1. Celtis L. (hackberry)
Plants trees or
shrubs to 35 m tall. Bark smooth or with irregular corky warts and/or ridges,
pale gray or almost white. Twigs sometimes slightly zigzag, the winter buds
axillary, usually with several overlapping scales (naked elsewhere). Leaves
spirally alternate in several planes, not 2-ranked. Leaf blades lanceolate,
ovate, or triangular, slightly to strongly asymmetric at the base, widest
1/5–1/3 from the base, the secondary veins turning aside and anastomosing
before reaching the entire or toothed margins, the basal pair longer and more
prominent than the others, thus the blade appearing more or less palmately
3-veined from the base. Flowers staminate and apparently perfect, appearing
before the leaves, the staminate flowers in small, dense clusters at the base
of current-year’s growth, the perfect flowers solitary in the axils of the
lowest expanding leaves of the current season. Calyces 2–3 mm long, shallowly
to deeply 4(5)-lobed, but the lobes usually shed early, green to greenish
yellow, turning brown after flowering. Stamens with the anthers yellow to
greenish yellow. Fruits drupes, the outer layer thin, glabrous, smooth,
sometimes somewhat glaucous; the middle layer thin, fleshy, sweet-tasting, the
seeds covered by a large, stony endocarp. About 70 species, worldwide.
The genus Celtis
is taxonomically very difficult. All characters are variable and identifications
are best made using several characters in combination. In the past, this
variability was often explained by postulating high levels of hybridization
between species. Recent studies have shown that this is false. It is difficult
to cross the species in the garden (Whittemore and Townsend, 2007) and hybrids
among sexually reproducing taxa are rare or absent in nature (Whittemore,
2005). Instead, C. pumila is apomictic, the seeds developing without
pollination, and as in many apomicts there is a great deal of variation from
population to population. Interestingly, it is possible to obtain hybrids by
placing pollen from sexual species on stigmas of apomictic plants (A.
Whittemore, unpublished), and some puzzling forms found in the wild may have originated
by pollination of apomictic C. pumila by pollen from C. laevigata
or C. occidentalis.
The bark of
native Celtis species is characterized by the formation of distinctive
corky warts and/or ridges. The species differ in the amount and distribution of
corky outgrowths, but the degree of cork development varies from tree to tree
and varies with the age of the tree, so this character is not always reliable
for species identification. Cork also forms in response to injury, so trunks
that have been damaged may form corky outgrowths in unusual places.
Leaves from
different parts of the plant or at different stages in its life may be very
different in form. In particular, leaves from vigorous leading shoots are much
larger than leaves from lateral branches and may differ strikingly in shape and
marginal toothing. The shape, texture, and toothing of juvenile leaves may be
very different from leaves of adult plants. Unless otherwise stated,
descriptions apply to a leaf that is subtending a flower or fruit. Vegetative
plants or vegetative, leafy twigs in the herbarium are not always identifiable.
The fruit of
hackberries is eaten by many species of mammals and birds. The flesh of the
fruit is sweet, with a pleasant flavor, but the stone is large and there is
relatively little flesh in the drupe. Some scholars have suggested that the
nettle tree of Europe and the Near East, C. australis L., was the lotus
of Homer’s Odyssey, the fruit eaten by the Lotus-Eaters of North Africa.
Others have felt that Homer’s lotus was more likely a species of Ziziphus
Mill. (Rhamnaceae). Fruit of our native Celtis often remain on the tree
long into the winter, and sometimes a few fruits even remain on the tree until
the following spring.
Hackberry wood
is soft and it is not widely used. It is sometimes used for furniture and
handcrafts, where its light color is valued.
Some species,
especially C. occidentalis, are used in horticulture, as they are
fast-growing trees with great tolerance to heat, cold, drought, periodic
flooding, rapid changes in temperature and moisture, and poor soils. They are
widely grown in dry areas with extreme temperature fluctuations (especially the
western Great Plains and the intermountain western states), but their use in
Missouri is limited by their susceptibility to several pathogens (Elias, 1970),
especially witch’s brooms (caused by mites [Eriophyes sp.] and powdery
mildew [Sphaerotheca phytoptophila Kellerm. & Swingle]) and leaf
galls (caused by hemipteran insects, Pachypsylla spp.). These pathogens
do not harm the plant, but they cause cosmetic damage and reduce its popularity
as a shade tree. Hackberries also tend to become weedy in garden settings.