ALBURNUM, in plants, is that part of the stem of trees which timber-merchants call Sapwood. It is the newly-formed unchanged wood lying immediately below the bark, and is always of a very light colour. It is the principal channel through which the crude sap is conveyed from the roots into the leaves, and is, therefore, an indis pensable part in all exogenous trees. [ExoeExs.] It consists of deli cate fibrous tissue ; in which respect it differs from Heartwood, or Duranten, in which the tissue is combined with solid secretions, the nature of which varies with species. It is probably on the latter account that heartwood is so much more durable than sapwood ; for simple fibrous tissue is in itself very perishable, and it only ceases to be so in consequence of the presence of secretions of a less destruc tible character.
While many plants have the alburnum and heartwood distinctly separated, there are others, technically called Whitewooded Trees, which consist of nothing but alburnum. This arises from their not forming any solid secretions which can give durability to the central parts ; hence all such trees are quickly perishable, and are generally unfit for any but temporary purpoaes.
ALCA (Cuvier), the Auk, a genus of Web-Footed sea-birds, which has a singularly-formed bill, being very broad when viewed laterally, straight towards the base, but much curved towards the point. Both the mandibles are half covered by projecting feathers, and furrow€d near the point. The upper mandible is crooked, and the under for ms a projecting angle. The nostrils are towards the middle of the sides of the upper mandible, being very narrow and almost closed by a membrane covered with feathers. The legs are short, and placed far back, so that the birds when standing have their backs nearly perpen dicular. There are only three toes fully webbed, the back toe being wanting. The claws are somewhat pointed. The wings are short, and the first quill is as long as the second, or perhaps a little longer.
Only two species are known, the Great Auk (Alca impennius), and the Razor Bill (Alec Torda), both natives of the British Isles. [Aux.]