Houstonia
(named after Dr. W. Houston, 1695-1733, a writer on American plants). ORD. Rubiaceae. A genus comprising about twenty species of hardy herbaceous perennials, for the most part natives of North-western America. Flowers white, purple, or blue, dimorphous. Leaves opposite, broad or narrow. Houstonias are admirable little plants for growing between large stones on rockwork, where they will flower nearly all the year round. A compost of leaf soil and sand, rather moist, is most suitable. H. caerulea forms a pretty pot specimen under cold frame treatment, and may be used with good effect for surfacing the pots in which other hardy bare-stemmed plants are grown. Propagated by careful divisions, in autumn; or by seeds. H. caerulea (blue).* Bluets. fl. usually elegant, light blue, sometimes white; peduncles one-flowered, elongated; corolla salver-shaped, 1/2in. across. Early summer. l. ovate-lanceolate, attenuated at the base; radical ones spathulate, a little hairy. Stem erect, dichotomous. h. 3in. to 4in. Virginia, 1785. (B. M. 370.) H. longifolia (long-leaved). fl. pale lilac; stamens inclosed. August. l. linear-oblong; radical ones tapering at base and ciliated; stipules broad-ovate, entire or bi-tridentate. h. 6in. 1828. (B. M. 3099.) H. serpyllifolia (Thyme-leaved).* fl. white; peduncles terminal, one-flowered, elongated. June to August. l. spathulate, rather hairy. h. 3in. 1826. (B. M. 2822.)