Having mentioned in my previous post that the epiphytes on the top and bottom surfaces of a Potamogeton polygonifolius leaf were different, I have produced a companion piece to the painting I showed in that post. The new painting is of the lower surface, and shows a greater number of diatoms than are present on the upper surface. In order to explain why this is the case, it is helpful to look at the structure of the Potamogeton leaves. The first image, therefore, shows a section through a leaf. It is quite a thick section but we can see the upper epidermis, the palisade mesophyll cells below this, which have plenty of chloroplasts in order to capture the sunlight that the plant needs for photosynthesis. Below this, we can see parenchymous tissue arranged to create some large internal air spaces which contribute to the leaves buoyancy. Finally, at the bottom, there is a single layer of epidermal cells. All this is crammed into a thickness of about half a millimetre.
Part of a section of a leaf of Potamogeton polygonifolius. The leaf vein is on the left, thinning to the leaf blade on the right. The leaf blade is about half a millimetre thick. The picture at the top of the post shows an artist’s impression of diatoms and Chamaesiphon cf. confervicolus on the lower surface of a Potamogeton polygonifolius leaf.
Viewed from the underside, these parenchymous tissues create polyhedronal chambers, ranging from about 100 to 200 micrometres (a tenth to a fifth of a millimetre) along the longest axis. There are also a few stomata scattered across the leaf surfaces (see the right hand image below).
With this in mind, take a look at my impression of the epiphytes growing on the lower surface of a Potamotgen polygonifolius leaf. There are a number of cells of Chamaesiphon cf confervicolius, as seen on the upper surface, but there are several cells of the diatom Achnanthidium minutissimum, growing on short stalks, plus a few long, thin cells of Ulnaria ulna, growing in small clusters on the leaf surface (there were a few other species present, but such low numbers that I have not included them here). It might seem strange to think of two surfaces of a leaf having such different communities of epiphytes but that’s because we’re thinking like large land-dwelling organisms, not like algae. The longest alga visible in the image of the leaf underside is Ulnaria ulna, at about a 10th of a millimetre in length. Therefore, to get a realistic impression of the two images, we really need to put a distance of five of these between them, and then pack the gap with chloroplast-rich mesophyll cells inside the Potamogeton leaf. Allowing for foreshortening, this distance is about five times the height of the image.
The structure of a Potamogeton polygonifolius leaf viewed from the underside. The left hand image (100x magnification) shows a leaf vein running diagonally across the lower right hand side along with the polyhedron-shaped chambers; the right hand image (400x magnification) shows the outline of one of these chambers superimposed behind the epidermal cells with a stomata with two guard cells visible just above the centre. Scale bar: 20 micrometres (= 1/50th of a millimetre).
The epiphytes on the upper surface of the leaf get first dibs at the meagre Pennine sunlight, which then has to pass through the upper layers of the Potamogeton leaf, where the mesophyll cells will continue to feast on the tastiest wavelengths, leaving relatively meagre pickings for the epiphytes that hang around on the underside of the leaf.
Chlorophyll, the molecule that makes plants green, absorbs light over a relatively narrow range of wavelengths – predominately red and blue – and this means that there are plenty of other wavelengths awaiting an organism with different pigments. Diatoms have chlorophyll, but they also have some carotenoids (principally fucoxanthin) that grabs energy from the green part of the visible light spectrum (which is reflected, rather than absorbed by chlorophyll) and passes it to the cell’s photosynthetic engine. Having this capability means that they can survive in relatively low light, which is why we see more diatoms on the underside of the Potamogeton leaf than on the top.
And that, best beloved, is the story of how Potamogeton got its epiphytes …
Some other highlights from this week:
Wrote this whilst listening to: more Bob Dylan. I’ve got to the mid-70s, which means the live version of Like a Rolling Stone on Before the Flood plus the great Blood on the Tracks. Also, as I was reading Ian Rankin, I listened to John Martyn’s Solid Air.
Cultural highlights: we’re watching the BBC adaptation of Sally Rooney’s Normal People
Currently reading: Ian Rankin’s Rather be the Devil.
Culinary highlight: A rather fine vegetarian chilli, from Felicity Cloake’s column in The Guardian last week. Served with corn bread, using a recipe we got from a hand-me-down American housekeeping magazine during our time in Nigeria.