000

A vascular bundle in a transverse section of a grass leaf, stained with the fluorochromes Calcofluor M2R (blue fluorescence = cellulose) and auramine O (yellow fluorescence = lignified cell walls). The red fluorescence is chlorophyll autofluorescing red in the blue excitation beam of the microscope.

The two big 'eyes' in this 'smiley face' (which is typical of a monocot vascular bundle) are metaxylem elements that transport water through the leaf. The bright blue fluorescence in the 'mouth' of the 'smiley face' is phloem, composed of larger sieve tubes and smaller rectangular (in cross section) companion cells, which together transport sugars, made by photosynthesis, out of the leaf. The bright yellow cells forming the neck of the 'smiley face' are lignified, providing a measure of rigidity in the leaf, and the band of cells along the bottom of the section are epidermal cells covered by a cuticle.

Seaweed Sexual Reproduction: a chancy business

noreply@blogger.com (Phil) — Fri, 24 Feb 2012 19:11:00 +0000

This is a transverse section of the conceptacle of a brown seaweed Fucus sp., commonly known as saw wrack. It has been stained with a fluorescent dye called anilino-naphthalene-sulphonic acid.

Brown seaweeds in the genus Fucus are common in the intertidal zone. Two species are visible here - saw wrack Fucus serratus with a saw-tooth edge to the fronds and bladder wrack F. vesiculosus with smooth frond edges and paired flotation bladders. In spring they make rapid new growth and enter their reproductive phase, producing swollen receptacles at the end of the fronds.

The receptacles are covered in large numbers of small swellings called conceptacles, each of which opens via a minute pore called the ostiole (double click image to enlarge).

This is a section through a receptacle showing two conceptacles developing inside. This is from a female conceptacle. The radiating, elongated filament-like structures are sterile hairs (paraphyses) and the club-shaped structures are oogonia, each of which produces eight eggs (oospheres)....

...... and here is an egg (oosphere) being liberated from an ostiole into the surrounding water. Inside the conceptacle some oogonia are still dividing by meiosis to produce oospheres - you can see the cell walls forming.

When the conceptacles are mature eggs and vast numbers of swimming male cells (antherozoids) are liberated into the water of the rising tide - most prolifically during spring tides - and at high tide the eggs are fertilised, if they are lucky, and carried away by the falling tide. If they're luckier still the fertilised zygotes attach to a rock and develop into a new seaweed. The clusters of small bright yellow structures that you can see here amongst the rounded oospheres are the antheridia that produce the antherozoids - this conceptacle is hermaphrodite, showing that it came from spiral wrack Fucus spiralis; saw wrack and bladder wrack have conceptacles that are either male or female.

You can find images of thin sections and male and female conceptacles of fucoid seaweeds here, more detailed information on their structure and life cycle here and more on Fucus and other seaweeds here.

Better than Cannabis.....

noreply@blogger.com (Phil) — Wed, 11 Jan 2012 17:51:00 +0000

There was a time – before the advent of synthetic fibres based on plastics and petrochemicals - when the wealth and security of nations depended on tough, coarse plant fibres that provided rigging for sailing ships and the raw material for countless other essential objects - like sacks and sails for example - so explorers were always on the look-out for new supplies of this strategic material.

Joseph Banks, travelling on Captain Cook’s first voyage to the South Seas in 1769, had high hopes that he might make a fortune from growing New Zealand flax Phormium tenax that he found in those Antipodean islands, as a substitute for cannabis fibre which, up until then, had provided most of the fibre for rigging naval vessels. Maoris made their traditional textiles from the Phormium fibres but Banks envisaged a thriving industrial market for the product, whose fibres are much stronger than those of cannabis, and an attempt was made to use convicts to grow the plant as a fibre crop on Norfolk Island. Banks was destined to be disappointed - you can read an account here - but it did become an important source of fibre for rigging in the 19th. century..

The image above shows a transverse section of a New Zealand flax leaf, using the fluorescent dye auramine O to stain the lignified fibres, which show up as the transverse yellow-green bands in the image. I've turned the natural orientation of the leaf 90 degrees clockwise, to fit the page.

The thick-walled fibres have a tiny central cavity (the lumen), which is typical of sclerenchymatous fibres.

The transverse red bands are photosynthetic parenchymatous cells - chlorophyll fluoresces red in the blue light that was used to illuminate the specimen.

The bright blue cells are bundles of thin-walled phloem, which has no lignin in its walls, and the brighter yellow cells surrounding the phloem will be lignified xylem, conducting water.

The lower surface of the leaf, to the left of the image, has a lignified hypodermis, below the epidermis.

The natural function of the fibres and lignified hypodermis is to provide structural rigidity for the long, narrow, sword-shaped leaves, which are held upright in the living plant, which is illustrated below (public domain image from Wikipedia Commons http://upload.wikimedia.org/wikipedia/commons/a/a5/NZflaxPiha02.jpg

Today Phormium tenax is mostly grown as a decorative garden plant.

Micro-rock-pooling in Winter.

noreply@blogger.com (Phil) — Fri, 30 Dec 2011 15:32:00 +0000

It's too cold in winter to spend a lot of time paddling around in rock pools but you can always take a few samples of seaweed home on a jar of seawater and have a look at the smaller inhabitants under the microscope. These two, each about a millimetre long, were in a sample of Corallina officinalis seaweed. The upper specimen is an unusually bristly acarine mite, found clambering through the seaweed fronds. You can see more acarine mites by clicking here.

This is a minute flatworm, with two very simple eyes, found gliding over the surface of the seaweed, propelled by thousands of cilia that are only visible at high magnification under the microscope. You can see another marine flatworm, in more detail and with a movie of the cilia in action, by clicking here.

Sowing Wild Oats

noreply@blogger.com (Phil) — Fri, 23 Dec 2011 20:34:00 +0000

Some seeds need to be sown while others - like wild oat Avena fatua - sow themselves. This is a wild oat fruit (or, to be botanically accurate a caryopsis) in the dry state. It's equipped with a long awn (which is extension of the floret in which the fruit formed and in which it is shed), that's bent at a right angle about a quarter of the way along it's length.

When the caryopsis falls to the ground and gets wet - from a passing shower of rain, for example, that bent awn straightens, then bends again as it dries out. The picture above shows the same fruit, but now it's been moistened and the awn has straightened. As the awn bends and straightens it also rotates, because the awn is constructed from a helix of fibres that twist and generate torsion as they dry (see below).

The outer coat of the floret containing the caryopsis is equipped with this arrowhead of stiff hairs at the tip ....

.....which readily catch in fur and feathers and help disperse the seed, but also anchor it in crevices in the soil when it falls to earth.

There is also a beard of stiff hairs running up the groove in the caryopsis. As the awn rotates .....

.... with the expansion and contraction of this helical tube of spiral fibres that it's constructed from, it levers the caryopsis further into soil crevices. Those stiff hairs on the caryopsis help to anchor it in the soil, ratcheting it in ever deeper until it's in a moist enough position to germinate and put down roots.

This is a seed that sows itself.

The video below shows a group of wild oat caryopses writhing as their awns dry out and begin to rotate.

Breathing Space

noreply@blogger.com (Phil) — Sun, 27 Nov 2011 09:03:00 +0000

This is a thin section of the lower stem of water milfoil Myriophyllum sp. , stained with the fluorochrome calcofluor which binds to the cellulose of the cell walls and is fluorescing brightly in ultraviolet light. Marsh plants tend to be rooted in anaerobic mud and so have air channels (aerenchyma) that conduct oxygen down to the roots.

Working from the outside inwards in this section, there is a well defined single outer layer of very small cells forming the epidermis, then inside that lies the stem cortex with 17 air channels arranged around the central stele, which contains the phloem (brightest flourescence) and the xylem.

The stem and leaves of water milfoil. The small white structures in the leaf axils are the stigmas of the female flowers.

Plant Cuticles

noreply@blogger.com (Phil) — Thu, 10 Nov 2011 14:42:00 +0000

The surface of plants (with a few exceptions, such as those that live submerged under water) is covered with a tough, transparent, waxy layer called the cuticle, composed of cutin secreted by the layer of epidermal cells that it covers. The best way to see the cuticle is to snap the leaf of a drought-adapted succulent plant like Crassula ovata and pull one part of the leaf against the other, peeling away the cuticle, which covers the above ground parts like a wrapping of cling-film.

These cuticular peels, often with a single layer of epidermal cells attached, can be mounted on a microscope slide .....

..... and viewed to reveal the pattern of cells and the .....

...... stomata, which allow carbon dioxide to enter for photosynthesis. The two images above are of cuticular peels of spiderwort Tradescantia virginiana.

In this vertical section of a leaf, stained with fluorescent dyes, the cuticle appears as the bright yellow layer on top of the epidermal cells. In the centre you can see a single stoma in vertical section, with closed guard cells and its adjacent subsidiary cells, with a sub-stomatal cavity below leading to the loosely packed mesophyll cells, where photosynthesis takes place. The cuticle has a dual function - keeping water in and keeping the leaf surface dry.

In plants adapted to arid conditions (xerophytes), like this Aloe variegata , the cuticle is visible as an extremely thick transparent layer that allows very little water to escape from the leaf.

In plants that are subjected to frequent rainfall, like this nasturtium Tropaeolum majus leaf, fine grooves in the cuticle surface trap air below the water droplets, which then round-up under their own surface tension and simply roll off the leaf when it shakes in the wind. This is vital, as a wet leaf surface blocks stomata and prevents carbon dioxide from entering, slowing down photosynthesis. As the water rolls off the leaf it carries away dirt and dust, so the leaf cuticle is effectively a self-cleaning surface - a phenomenon known as the 'lotus effect', referring to the extremely hydrophobic self-cleaning leaves of sacred lotus. The silvery area under the central water droplet in the image above is caused by minute air bubbles, trapped between the water and the leaf surface.

The cuticle can also have a secondary defensive role, as seen in this painfully prickly leaf of the hedgehog holly Ilex aquifolium 'ferox', which is covered in cuticular spines. In general, leaves of evergreens, that survive for several years before they are shed, tend to have thick cuticles that protect the leaf against herbivore attack throughout their extended life.

Springtails

noreply@blogger.com (Phil) — Thu, 03 Nov 2011 12:11:00 +0000

Every time I lift the lid of our garden compost bin scores of these tiny insects, each smaller than the diameter of the head of a pin, leap around in all directions. They are members of the ancient insect order known as the Collembola - commonly called springtails - and feed on decaying vegetation. The darkness, warmth and humidity of the compost bin suits them perfectly. I think this species might be Folsomia candida, which is very common in gardens.

Most of the time they move slowly on those stumpy legs but when they are alarmed they hurl themselves into the air using an organ called a furcula under their tail .....

.... which you can see in this specimen. You could liken its action to a kind of exceptionally energetic pole-vaulting. The tip of the furcula is held in place by a clip-like structure called a retinaculum, but when the muscles in the furcula contract the clip suddenly releases its grip and the furcula flicks downwards and backwards, hurling the animal upwards and forwards.

You can find pictures of another springtail species here and a fine set of photographs for ID purposes here.

Was Sid, the Mystery Microscopist, a Scotsman?

noreply@blogger.com (Phil) — Mon, 31 Oct 2011 11:19:00 +0000

When I was exploring the old microscope slides belonging to the mysterious 'Sid' that I mentioned in my previous post I came across one that was clearly home-made and was labelled as porphyritic olivine basalt from the Lion's Haunch, Arthur's Seat, Edinburgh. The hill is the plug from the mouth of a long extinct volcano, exposed after softer sedimentary rocks were eroded from around the igneous core. Polarised light microscopy reveals the crystalline inclusions very nicely.

Similar rock samples were collected on the Apollo 15 mission to the moon in 1971

Arthur's Seat is the very conspicuous hill - some might call it a mountain - almost in the centre of Edinburgh. From the right angle it looks like a resting lion and this polished thin section of igneous rock came from the 'Lion's Haunch'. Of couse, there's no way of telling whether Sid, who was apparently a chemist, prepared this specimen himself or swapped it with a geologist friend - microscope slide exchange clubs were once common - but he clearly had an interest in Scottish geology, so may have been a Scotsman......

Mystery Microscopist

noreply@blogger.com (Phil) — Fri, 30 Sep 2011 21:29:00 +0000

Many years ago, when I was a student, I was given this box of old prepared microscope slides. I can't remember who the donor was but they've been at the back of a cupboard for years and only emerged when I was having a clear-out, a couple of months ago.

They date from around the time of the First World War. Some were commecially prepared by the firm of Watson & Son, of 313, High Holborn, London - labelled with beautiful handwriting, in mounts that were so well ringed with shellac that they have remained in perfect condition for almost a century.

Others were prepared by the original owner, who I'm guessing must have been a chemist because many are cystalised mounts of chemical compounds, intended to be viewed with polarised light, made from substances that would only have been available to a professional chemist - possibly a plant biochemist because quite a number of the crystals are naturally-occuring plant compounds that he might have extracted and purified himself. One contains the only clue to the identity of the mystery microscopist, because he has written his name in chemical crystals on the slide - 'SID'.

Sid would have looked at these specimens with a rather primitive instrument called a Fox Polariscope - so Sid probably wouldn't have seen the images in quite the same vibrant colours that you can see here, achieved with a modern polarising microscope. The specimen above is strychnine...

... this is floridzin, an alkaloid from apple roots...

...as is this one, too.

This is mercuric cyanide

Salignin...

Coumarin, the compound responsibe for the scent of new-mown hay...

Salicin, extracted from willow bark and the precursor of salicylic acid ,better known as aspirin...

... and ammonium bitartrate

Hooked on Hops

noreply@blogger.com (Phil) — Fri, 23 Sep 2011 20:01:00 +0000

Hops Humulus lupulus have an impressive ability to climb supports - either up other plants or, in the case of cultivated hops, up poles in hop gardens. Charles Darwin devoted a lot of time to studying the way in which their shoot tips rotate as they grow (by the process of circumnutation), seeking out objects to coil around (you can read more about his experiments here). There's more to hops' climbing ability than circumnutation and rapid growth, however - their stems are clothed in very distinctive epidermal hairs (trichomes) that act as grappling hooks, securing their grip on supporting structures.

The hop trichomes that are adapted for climbing have a very distinctive anvil shape - you can see them here, at low magnification, on either side of a hop leaf petiole.

At higher magnification the anvil shape is very distinctive, something noted ....

.... by the botanist Anton Kerner von Marilaun in his Natural History of Plants (1895).

Hops have been cultivated for centuries, primarily for the resins produced by their epidermal glands, mainly at the base of the bracts in the female flowers but also on other parts of the plant, including the underside of the leaf. In the photograph above you can see the minute gold drops of resin on the lower surface of a hop leaf. The resins are converted to bitter isohumulones during the brewing process, adding a distinctive flavour to beer.

Bacterial Root Nodules

noreply@blogger.com (Phil) — Mon, 22 Aug 2011 11:40:00 +0000

These are bacterial root nodules on the root of runner bean Phaseolus coccineus. Each nodule contains a population of Rhizobium bacteria that are capable of converting atmospheric nitrogen into soluble forms of nitrogen that the plant can use for growth - which is what makes this symbiotic association between plant and bacterium so valuable for agriculture. In annual legume crops, once the bean crop has been harvested the root nodules decay and release nitrogen in the soil, where it can give a yield boost to following non-legume crops in the crop rotation - like wheat, for example.

In this image one of the nodules has been cut in transverse section and stained with the fluorochromes calcofluor and auramine O. The plant root, with its xylem vessels visible, is at the top. The bacteria filling the root nodule, encased in blue-stained plant cells, are stained yellow. The Rhizobium bacteria in the soil penetrate through a root hair, trigger proliferation of the host plant root cells to form a nodule and multiply within. Healthy root nodules are pink when you cut them open due to the presence of leghaemoglobin which, like haemoglobin in mammalian blood, absorbs oxygen. This is important because oxygen would otherwise inhibit the enzymes in the nodule that 'fix' nitrogen into soluble forms. Bacterial nodules that are not pink when you cut them open are likely to be parasitic on the host plant, rather than symbiotic.

This is the difference that nodulation makes. The plant on the right has effective nodules, the one on the left doesn't. The interaction between plant and bacterial strain is complex; for any give crop cultivar, different bacteral strains will show varying degrees of effectiveness in boosting crop yield and different crop varieties perform best with different bacterial strains. Deliberately inoculating seeds with effective Rhizobium strains can produce significant yield benefits, although there is no guarantee that any particular inoculum will persist in a soil type or location where it's not a naturally-occurring strain amongst the existing soil microbial community.

Out of Sight,Out of Mind...

noreply@blogger.com (Phil) — Wed, 10 Aug 2011 20:57:00 +0000

Roots are the most neglected parts of plants, perhaps because they are out of sight and - superficially at least - lack the intrinsic aesthetic beauty of the above-ground parts. For most (although not all) plants they are vital structures and - when you look really closely - they have an intricate beauty of their own.

Root tips are sensitive gravity detectors, ensuring that the root always grows downwards into the soil. This root was held in the horizontal plane for less than an hour before it redirected its growth downwards. Behind the root tip you can see the point where the root hairs develop, with newly initiated root hairs just visible nearest the root tip but becoming longer as you move away from it. Further back still the root hairs die away continually and each has a life span of just a day or two, but they are continually replaced as the root penetrates further into the soil. The passage of the root through the soil is assisted by lubricating mucilage produced by the root tip, whose surface cells slough off. The mucilage also supports a bacterial microflora that helps the root acquire nutrients and may provide some protection from disease-producing organisms.

This is a root tip sectioned vertically and stained with a fluorescent dye called DAPI. If you click on the image to enlarge it the details will be a little clearer. The brightly fluoresescing dots are the nuclei, one per cell, and you can see the files of cells produced by sequential cell division followed by cell elongation, which pushes the root ever-further into the soil.

This is a root in transverse section, further back from the tip than the previous image, in the middle of the root hair zone. It has been stained with a fluorescent dye called calcofluor, which makes the cellulose cell walls fluoresce blue in ultraviolet light. From the outside inwards, you can see the long root hairs, each a single cell that arises from the root epidermis (surface layer of cells). Next inwards lies the root cortex, which constitutes the vast bulk of the cells, then in the centre you can see the stele - the cylinder of vascular tissue that transports water upwards to the rest of the plant and carries sugars and amino acides downwards to support the continued growth of the root.

The arrangement of the various cells and structures is more clearly visible here, at higher magnification. The large circles in the stele, top left, are xylem vessels that conduct water away from the root.

The root hairs, which are in intimate contact with the soil particles, absorb water and soluble minerals that are transported through the root cortex, both from cell-to-cell within cell cytoplasm (the symplastic route) and through cell walls and the spaces between cells (the apoplastic route), to the stele in the centre of the root.

Once the water reaches the stele it encounters a single layer of cells called the endodermis, that sheaths the stele. The walls of the endodermal cells contain a substance called suberin which renders them impermeable, so water that arrived via the apoplastic route is forced into and through the cytoplasm of these cells, where dissolved minerals are selectively removed. You can see the suberin deposits, known as the Casparian strip, as the orange staining in the single ring of cells that lies between the blue and the yellow cells in the section of a stele above. Some water also passes unimpeded through specialised passage cells in the endodermis - if you follow the ring of cells with the orange stained Casparian strip in their cell walls around the stele in the image above, you'll notice a few passage cells with no orange-stained suberin deposit in their walls.

Almost all the water taken up and transmitted via both routes, via the cytoplasm of the endodemis cells or via their passage cells, then enters the dead xylem cells that carry it aloft in the water column that is drawn upwards by transpiration from the leaves.

When gardeners buy plants in garden centres there's a great temptation to simply dig a hole and plant them, without teasing out the pot-bound roots or cultivating the soil around the planting hole, but a little tender, loving care for root systems pays great dividends: the vigour of the plant above the soil depends on the health of the roots, hidden below the surface.

Banana Stellate Parenchyma

noreply@blogger.com (Phil) — Sat, 18 Jun 2011 20:18:00 +0000

These beautiful cells come from the midrib of a banana leaf. Each is shaped like a 6- or 7-armed star, with its arms joined to the arms of surrounding cells, forming a lattice of cells. This form of tissue is known as stellate parenchyma and you can find another example here. The image was produced using polarised light and the brightly coloured birefringent objects inside the cells are calcium oxalate crystals inside the cell vacuole. You can see further examples of calcium oxalate crystals, including a video of their Brownian motion inside a cell, if you click here.

To find these cells you need to look inside the midrib of a banana (Musa sp.) leaf .....

by cutting transversely across the midrib, which reveals this internal pattern of strenthening tissue filled with very delicate, transverse plates of glassy cells ....

... then dissect out one of these plates of cells and mount it on a microscope slide.