Opuntia Rafinesquii (Hardy Prickly Fig).—­A dwarf hardy Cactus with sulphur-coloured flowers, produced from June to August; very suitable for dry spots in rock-work.  It grows best in peat with a little sand, and is propagated by separating the branches at a joint, and allowing them to dry for a day or so before putting them into the soil.  Height, 2 ft.

Orange, Mexican.—­See “Choisya.”

Orchids.—­The four classes into which these charming and interesting plants are divided may be described as (1) those coming from the tropics, (2) from South Africa, (3) from the South of Europe, and (4) our native varieties.  The first require a stove, the second a greenhouse, the third and fourth slight protection during winter.  As their natural character differs so widely it is necessary to ascertain from what part of the globe they come, and to place them in houses having as near as possible the same temperature and humidity as that to which they are accustomed.  The pots in which they are grown should be filled with fibrous peat and sphagnum moss, largely mixed with charcoal, and abundant drainage ensured.  They are propagated by dividing the root stocks, by separating the pseudo-bulbs, and, in case of the Dendrobiums, by cuttings.  Orchis Foliosa (Leafy Orchis) may be grown in the open ground in good sandy loam.  When once established it is best not to disturb it, but if needed it may be increased by division, after the tops have died down.  Orchis Fusca (Brown Orchis) may likewise be planted in the open, in a sheltered position, in fine loam and leaf-mould, the soil to be well drained, yet constantly moist.

Origanum Pulchellum.—­Popularly known as the Beautiful Marjoram, this plant is useful for cutting for vases.  It is perennial and hardy, and thrives in a dry situation with a sunny aspect and in a sandy soil.  The bloom is in its best condition in October.  The rooted shoots may be divided in spring or almost at any other period, or it may be propagated by taking cuttings in summer.  Height, 1-1/2 ft.

Ornithogalum.—­O.  Arabicum bears a large white flower with a shiny black centre.  It is a fine plant for pot culture, or it may be grown in water like the Hyacinth.  It may be planted in the open early in spring in sandy loam and peat.  Take it up before the frost sets in and store it in a dry place, as it requires no moisture while in a dormant state.  In September the flowers are produced.  Height, 6 in.  O. Umbellatum (Star of Bethlehem) is a pretty little flower often found in English meadows, is quite hardy, and once established may be left undisturbed for years.  It throws up large heads of starry flowers, which are produced in great abundance.  While in a dormant state the bulbs should be kept almost dry.  It is propagated by off-sets; flowers in May.  Height, 1 ft.

Orobus.—­These hardy perennials bear elegant Pea-shaped blossoms.  The plants will grow readily in any light soil, and are easily increased by root-division in the spring, or by seeds.  They flower in June.  Height, 2 ft. to 3 ft.