This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
This plant is very rare, occurring in Unst. The stem is tufted, prostrate, then ascending. The leaves are inversely ovate, densely hairy, dark-green, tinged with purplish-black (hence nigrescens). The flowers are white, large. The seeds are wrinkled. The plant is 3-5 in. high, flowering from June to August, and is a herbaceous perennial.
The habitat of this plant is lofty alpine and subalpine rills, wet gravelly spots, as Ben Lawers, Ben Nevis. The habit is prostrate. The plant is almost devoid of hairs. The stem is weak, has alternate hairy lines, and is slender, leafless, branched below. The leaves are small, oblong, lance-shaped, elliptic, distant, blunt, light-green more or less on one side, and sickle-like. The flowers are white, in cymes of 1-3, terminal. The bracts are herbaceous, glandular or smooth, with a broad membranous margin. The sepals are linear to oblong, spreading, i-nerved. The corolla is large, with petals 2-fid. The stalks are downy. There are 3 styles. The capsule exceeds the sepals. The plant is 3-6 in. high, and flowers in July and August, being a herbaceous perennial.
The habitat of this plant is rocky tops of Scottish mountains, as the Breadalbane range, Ben Hope, etc. The plant has the rosette or cushion habit. It is looser in habit than the last, yellowish-green or purple. The stems are densely-tufted, many, with short hairs (hence hirta). The leaves are limp, awl-like, crowded, blunt, 3-veined. The flowers are white, solitary. The flowering shoots are downy, terminal, with three pairs of leaves. The sepals are acute, 3-veined, ovate, lance-shaped, with a membranous margin. The flower-stalks are 1-flowered. The petals are inversely ovate, lance-shaped, narrow below, shorter than the calyx. There are 4 styles. The valves of the capsules are 3-5. The seeds are rounded. The plant is 1-2 in. high, flowering in July and August, and is a herbaceous perennial.
Bog Sandwort grows on the banks of rills and calcareous bogs. The habit is prostrate or ascending. The stems are loosely tufted, and are ascending, slender, smooth. The leaves are slender, thread-like, without veins, half-round in section, blunt. The flowers are white, 1-3, on slender flower-stalks, the sepals ovate to lance-shaped, acute, when dry 3-veined, the petals oblong, oval, narrowed below, as long as the calyx. The capsule is ovoid. The seeds are kidney-shaped, rough on the disk. The plant is 2-3 in. high, flowering in June and July, and is a herbaceous annual.
The habitat of this plant is limestone cliffs, bare calcareous spots. The habit is prostrate or ascending. The stems are matted, numerous, much-branched, rough, long, downy, with short hairs turned down, when dry angular. The leaves are oblong to spoon-shaped, fringed with hairs (hence ciliata), blunt, i-nerved. The flowers are white, solitary as a rule, with leaf-like bracts. The sepals are oblong to lance-shaped, more or less acute, with membranous borders, with 3 hairy ribs, shorter than the petals. The petals are spoon-shaped, ovate, slightly clawed. The capsule is ovoid, as long as the sepals. The seeds have no aril. The plant is 1-6 in. high or long, flowering from May to August, and is a herbaceous perennial.
Arctic Sandwort (Arenaria norvegica, Gunn) - - The habitat of this plant is loose barren gravel on serpentine hills, stony places. The plant is almost smooth, with denser, shorter leaves than the last.
The habit is prostrate, then ascending-. The plant is not hairy, or sparingly so. The stems are much-branched, short, angular when dry. The leaves are spoon-shaped, inversely ovate, fleshy, not fringed with hairs. The flowers are white, 1-3, terminal. The sepals are ovate, acute, with 3 smooth, faint ribs, not so long as the capsule. The petals are longer than the calyx, ovate, shortly-clawed. The flower-stalks have very short, turned-down hairs. The capsule is oblong, not narrowed above, and the teeth are erect. The seeds are dark-brown, tubercled. The plant is 1-4 in. high, flowering in July and August, and is a herbaceous perennial.