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.
Trailing St. John's Wort (Hypericum humi-fusum, L.). - The habitat of this plant is gravelly and heathy places, roadsides, commons. The plant is prostrate in habit (hence humifusum). The stems are numerous, prostrate, then ascending-, with 2 raised lines, or 2-edged, slender. The leaves are oval to oblong, blunt, hairless, minutely dotted, transparent, with black dots on the borders below, the border rolled back. The branches are numerous, flattened, curved upwards, leafy. The flowers are yellow, in forked cymes. The sepals are glandular, toothed, with black dots below, unequal, entire, 3 oblong, blunt, with a short point, 2 lance-shaped. There are 15-20 stamens. The styles are very short. The plant is 4-10 in. high, flowering in July and August, and is a herbaceous perennial.
Narrow-leaved St. John's Wort (Hypericum linariifolium, Vahl).- The habitat of this plant is dry rocky banks, granite rocks. The habit is erect or ascending. The stems are more or less round in section, many, leafy. The leaves are linear, blunt, the margins rolled back. The flowers are yellow, few, in cymes. The sepals are unequal, lance-shaped, acute, with glandular teeth and black dots below. The petals are twice as long as the sepals, with black marginal glands. There are few stamens (30). The styles are short, half as long as the capsule. The plant is 4-12 in. high, flowering in July and August, and is a herbaceous perennial.