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A large genus of shrubs, mostly deciduous, but sometimes acquiring the character of an evergreen from the colour of the young branches. They vary from dwarf and prostrate plants a few inches high to tall ones with a stature of over 20 ft. In a wild state they are found almost exclusively in Europe, but a few reach the western borders of Asia and the southern shores of the Mediterranean. With but one exception among cultivated hardy species (G. monosperma, with white flowers), the blossom is of some shade of yellow, and all have the pea-flower (or papilionaceous) form. The leaves are simple or trifoliolate, often so small and few as to be negligible; in these cases the work usually done by leaves devolves upon the green branches.
As garden shrubs some of the genistas, such as G. aetnensis, hispanica, pilosa, and tenera, are in the very front rank, and are all worth growing. They are easily accommodated and do not require a rich or manured soil. A sunny position (for most of them are essentially sun-lovers) and a well-drained, light loam suits them best. Whenever possible, genistas should be raised from seed, as plants so obtained are usually healthier and longer-lived than cuttings. Still cuttings are frequently employed. They are taken in late July or August, and dibbled in very sandy soil in frames, usually pushing roots the following spring. The taller species are all improved by shortening back several times in the young state to induce a bushy habit. They transplant badly after a few years, and should be given permanent quarters early, or else grown in pots. (See also Cytisus.)
A considerable number of tender or half-hardy species have been, and continue to be, introduced from the south of Europe and the islands of the Mediterranean. Many of them can be cultivated in the Scilly Isles, but they are of no use for the ordinary climate of Great Britain. The species dealt with in the following pages include all in cultivation that are really hardy.
Genista and Cytisus are closely allied, and many species have been assigned to one or the other genus according to which differential character is given most weight. The majority of the species concerned conform to the following distinctions: Cytisus has the upper lip of the calyx shortly toothed and the seeds are strophiolate, i.e. there is a wart-like swelling at the base of the seed, known as the strophiole or elaiosome; in Genista the upper lip of the calyx is deeply cut into two lobes and the seeds lack a strophiole. However, there are a number of species which are in one or more respects either anomalous or controversial, and the modern tendency is to remove these to separate genera, so making it possible to draw a sharper line between Genista and Cytisus. Thus C. monspessulanus and its allies (a group most numerously represented in Madeira and the Canary Islands) have certain 'Genista' characters such as a deeply divided upper calyx-lip, and are on that account sometimes placed in the segregate genus Teline. In C. nigrescens the seeds have a very small strophiole and in this respect the species inclines towards Genista; but it would be out of place in that genus and has been treated as a separate genus – Lembotropis. Both these genera are recognised in Flora Europaea. It should be added that commonly the leaves in Genista consist of a single leaflet, whereas in Cytisus they are usually trifoliolate. Unfortunately there are so many exceptions, as well as many species that are virtually leafless, that this difference is of little value as a spotting character, though it holds good for the species most frequent in gardens.
The species treated below are mostly retained in Genista in Flora Europaea. But the anomalous G. horrida (and allied species not treated here) are removed to the genus Echinospartum, in which the branches are opposite as in G. radiata and its allies, but the calyx is inflated, as in Erinacea. Two other segregate genera recognised in Flora Europaea, but here included in Genista, are Lygos (syn. Retama) and Chamaespartium. In the former, in which G. monosperma, G. sphaerocarpa, and G. raetam would belong, the pods are inflated and split tardily or not at all. To Chamaespartium would belong G. sagittalis and the closely allied G. delphinensis, of which the distinctive character is the flattened and winged stems.