Class III. Exogenae, DC.—Exogens.
Characters.—Trunk, consisting of bark, wood, and pith, placed one within the other, the pith being innermost. Bark, composed of strata (the younger and inner being called endophloeum or liber), each usually increasing by the deposit of new matter on its inner side. Wood, consisting of ligneous strata, traversed by medullary rays, and increasing by the deposit of new woody matter on its outer side (exogenous growth); the older and inner strata are called duramen, or heart wood; the younger and outer strata are termed alburnum, or sap wood. Leaves usually articulated with the stems; their veins commonly branching and anastomosing (netted, reticulated). Flowers, if with a distinct calyx, often having a quinary, sometimes a quaternary, rarely a ternary arrangement. Embryo with two or more cotyledons (dicotyledonous); if two, they are opposite; if more than two (polycotyledonous), they are verticillate, radicle naked, i. e. elongating, without penetrating any external case (exorrhizous).
This class includes two sub-classes: 1. Gymnospermae, or naked-seeded exogens; 2. Angiospermae, or covered-seeded exogens.