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.
Stem when perennial consisting of a central pith, concentric layers of wood, and a separable bark; and increasing in size by additions between the wood already formed and the bark. Leaves net-veined. Seeds with 2 opposite entire cotyledons, or rarely deeply divided, as in some of the Coniferce. Parts of the flower generally in fours or fives.