1. Alnus Mill.
Plants shrubs or
trees, sometimes forming thickets. Young growth (twigs, leaves, and
inflorescences) with a sticky or resinous coating. Twigs 1.5–2.0 mm
thick, dark purplish brown, usually hairy, the pith more or less triangular in
cross-section. Buds stalked, with 2 or 3 scales. Leaf blades elliptic, rhombic,
or very broadly obovate to almost circular, the tip broadly or bluntly pointed
to shallowly notched, the undersurface green, hairy (at least along the veins),
but not or scarcely felty to the touch, the lateral veins 6–11 on each
side of the midrib and sometimes branched. Stamens 4, undivided. Styles
persistent at fruiting. Fruits samaras with 2 small lateral wings, brown,
arranged in conelike aggregates of mostly 50–120 fruits. Bracts fused
into a relatively flat structure, unlobed or very shallowly 5-lobed woody
scales that overlap and more or less conceal the fruits, the scales persistent
on the axis after the fruits are dispersed. About 25 species, North America to
South America, Europe, Asia.
Alders are very
important ecologically for their ability to fix nitrogen, that is, to transform
inert gaseous nitrogen from the atmosphere into nitrates and other compounds
useful to living things. Like other nitrogen-fixing plants, the chemical
reactions occur in root nodules that contain symbiotic microorganismsin
the case of Alnus, bacteria (actinomycetes) in the genus Frankia.
The leaves and bark are rich in tannins, so alders have been used for tanning
leather; in traditional medicine they have been used to treat various kinds of
infections and inflammations, especially of the skin (Moerman, 1998). The
foliage and bark also yield dyes; depending on the parts of the plant used and
the method of preparing and applying the dye, they can be yellow, red, or
black. The conelike infructescences of some species have been marketed in
handicrafts and jewelry as miniature pinecone substitutes.