Aphids

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 .....

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......

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

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

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 .....

A

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

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.....

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.

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

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.