ALDER  , a genus of

plants (Alnus) belonging to the order Betulaceae, the best-known of which is the common alder (A. glutinosa) . The genus comprises a few species of shrubs_ or trees, seldom reaching a large size, distributed through the North Temperate zone, and in the New World passing along the Andes southwards to Chile . The British species A. glutinosa is confined to the Old World . This tree thrives best in moist soils, has a shrubby appearance, and grows under favourable circumstances to a height of 40 or 50 ft . It is characterized by its short-stalked roundish leaves, becoming wedge-shaped at the base and with a slightly toothed margin . When young they are somewhat glutinous, whence the specific name, becoming later a dark olive green . As with other plants growing near water it keeps its leaves longer than do trees in drier situations, and the glossy green foliage lasting after other trees have put on the red or brown of autumn renders it valuable for landscape effect . The stout cylindrical male' catkins are pendulous, red-dish in colour and 2 to 4 in. long; the female are smaller, less than an inch in length and reddish-brown in colour, suggesting young fir-cones . When the small winged fruits have been scattered the ripe, woody, blackish cones remain, often lasting through the winter . The alder is readily propagated by seeds, but throws up root-suckers abundantly . It is important as coppice-wood on marshy ground . The wood is soft, white when first cut and turning to pale red; the knots are beautifully mottled .

Under water the wood is very durable, and it is there-fore used for piles . The supports of the Rialto at

Venice, and many buildings at Amsterdam, are of alder-wood . Furniture is sometimes made from the wood, and it supplies excellent charcoal for gunpowder . The bark is astringent; it is used for tanning and dyeing . ALDER-FLY, the name given to neuropterous insects of the family Sialidae, related to the ant-lions, with long filamentous antennae and four large wings, of which the anterior pair is rather longer than the posterior . The females lay a vast number of eggs upon grass stems near water . The larvae are aquatic,active, armed with strong sharp mandibles, and breathe by means of seven pairs of abdominal branchial filaments . When full-sized they leave the water and spend a quiescent pupal stage on the land before metamorphosis into the sexually mature insect . Sialis lutaria is a well-known British example . In America there are two genera, Corydalis and Chauliodes, which are remarkable for their relatively gigantic size and for the immense length and sabre-like shape of the mandibles .