Sunday, June 19, 2011

Scenedesmus algae

I am an amateur naturalist trying to discover everything living in my garden.

Some time ago I wrote about a puddle I spotted in my garden. Of course, I bought it into my house in a fishtank (doesn't everyone do this with their garden puddles?!).  It was only last week, nine months on, that I finally returned it to the great outdoors. Amazingly it was still brimming with microscopic 'pondlife', although it had become rather choked with the 'sludgy' cyanobacteria I wrote about here.

Over the months I repeatedly examined my puddle under the microscope. Photo 1 shows one of the lifeforms I found.  This one was rather common in the early days, but later seemed to disappear.

Spot something tiny and green under the microscope and its either algae or cyanobacteria. Cyanobacteria tend to be featureless, lacking detailed internal structure, notably a nucleus. The cells here have nuclei however, helping to identify them as algae. (The nuclei don't show up very clearly in photo 1 but by squinting I think you'll be able to see the spherical features towards the centres of the upper three cells).

Turning to my (borrowed) copy of the hefty "The Freshwater Algal Flora of the British Isles" (John, Whitton, Brook) - a 700-page light bedtime read! -  I was able to identify my algae as a member of the Scenedesmus genus. Characteristic features are the obviously 'pointy' crescent shape. Also, although I occasionally found single, isolated cells, very often I found 4 (as here) or 8 cells together (about which more shortly...).

Scenedesmus algae are part of the Chlorophyta (= the green algae). They are all freshwater. According to the book above as many as 200 different British species have been reported. However, it's been found that individuals from a species can grow into a variety of different forms as a result of different environmental stresses, so there's suspicion that many of these 200 'species' may not be unique. Furthermore the authors explain that recent scientific studies powered by advances in electron microscopy and modern-day biochemistry are pointing to a need to significantly rethink the traditional classification of many species. (This is happening everywhere in biology these days - see my posting on mushrooms here for example). All this means that although the authors give a key to the 42 British species of Scenedesmus algae they recognise, I've not attempted to pin down the species-identity of mine.

As I've said many times, I'm rarely disappointed when it comes discovering that there is some fascinating or unusual feature in the lifestyle of any creature I come across. Turning to my copy of R.E.Lee's Phycology (a present Christmas-last) it turns out that the fact I frequently saw 4 or 8 algal cells together was no accident.  If Scenedesmus is grown in a tank free from predators (such as grazing water fleas) it grows as single isolated 'unicells'. (If you're an algal cell wanting to maximise the amount of sunlight and nutrients reaching you its preferable to keep your distance from neighbours who might otherwise shield or shadow you). However, introduce some predators into the tank and, amazingly, Scenedesmus cells switch to growing in small groups as an anti-grazing defence! In the jargon, a group is known as a coenobium. Some groups also grow long spines, although interestingly the book implies that these are flotation devices to help the colony stay in the light, rather than anti-predator devices per se. The algae are able to detect the presence of predators by detecting chemicals ('infochemicals') in the water that leach from the digestive tract of the predators. Another of nature's tiny miracles!

Friday, June 10, 2011

A Mock Orange Tree - Philadelphus coronarius

I am an amateur naturalist trying to learn something about everything living in my garden.

Photos 1 and 2, taken a few days ago, shows the Mock Orange (Philadelphus) bush that grows at the back of my garden. It is a large plant (maybe 4m x 4m) and a fabulous site in early summer.

And yes, it smells as gorgeous as it looks! A rich honey/jasmine aroma that wafts across my lawn on summer evenings.

Philadelpus has long been popular with gardeners and plant nurseries stock numerous artificial cultivars. The Mock Orange (genus Philadelphus) and 'true' Orange (genus Citus) are really only distantly related. The genus Philadelphus is part of the large Hydrangeaceae family of plants.  I found a species-key here, and my plant keys out as Philadelphus coronarius.

Seeing a thin dusting of yellow pollen on many of the leaves I was inspired to get out my trusty student microscope. I enjoy fiddling around with microscopes and I'm a little surprised that in many years of doing so I've never before looked at pollen. For any beginnner like me who wants to have a go, there are a few tips it may help to know:

Firstly, something I hadn't previously realised is that plants release their pollen in a dehydrated state (15-35% water content is typical [ref.1]). I guess (but don't know) they do this to keep their weight low and so assist their transportation by wind or insects. Also, desiccation may help prolong the active lifespan of the grain. Dehydrated and hydrated grains can look significantly different (see photo 3).

A second thing it helps to know is that pollen grains often have a waxy surface coating. This can be a nuisance for microscopy as it may cause grains to stick together. It also obscures fine surface features of the grains.

Fortunately, both rehydration of pollen grains and removal of their waxy layer is easily achieved by simply wetting them with a few drops of alcohol.

Once you've looked at your pollen slide you can of course simply throw it away. Microscope users will know however, it is possible to make a collection of semi-permanent slides by encapsulating specimens in glycerine jelly. You can buy glycerine jelly especially designed for pollen. The jelly contains red dye that stains the grains and makes it easier to see fine details on their surfaces.

Anyway, photo 3 shows the results of the above: circular/triangular pollen grains about 12microns across.

Seeking to learn some more about pollen, I came across a nice review paper by Edlund here [ref.1]. The paper highlights various areas where the science behind pollen is unexplored or only partly understood. Take for example the functioning of the outer coating of pollen grains (the exine). This layer can be extremely ornate. Often it is riddled with cavities containing exotic plant proteins. When a dehydrated pollen grain lands on the 'female' stimga in the centre of a flower of the same species, something about the surface of the grain causes it stick fast, when pollen from a different species doesn't. The science behind this 'selective adhesion' is only partly understood.

Once a grain has stuck, chemicals are exuded by the stimga that rehydrate the pollen in a matter of minutes. Once again, this can be exquisitely selective. Two species of pollen grain can be put, side-by-side, onto a single stigma, and only the pollen grain from the correct species will be rehydrated. How nature manages to pull off this clever stunt is again something of a mystery.

With the grain re-hydrated, it germinates and sprouts a single tube that grows its way down the stimga. (There's a fun article by Chris Thomas here that describes how to observe pollen tubes by sprouting grains on a piece of onion skin).  Eventually the pollen tube contacts with a female egg at the base of the stigma and the pollen grain sends its DNA down the tube to fertilise the egg. (Pollen grains are a mechanism by which DNA is carried between plants. Its wrong to think of them as 'male sperm' however since a pollen grain is mostly comprised of bundles of 'normal' vegetative plant cells.)

Some plants rely on wind to spread their pollen. Others, animals and insects. Something new I learnt was that one plant- Lagerstroemia - is so keen to attract the latter it produces two types of pollen: a sterile, yellow, feeding pollen and a fertile, blue one.

My first microscope observations of pollen were great fun and I learned a lot. It turned out my Mock Orange had an another interesting microscopic feature for me. What it was however, will need to wait for another posting.

[1] Pollen and Stimga Structure and Function, A.F. Edlund, R. D. Swanson, Preuss, The Plant Cell 16:S84-S97 (2004)

Saturday, June 4, 2011

Hedge Bindweed Calystegia sepium

I am an amateur naturalist trying to discover everything living in my garden.

Photos 1 and 2 show some Hedge Bindweed (Calystegia sepium) - a weed that pops up frequently in my garden.

My copy of The Englishman's Flora (Geoffrey Grigson) lists dozens of alternative names for this common plant, from the pretty Rutland Beauty, Shimmy-and-Buttons and Robin-run-the-Hedge,to the sinister  Devils Garter, Strangleweed and Devil's Guts.

A name not in the book is the one my mother taught  - Granny Pop the Bed - so called because if you squeeze the green base of the trumpet shaped head (see photo 2) the white flower pops out. It's not a very convincing pop it has to be said, but hey, when you're six its great!

In the jargon, the green flower base is called the calyx (I've labelled this in photo 2). The 'leaves' that make it up are called sepals. C.sepium also has an outer epicalyx.

When I first started this posting I took it for granted  that a web search would turn up scores of scientific papers on my common weed. As it turned out I struggled to find any! I did come across a research paper [1] on the apparently unusual lectin (a protein) biochemistry chemistry of my weed, but the subject matter was rather technical and I'm not expert enough to do it justice here. This aside most of the material I did manage to find concerned the related Field Bindweed (Convolvulus arvensis), a target for frequent study because it is a major weed of arable crops. (Field- and Hedge Bindweed can be readily distinguished by  knowing that Field Bindweed doesn't have an epicalyx).

This lack of literature meant that for a time I was left wondering what to say in this post, but then I recalled an unusual fact concerning Bindweed's spiral growth which I'd read about some time ago (I don't remember where). Photo 1 shows a plant climbing a garden cane. As it climbs the stalk is seen to spiral in an anticlockwise direction (as viewed from above). What's interesting is that C.sepium always spirals anticlockwise. (I've even been around my garden checked! Indeed once you know this fact, its hard resist the temptation to check the spiral of every Hedge Bindweed you see anywhere!).

This feature of always twisting one way  turned out to be rather a rich topic for exploration. Amongst others I was led to a paper by Thitamdee [2] on the origins of spiral forms in plants:

The authors' studies focused on plant microtubules.  These are molecular sized rods found in both plant and animal cells (they've received mention on my blog before, here). Its been discovered that large numbers of these rods decorate the surface of plant cells (like matchsticks stuck on a balloon). The rods do not lie randomly on the surface of the cells however, rather they order themselves so as to line up along a common direction. There's some amazing video of real microtubules jostling about on cell surfaces on the webpage of Indiana University's Shaw Lab. here.  Now, to continue the balloon analogy, imagine having a lot of matchsticks densely glued to the surface of one of those sausage shaped party balloons. Imagine the matches are all lined up so as point around the short, circular axis of the balloon (like hoops around a barrel). Next, imagine blowing more air into the balloon. Though I haven't actually done the experiment, I hope its reasonable to suggest that the rigid matches would make it more difficult for the balloon to swell  in circular cross-section (get 'fatter'), and instead the balloon would grow more freely lengthwise (get longer). This is exactly what aligned microtubules are believed to do for plant cells  i.e. cells that would otherwise grow and expand as simple spheres are instead constrained to grow and expand along a preferred direction. This is useful because it allows the plant to create e.g. long, thin cells suitable the plant stalk. (Actually, strictly its not microtubles themselves that constrain the growth of the cell walls,  rather the microtubles appear to act as markers for the laying down of a secondary stiffening material - cellulose - but the principle's the same)

Now, Thitamdee were studying a cress plant called Arabidopsiss. This is famous amongst botanists as the plant for genetic studies worldwide. Normal Arabidposis plants don't spiral,  they grow straight. Furthermore, when scientists looked at the microtubules on cells in the stalk they found them to be arranged exactly as in the description above (i.e. 'hoops around a barrel')

What Thitamdee discovered however, was that a mutation in a single gene can cause a change in the way microtubles on cells in the stalk of an Arabidopsis plant arrange themselves. Specifically, they observed mutations that caused the microtubules to shift from being aligned all-parallel to the cell circumference ('hoops around a barrel') to instead all lying on the cell surface at an angle to the long axis of the cell.

And what did these mutant plants, with their slanted microtubles, do? Yep, grow in a spiral!

The importance of this work is that it implies an explanation for why some plants spiral and some don't, and furthermore why, for many species, every individual must spiral in the same direction: Things are dictated by the angle at which microtubles are aligned on the cells. This in turn is hardwired by the plant's DNA.  Perhaps an ancient ancestor of Hedge Bindweed grew straight. At some point however a gene mutation arose that caused the microtubles to align at some new angle. With this angle fixed by the DNA, the Bindweed's fate was fixed; Subservient to the constraining forces acting on its cell walls, it was doomed to spiral, and always in the same direction, this being dictated by the angle of microtubule alignment (though what this is specifically I don't know -  I haven't found any reference to suggest the microtuble alignment of C. sepium specifically has been studied).

To end, a bit of humble pie. When I first recognised the anticlockwise spiralling of Bindweed, I admit I thought I was rather clever in having uncovered some little known fact... until, that was, I discovered that my supposed 'little known fact' even had its own popular 1950's song!

The fragrant honeysuckle spirals clockwise to the sun, 
And many other creepers do the same.
But some climb anti-clockwise, the bindweed does, for one,
Or Convolvulus, to give her proper name... 

( from Misalliance by Flanders and Swann, ).

The Crystal Structure of the Calystegia sepium Agglutinin Reveals a 
Novel Quaternary Arrangement of Lectin Subunits with a Prism Fold,  Y Bourne, The Journal Of Biological Chemistry, 279(1),pp. 527–533, 2004
[2] Microtubule basis for left-handed helical growth in Arabidopsis, S. Thitamadee, K. Tuchihara & T. Hashimoto, Nature, 417, p.193, 2002.

Thursday, June 2, 2011

A Grey Squirrel Sciurus carolinensis

I am an amateur naturalist trying to learn something about everything living in my garden.

Photo 1 (strictly I took this particular photo in a local park) shows a sporadic visitor to my garden, and my third mammal, the Grey Squirrel (Sciurus carolinensis).

To learn something about them I have been reading Squirrels by Jessica Holm (Whittet Books).

Together with the Red (Sciurus vulgaris) the Grey is one of two species of squirrel found in Britain. It is a North American import. The first pair was released by a Mr Broklehurst in the county of Cheshire in 1876. Famously, the Grey has thrived (indeed, they are legally classified as vermin), whilst the once common Red is today a protected species clinging on in a handful of isolated locations (I have only ever seen one myself- in Cumbria).

Why the populations have changed in this way is not entirely understood. It is often said (indeed, before reading Dr. Holt's book I too had lazily assumed) that the Greys have 'driven' the Reds from their 'territories'. This is false on two counts however: Firstly, in woodlands where both have been studied together its found that Reds and Greys do not show any undue aggression towards one another. Secondly (and a surprise to me) squirrels aren't territorial animals. The life of a squirrel is a 'roaming' one (though generally confined to some home range of a kilometer or so). Rather than all-out interspecies hostility, it seems that the Red population may have declined as a combination of diseases passed on by the invaders and because the smaller size of the Reds means they are less able to gather food in areas where the more avaricious Greys are eating much of it up. Totally, more study is necessary however.

A few additional things of interest I picked up from my reading are firstly that Reds and Greys normally carry distinct species of flea (Monopsyllus sciuronum for the Reds, Orchopeas howardii for the Grey). Mother Nature is nothing if not an expert in specialisation! Secondly, watching a squirrel work through a pile of nuts, it will sometimes be observed to discard one without opening it. These turn out to be bad nuts with withered kernels. How the squirrel determines this with the nut still in its shell is rather impressive. It weighs them in its paws. A neat party trick!

To end, the literature abounds with Squirrel poems and Beatrix Potter quotes, but for me there is only one winner of the prize for top squirrel literary moment:

The squirrels pulled Veruca to the ground and started carrying her across the floor.
"My goodness she is a bad nut after all" said Mr Wonka, "Her head must have sounded quite hollow" [...]
"Where are they taking her?" shrieked Mrs Salt.
"She's going where all the other bad nuts go" said Mr Willy Wonka. "Down the rubbish chute."
[Roald Dahl, Charlie and the Chocolate Factory]