This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Remains of this plant have been found in the Preglacial beds in Norfolk and Interglacial beds in Sussex, testifying to its antiquity. It is to-day found in the Temperate Northern Zone in Europe, North Africa, Siberia, Western Asia. In the Peninsula province it is absent from West Cornwall, and North Hants and E. Sussex in the Channel province. In S. Wales it is absent from Glamorgan, Pembroke, and Cardigan, and it occurs in N. Wales only in Flint and Denbigh, and not in Mid Lancs, but south of this it is general. It is uncertain whether it occurs in North England or Scotland, north of York, except in Stirling.
The Great Chickweed is a good index of wet soil, for it cannot grow away from water. The plant is a hygrophile, without being a marsh or bog plant. It is found growing in tall clumps along a shaded ditch-side on the roadside, or lining the margin of pool or lake, or more extensively by the side of a stream or river. Here, itself white-flowered, it vies with the large puce flowers of the Great Hairy Willow Herb, or perchance with Water Bedstraw or Water Figwort.
This is the largest and finest of the Stitchworts, being tall and leafy, but slender, brittle, and prostrate below, then ascending. The leaves are heart-shaped, with a long point, the lower stalked, the upper stalkless, and hairy along the margin. The branches are alternate, and the plant supports itself by aid of the surrounding vegetation.
The flowers are large, white, in the axils, single, and distant; the petals are divided to the base, longer than the calyx. The capsule is larger than the calyx, on turned-back flower-stalks, and opens by five clefts. The seeds are reddish-brown and rough, about sixty in each capsule.
The Great Chickweed is often as much as 3 ft. high. The flowers are in bloom in July and August. It is perennial, and may be increased by division. The flowers are larger than in Cerastium triviale, but of the same size as in Cerastium arvense and Stellaria Holostea. The number of insect visitors and the arrangement of stamens and pistil is intermediate, and as favourable to cross- as self-pollination. The flowers are proterandrous, the anthers ripening first. When no insects visit the flowers the stigmatic lobes spread out and become covered with pollen from the anthers. There are usually five styles, sometimes three. The insects that visit it are Diptera (Syrphidae, Muscidae), Coleoptera (Nitidulidae), Thysanoptera (Thrifts), Hymen-optera (Apidae).