I’ve been visiting Cassop Vale National Nature Reserve for about thirty years now, mostly to visit the small lake that lies at the foot of the Permian limestone escarpment in County Durham. I remember one early visited yielding a handful of floating vegetation that included both of Britain’s free-floating liverwort species – the memory is seared into my memory because Rev. Gordon Graham, the local botanical expert, didn’t believe one of my records. In my ignorance I had recorded Riccia fluitans without knowing that it had never before been recorded from County Durham so I had to go and get another sample in order that he could verify it for himself.
On the cold Saturday morning in January when I visited, the pond was fringed with the dead stems of sedges and the water was icy cold on my skin. Yet there was life here, even at this most inhospitable time of year, including quite prolific growths of floating plants, especially the tiny leaflets of common duckweed, Lemna minor, in amongst the patches of sedge. The individual plants are just one or two millimetres in diameter, each with a root hanging down into the water for a few millimetres beneath them.
As ever, with my trips, the point of this visit was go beyond what I could see with my eyes even if, as is the case with Lemna, my starting point was, itself, only just visible. But by the standards of the microscopic world with which I normally deal, Lemna is huge. So huge, in fact, that I could not use my conventional microscope slides and coverslips and, instead, needed a cavity slide (one which has a shallow, round depression at the centre). Now I could put it under my microscope and explore the hidden world of Lemna in more detail.
Scattered across the underside of the leaf, I could see many elliptical outlines, each yellow-brown in colour. These were cells of Cocconeis placentula, an extremely common alga, belonging to a group called the diatoms. They are fast-growing species that thrive in many habitats, but the surfaces of aquatic plants often present the open spaces where this species can grow and multiply without competition from other algae. There were also a few needle-like cells belonging to another type of diatom, Fragilaria rumpens, growing upright from the leaf surface. All these are growing on the underside of the Lemna, so my picture looks as if it is upside down. This habit means that these diatoms are living in a shady underworld after the Lemna plants have filtered out the light that they need. But this should be no surprise as diatoms, worldwide, are species that thrive in the cool, dark depths of lakes and oceans. January in County Durham presents far less of a climatic challenge to these tiny organisms than it does to grown men who should have grown out of pond-dipping years ago.