This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Deciduous shrubs or small trees, producing their flowers before the leaves. Male flowers devoid of perianth, arranged in clustered pendent catkins. Stamens 4 or 8. Female flowers minute, few together, in lateral and terminal bracteolate clusters. Fruit by abortion 1-celled and usually 1-seeded, more or less enclosed in the enlarged leafy involucre, whence the generic name, from Kopvs a helmet. There are about seven species distributed over the temperate regions of the north. None of them are, strictly speaking, ornamental; but C. Avellana, the
Hazel, enters so largely into the composition of our copses and plantations, that it deserves mentioning here. From this have sprung the varieties producing the Spanish, Filbert, Cob, and other nuts. There is also a variety in which the large leaves have a purplish blotch in the centre.