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.
The habitat of this plant is moist places. The plant differs from H. quadrangulum (q.v.) in the leaves, which are narrower, and the sepals, which have small teeth; and the latter are turned back, with many black dots on the outside. For other details see the above species, which this one resembles in other respects.
Square-stalked St. John's Wort (Hypericum quadrangulum, L.). - The habitat of this plant is copses, wet and moist places. The habit is erect. The- stems are 4-angled (hence quadrangulum), and erect. The leaves are broad, elliptic, ovate, oblong-, rounded, with few or no pellucid dots or glands, blunt. The upper leaves are half-clasping, the veins and net-like connections being pellucid. The flowers are yellow, cymose, with sepals erect or bent back, oblong or lance-shaped. The petals are broad and glandular. The style is half as long as the capsule. The carpels have many vittae. The capsule opens by septa. The seeds are oblong. The plant is 1-2 ft. in height, flowering from July to September, and is a herbaceous perennial.
Hypericum tetrapterum, Fr. =H. quadraium, Stokes = H. acutum, Moench. - The habitat of this plant is moist places. The habit is as in the last. The stem is erect, 4-angled, with narrow wings. The leaves may be heart-shaped below, broadly ovate, oblong, with pellucid glands and dots, and reticulations. The flowers are yellow and form a dense cyme. The sepals are lance-shaped, erect, with glands, long-pointed. The styles are not as long as the capsule, which has many vittae. The plant is 1-2 ft. in height, and flowers in July and August, being a herbaceous perennial.