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.

Travelling light

This strange object, magnified one hundred times under the microscope, is a single seed of a common spotted orchid Dactylorhiza fuchsii. The lower photo shows a couple of the orchid’s seed capsules, with the dust-like seed laying on the paper below.

Unlike seeds of oak and horse chestnut, which send their seeds out into the world with a large food store surrounding the embryo, orchids have a much more minimalist approach to equipping their embryos for future survival. The orchid embryo – inside the darker object in the centre of the seed in the top photo – has no food store and is housed in a fragile papery coat, just one cell thick. The whole seed is no larger than a speck of dust and is so light that it can be swept up by air currents and wafted long distances – orchid seed could easily be blown across the English Channel, for example. So, unlike heavy seeds with a large food that are unlikey to disperse very far from the parent plant, orchid seeds are great travellers heading for random destinations and this accounts for their tendency to suddenly appear in unlikely places – lawns, roadside verges, industrial spoil tips, to name but a few. A large orchid flower spike will produce tens of thousands of these minute seeds, but only a tiny fraction will ever achieve the next critical step in the life cycle – landing on soil that contains the essential mycorrhizal fungus that will link up with the germinating seed and provide the embryo with the nutrients that it lacks until the seedling is large enough to produce leaves and survive on its own. After that the orchid's roots returns the favour by supplying its partner fungus with nutrients for the rest of the orchid's life. Early growth of the orchid seedling is slow and its leaves passes unnoticed - until it's large enough to produce a spectacular flower spike........and to read about the next step in the life cycle - pollination of the flowers - take a look at  http://cabinetofcuriosities-greenfingers.blogspot.com/search/label/orchids

Psychedelic Citrus

Above and top: spheres of citrus oil trapped

amongst the cells in orange peel x100

Vertical section through an oil gland in orange peel.

In the intact gland oil accumulates in the central cavity

and leaks out onto the surface of the fruit via a minute pore. Polarised light. x40

The shiny flash-shaped patches in the surface of the orange peel,above the white pith, are the oil glands full of intensely fragrant oil.

Pleasure from eating comes from a combination of the taste and smell of food , and when it comes to oranges these two factors are subtly different but complementary. The distinctive taste comes from the soft acidic flesh, which only has a relatively faint aroma compared with the intensely fragrant peel. To see what I mean sniff an orange segment, then compare its scent with the rind by holding a small piece of peel under your nose, surface towards you and squeezing the peel hard. You’ll feel the citrus oils that are squirted from the flesh against your top lip and will experience a very strong citrus smell. The citrus oils are concentrated in hundreds of microscopic glands under those tiny dimples that cover the orange skin. This lunchtime I cut a thin vertical sliver through the peel of my orange and took a look at it under polarised light, which creates the psychedelic interference colours that you can see in the top three images. The central cavity of the oil gland is where the citrus oil accumulates as the fruit grows, but it leaked away when I cut the sliver of peel with a razor blade. The top two pictures show the residual citrus oil droplets that were trapped in the peel sliver, looking like those globules in 1960s lava lamps. We throw the fragrant but inedible orange peel away, but citrus oils have many commercial applications, in food flavourings and in the fragrance industry. In nature, their role is probably as a defence against insects that might otherwise burrow into the fruit. You can read more about citrus oils at http://www.aromaticplantproject.com/articles_archive/citrus_essential_oils.html

A Mouth Full of Stones

Pear stone cells, x400, polarised light
Pear stone cells, x400, interference contrast optics, showing the plasmodesmatal channels that connect one cell to the next. Notice the enormous thickness of the cell walls and the tiny internal cavity (lumen) that housed the cytoplasm of the cell when it was alive
A cluster of pear stone cells - all dead - surrounded by thin-walled living cells of the succulent pear flesh, x100
Clusters of stone cells in a thin section of the pear I ate for lunch. The green layer on the left is the skin of the fruit. x40. Dark field illumination.

A nice pair of pears

There are two distinctive features of pear fruit that makes it instantly recognisable from the first mouthful – a wonderful flavour and those gritty little particles in the pear flesh that get stuck between your teeth. The particles are stone cells, technically known as sclereids, which are clusters of dead cells with amazingly thick walls. The second photograph from the bottom is a very thin section through a piece of the pear that I had for my lunch today – those bright clusters are the stone cells. Next up, a cluster of these cells at higher magnification, viewed with polarised light and revealing the interference colours created by their thick, birefringent walls. The top two photograpsh show the individual cells at much higher magnification – with polarised light (top) and using interference contrast optics (below). In each you can see the enormous thickness of the walls, perforated by the channels called plasmodesmata that connected the cytoplasm of one cell with that of the next when they were alive. The function of these stone cells is uncertain, but they might have something to do with water supply to the rosettes of thin-walled living cells around them that inflate as the fruit grows. Generally speaking, consumers prefer pears that aren’t gritty, so pear breeders try to produce varieties without too many stone cells.

The Wonder of Whiskers

Groundsel flowers self-pollinate and run to seed with remarkable speed. Each seed is carried aloft on a parachute (pappus) of hairs...but there are other, much smaller hairs with a different function attached to the seed coat itself....

The seed coat also has a covering of microscopic hairs, far smaller that the parachute hairs, that rapidly extend outwards at right angles from the seed coat when it becomes wet. x40
A forest of minute seed coat hairs, at higher magnifcation (x100)

A single seed hair, magnified x400, using interference contrast microscopy that reveals hidden structures within the cell that the hair is formed from. Each hair is made up of a woven spiral column of threads, like a piece of string........or a water-absorbing wick?

Groundsel Senecio vulgaris is one of the most successful colonisers of cultivated land, thanks to its capacity to grow rapidly, flower quickly and disperse its seeds far and wide on tiny hairy parachutes that will carry them over large distances. But it has another winning adaptation for rapid invasion of ecological niches too, that I only noticed by accident recently. The Latin generic name of groundsel – Senecio – means ‘old man’ and alludes to its parachute of whiskery, silvery hairs that transport the seeds on the breeze. But, as I discovered yesterday, groundsel has a hidden complement of hairs that are invisible to the naked eye that have a quite different function: anchorage and water uptake. I’d put some groundsel seeds in a drop of water on a microscope slide, to photograph their parachute hairs, and was amazed to see a mass of writhing, much tinier hairs begin to extend from the coat of the seed, until hundreds were extended at right angles to the seed coat surface. When the seed is dry these are pressed so close to the seed coat that you don't notice them, but once it’s wetted they extend. What’s their function? Well, my guess is that they anchor the seed to a damp soil surface and then act as wicks, conducting water to the seed and speeding up germination. Notice how, at high magnification, each hair resembles a woven piece of string. Groundsel seeds germinate and establish themselves as seedlings incredibly quickly, and my guess is that this surface seed coating of ‘wicks’ is a crucial adaptation for helping airborne seeds that have landed on the soil surface to take up water and germinate as quickly as possible.

Thistledown

Individual branches at high magnification. The yellow, purple and blue colours are generated by using polarised light

The single-celled side branches resemble the barbules of a feather

Fine, single-cells hairs grow from the branches and form a light, overlapping mesh

Newly-shed thistledown, with seeds attached

We’re entering the season when plants are on the move, in the guise of seeds carried in the guts of birds that have eaten fruits, or hooked into the fur of animals, or carried on the wind. Thistledown – the very evocation of lightness and aerial buoyancy – can carry seeds miles, tens of miles, maybe even hundreds of miles on the breeze. Plants may be rooted to the spot, but their propagules are some of our planet’s most mobile travellers. Last week I was standing on a hillside in Weardale over five hundred metres above sea level, watching a continuous stream of thistledown drift past, swept up by thermals from thistle seed heads in the pastures down in the valley below. Under the microscope the main branches of thistledown (in this case from spear thistle Cirsium vulgare) can be seen to be made up of multicellular arms that with long, single tubular cells branching off, like the individual barbs of a feather, to produce an overlapping mesh that is light enough to provide enough lift to carry a seed aloft on a thermal. The colours here have been generated by the use of polarised light microscopy. Double-click for larger images.

Seed sculpturing

Fruits and seeds are often aesthetically pleasing natural objects but they reveal a whole new layer of complexity under the microscope. This is the ripe seed capsule of red campion Silene dioica which has just split open at the tip, with the segments at the apex rolling back to form a ring of teeth. Inside are the seeds, each bearing a complex surface pattern on their seed coats. Fruit and seed structures are often doagnistic characters within plant families and many species in the campion family (Caryophyllaceae) have capsules like this that split open to form a ring of teeth, that open and close depending on air moisture levels, protecting the seeds during wet weeather and opening to allow them to be shaken out when the sun shines. You can see another example at http://cabinetofcuriosities-greenfingers.blogspot.com/2009/07/exquisite-seed-dispenser.html