Wagging tongues of ferns and salty yarns

Hart’s tongue fern, thriving in calcareous woodland soil. Photograph: Phil Gates

This section of moist, shady, wooded bank above the footpath, extending for perhaps 150 metres, is covered with the largest concentration of hart’s tongue ferns I have ever seen. This fern, Phyllitis scolopendrium, dominates because it thrives in calcareous woodland soils over limestone and the conditions here are perfect.

This morning, as I approached, hundreds of long, undulating, emerald-green tongues wagged in the breeze: if these plants needed a collective name “a gossip” of hart’s tongues would do nicely.

Juvenile hart’s tongues, among mosses embedded in tufa. Photograph: Phil Gates

I turned over a frond and saw the long, slit-like rows of spores, then searched under a rocky overhang for signs of germination. There were scores of juvenile ferns, their tiny tongues poking from the crevices where wind eddy currents had wafted spores into contact with the wet rock.

But there was also something very unusual going on here. The surrounding mosses, clinging to the rock, were becoming fossilised. Tufa, that strange, porous, sedimentary rock produced when trickling lime-laden water evaporates and deposits its salts, was being formed.

I pulled off a small grey piece and rubbed it between my fingers; it was gritty and as it crumbled I could see, under a hand lens, fragments of moss stems and foliage. Water was being trapped by capillary action between the minute overlapping moss leaves long enough for it to deposit its load of calcium salts, enclosing the plants in a stone straitjacket. New moss shoots were barely managing to grow from the concretion.

Moss shoots weighed down by calcium salts from lime-laden water. Photograph: Phil Gates

This process will be familiar to anyone who has visited the petrifying well at Mother Shipton’s well near Knaresborough, in Yorkshire, and seen everyday objects turned to stone by dripping, lime-laden, water. It might have been going on at a microscopic scale under this rock for decades, a race every spring between seeping water’s stony embrace and the new growth of moss shoots.

The new hart’s tongues are immune to the threat because they are shiny and water runs off them quickly before it can precipitate salts. But the intricate, absorptive, structure of these mosses condemns them to a perpetual struggle to escape the stony fate that continually overwhelms their lower stems.

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