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A large and well-marked genus, composed mainly of herbaceous plants, but comprising also a good number of shrubby and sub-shrubby species, hardy in this country and of considerable beauty. The leading characters of these species are invariably opposite or whorled leaves, often dotted with pellucid glands; the five, rarely four, sepals and petals; the numerous stamens, often grouped into three or five bundles; the three or five, rarely two or four, styles; the capsular, usually more or less cone-shaped, fruits (which are, however, fleshy in H. androsaemum and in some species not in cultivation, and somewhat fleshy at first in H. × inodorum); and the cylindrical seeds. Hypericum is the largest genus in the small family Hypericaceae, which is by some botanists included in the Garcinia family (Guttiferae).
The hypericums rarely grow more than 4 or 5 ft high in this country, and most of them retain more or less foliage in mild winters; in severe ones they are deciduous. The stems of some of the species here described are only half woody, and naturally die back some distance every winter. Although the flowers are always yellow in these shrubby species, there is considerable variety among them either in size or depth of shade. The plants themselves vary much in foliage and general aspect.
In gardens, perhaps the chief value of the hypericums is in their habit of flowering during late summer and autumn, when comparatively few shrubs remain in bloom. Planted in groups, as the hardier species should always be, they also give during a large part of the year healthy masses of deep green or blue-green foliage. They are of the simplest culture, and all of them like a well-drained loamy soil and abundant moisture. All, except perhaps the American species, will grow on chalky soils. Many of them produce seeds, and none, so far as I am aware, will not increase easily by cuttings. These should be taken off in August, dibbled in pots of sandy soil, and placed in gentle heat. Species like H. calycinum and xylosteifolium, that produce creeping root-stocks, are very easily increased by division.
The revised treatment of Hypericum that follows owes much to the work of Dr N. K. B. Robson of the British Museum (Natural History), and we are further indebted to him for reading and commenting on the manuscript. The treatment of the species of the section Ascyreia is based on his notes published in Journ. R.H.S., Vol. 95 (1970), pp. 482-97. The nomenclature of the European and Anatolian species follows that adopted by the same author in Flora Europaea, Vol. 2 (1968), and Flora of Turkey, Vol. 2 (1967).
The following parts have so far appeared of Dr N. K. B. Robson's 'Studies in the genus Hypericum L. (Guttiferae)', in the Bulletin of the British Museum (Natural History) Botany Series:
1. 'Infrageneric Classification', Vol. 5 (6) (1977).
2. 'Characters of the Genus', Vol. 8 (2) (1981).
3. 'Sections 1. Campylosporus to 6a Umbraculoides', Vol. 12 (4) (1985).
The species treated in the main work fall into the following sections of Dr Robson's classification:
H. revolutum (see this supplement).
H. balearicum, the only species.
Horticulturally, this is the most important section and to it belong all the species in the main work and this supplement not mentioned under other sections.
H. androsaemum, H. grandifolium (see this supplement), H. × inodorum. The only other species, H. foliosum, is allied to H. grandifolium:
H. xylosteifolium, the only species.
H. olympicum, H. polyphyllum, the only species.
H. cerastoides, the only species.
H. coris, H. empetrifolium, H. ericoides. There are two other species in this section.
A large American section, of which those treated in the main work are: H. buckleyi, H. densiflorum, H. frondosum, H. galioides, H. hypericoides (Ascyrum hypericoides), H. kalmianum, H. nudiflorum, H. prolificum.
H. aegypticum. There are two other species in this section.
Of these sections the first five mentioned are treated in the third part of Dr Robson's treatise, referred to above.