From Moth to Sloth: Interpreting Plant
Morphologies in the Context of
At times, an as-yet-unknown factor, such as an elusive pollinator or
seed disperser, is implied by morphological adaptation in coevolved
plants. A well-known, classic example is the flower of the Malagasy
orchid (Angraecum sesquipedale), also known as the “comet orchid,”
sporting an exceedingly long spur. “Good heavens what insect can
suck it” a puzzled Darwin (1862) remarked, and he postulated that
“in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches!” This description
was found to fit Morgan’s sphinx moth (Sphingidae: Xanthopan mor-ganii), whose Malagasy population received the subspecies epithet
praedicta (Kritsky, 1991). In support of Darwin’s hypothesis, the visit
of the unusually long-spurred flower by the moth with the unusually
long proboscis was eventually documented (Wasserthal, 1997).
In an age of extinction, however, it is well conceivable that the
animal “best fitted” to a given plant structure may never be found
because it has vanished from the face of the earth. A considerable
number of so-called anachronistic plants still show putative adaptations to ecological situations that are no longer current. This may
include fruit that, for seed dispersal and germination, needs ingestion by a large herbivore that is no longer extant in the ecosystem
in question. An illustrative, though disputed, case is the putative
mutualism that may have once linked the Mauritian tree Sideroxylon
grandiflorum (Sapotaceae) and the proverbially extinct columbiform
Raphus cucullatus, the dodo (Temple, 1977). Janzen and Martin
(1982) suggested that various Neotropical plants are adapted to dispersal by now extinct large frugivorous mammals. There has been
extensive discussion of ecological anachronisms in the ecology literature (Janzen & Martin, 1982; Guimarães et al., 2008; Johnson,
2009; Gill, 2014). Popular natural history writing (Barlow, 2000,
2001; Bronaugh, 2010) has paid attention to some species native
to temperate North America that may once have been dispersed by
megaherbivores such as mammoths, mastodons, or ground sloths.
A particularly striking example is Maclura pomifera (Raf.) C.K.
Schneid., the Osage-orange.
“A Bumpy Green Object That Looks
Like Neither Apple Nor Orange . . . ”
The Osage-orange is a small, deciduous, dioecious tree of the family
Moraceae, which includes mulberries, figs, breadfruit, and the colossal jackfruit. It grows to a medium-sized tree with a grayish to light
brown bark that sometimes has an orange tinge. Its ovate leaves are a
dark, shining green on the upper side and pale green on the underside. In the leaf axils there is a sharp single spine (Burton, 1990).
The nucleus of M. pomifera’s Holocene, pre-Columbian distribution was in southwest Oklahoma, southeast Arkansas, and
northeast and central Texas (Turner, 2010, p. 48). It is hardy as
far as USDA zone 5 and can be planted from southern Canada
as far south as Florida. Given the wood’s usefulness for bow making (which is reflected in the name “bois d’arc” or “bodark”) it
has expanded anthropogenically. Hundreds of thousands of miles
of Osage-orange hedges were planted in the United States in the
19th century as windbreaks, for erosion control (Burton, 1990, p. 426;
Fergus, 2002, p. 252), and as impervious living fences that were
“horse high, bull strong and pig tight” (Austin, 2004, p. 423). On
an anecdotal note, the “hedgeapple,” as it is called locally, is said to
have inspired the inventor of barbed wire (Austin, 2004, p. 423;
Nachtigall, 2008, p. 18).
True to its taxonomic relationship to the genus Morus, its leaves
were once used to feed silkworms (Kenrick, 1839). Today it is mainly
used as an ornamental, with its “showpiece” being the baseball-sized,
irregularly spherical, lime-chartreuse green fruit, a syncarp formed of
many small drupes (Figure 1). The “bumpy green object that looks
like neither apple nor orange” (Christian, 2009, p. 11) has a smooth,
waxy-rubbery surface and a fibrous interior of stringy pulp and small,
light brown seed. A characteristic feature shared with the fruit and
leaves of many other Moraceae is the milk sap, for which it is sometimes called Milchorange (milk orange) in German. The sap, with an
appropriate charm added, was applied in folk medicine to treat warts
(Watts, 2007, p. 281).
The fruit is generally considered unpalatable. As phrased by Flint
(1833, p. 47), “tempting as it is in aspect, it is the apple of Sodom to
the taste,” though opinions vary (Barlow, 2002, p. 120ff.). Notwithstanding its questionable gustatory qualities, the fruit offers several
rather pleasant stimuli, such as the haptic, tactile sensory impression
of its pebbly surface. Also noteworthy is the olfactory sensation of
its light aroma, reminiscent of citronella and orange peel. Paul Elmer
More (1864–1937) wrote in a poem published in 1890 (p. 26):
“Sweeter odor nor field nor forest offers / Than the scent of the osage
orange ripening / In October. Forsooth the air is nectar. . . .” The scent
is also said to ward off insects, and the fragrant green orb is sometimes
sold in farmers markets as cockroach repellent, like an oversized natural mothball (Peterson et al., 2002).
As is evident in any ornamentally planted specimen, the fruit falls
close to the mother tree’s stem, suggesting a dependence on zoochory
for dispersal. Most native animals in the tree’s natural distribution do
not eat the fruit. Those that do mostly destroy the seed by chewing.
Figure 1. A fruit of the Osage-orange (Maclura pomifera).
Note the rodent bite in the center of the fruit in the
foreground, showing that a seed has been eaten. A fallen leaf
of the tree is seen on the fruit lowest in the pile in the front
row and in the background.