1. Alnus Mill.
(alder)
(Furlow, 1979)
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.