Alburnum, that part of the stern of trees which timber merchants call sap wood, in contradistinction to heart wood. It is the newly formed wood, lying next below the bark, and is a delicate fibrous tissue, the principal use of which is to convey the crude sap from the roots to the leaves. It is, therefore, a necessary part of all exogenous trees. But it is of a very perishable nature, and only loses that quality when, being enveloped within exterior layers of the same substance, it becomes combined with other secretions, which solidify it and convert it into duramen, or heart wood. Most plants, and all trees valuable as timber, have the sap wood and heart wood distinct, the one forming the external layer, the other the core. Some, however, consist of alburnum only, and are known as whitewood, which are useless, or of use only for the most temporary purposes.