It tears laterally, in long, narrow plates forming around the trunk in
several vertical fissures, furrows, and slab-like ridges with projecting,
outcurving edges (Fergus, 2002, p. 184; Figure 1A, B).
The fruit is a spectacular sight: the flat, maroon pod reaches up
to 45 cm in length (Allen & Allen, 1981, p. 299) and warps, when
ripe, into an almost helical shape (Figure 1C, D). When the pod
has dried, the seeds come loose inside it, turning it into a natural
rattle. This botanical sound effect has been used in teaching young
children (Moomaw, 2013, p. 107), but it certainly can catch adults’
attention too. The limitations of dispersal without a mobile agent
are evident from pods piling up near the trunk in urban spaces.
Names like “honey locust,” “sweet locust,” and “honey shuck”
apparently refer to the fruit pulp’s sweetish taste. The pulp can contain up to 14% sugars (Allen & Allen, 1981, p. 299) and was used
by Native American peoples as a basis for fermentation and brewing of a beer-like beverage (Austin, 2004, p. 324). Conveniently
for teaching purposes, many of the pods remain on the tree
through winter and into spring. Unless the site is cleaned regularly,
some that have dropped late can still be found on the ground when
the next season’s flowers emerge (Figure 1C).
Apart from the pods, the tree’s most striking feature is certainly
its enormous thorns (Figure 1A, B). They normally measure, in a
typical “wild-type” specimen, between 10 and 20 cm in length
but may reach a staggering 40 cm (Lance, 2004, p. 154). Not surprisingly, the tree is blamed for flat tires (Logsdon, 2012, p. 111),
human injuries, and even casualties (Walker, 1917). There is also
the story – probably purely fictional – of alleged gangster David
McCanles forcing victims of his displeasure to climb up a honey
locust stem as a cruel and unusual punishment (Connelley,
1928). Another perfidious, though less ostentatious and probably
rather ineffective, method to inflict harm on others by means of
Gleditsia thorns is reported from the Deep South, where the thorns
were apparently used for voodoo ceremonies (Brown & Hand,
1977, p. 103).
The honey locust, however, has several other, more benign uses.
Its flowers are a nectar source for honeybees, and the stem provides a
durable timber for furniture making. It is also planted for shade and
shelterbelts and has proven a valuable urban street tree for its tolerance of drought, compacted soil, and pollutants (Gilman, 1997,
p. 288). A popular choice is the “unarmed” G. triacanthos var. inermis
and its cultivars “Shademaster” and “Sunburst,” the latter being a
“golden” selection with yellowish foliage, and both being not only
thornless but also mostly podless (Stoecklein, 2001, pp. 7, 15).
These are forms that students may frequently see in urban spaces,
but unfortunately they lack the very features at the center of the
learning activity described here.
Both large fruits and large thorns are believed to be adaptations to
the former presence of large herbivores, namely of the late Pleistocene.
The species is therefore thought to be shaped by past evolutionary
forces and fit for a biological environment that no longer exists
(Barlow, 2001, 2002; Bronaugh, 2010).
Several Old World Gleditsia species – such as Southeast Asian
G. australis, or G. fera – are equipped with equally impressive
thorns that may have protected them (until recently) against
browsing and debarking by extant large herbivores. However,
no literature was found as to whether any of these species is
browsed upon, or avoided, or their fruit eaten and seed dispersed
by Asian elephants.
Channeling the Mastodon
The exercise described here was carried out with students from
senior high schools (grades 10, 11, and 12; ages 16–18), students
of a school for mature students, and university students in teacher’s
training (M.Ed.) in biology and horticultural sciences, on a day visit
at the International Garden Exhibition (IGA) Berlin in summer 2017.
The IGA Park harbors a particularly striking specimen of
G. triacanthos. Students were guided to the tree without any comments. I initiated discussion with an open question like “What’s
going on here?” – asking only for observations, not background
knowledge. Following Corbishley (2015), participants familiar
with the object were asked not to reveal any details to the rest of
the group. Only one student – who, prior to entering university,
had completed vocational training as a landscape gardener – was
aware of Gleditsia, but was not familiar with any botanical detail.
Several students felt reminded of acacias and were told that the first
botanical description of G. triacanthos indeed called the species
Acacia americana (Austin, 2004, p. 323). One student remembered
having played with such pods as a child and mentioned the rattling
of the seeds.
Typically, students immediately interpreted the thorns as a
defensive structure. To induce “facilitated interaction” (Kador
et al., 2017) with the object, several guiding questions were asked,
like “What can you imagine, when looking at this tree and its
thorns, that this defense might work against?” Answers across
groups summed up to a colorful pan-global bestiary including
“raccoons,” “giraffes,” “moose,” “deer,” “roe deer,” “camels,” “birds,”
“woodpeckers,” “bears,” “panda bears,” “squirrels,” “rabbits,” “koalas,”
“monkeys,” and “dinosaurs.”
Given that the tree’s North American origin had been mentioned,
this revealed that general biogeographic knowledge was rather mixed.
Several students suggested that the thorns were a defense against a
climbing animal – until I argued that for smaller animals the large
thorns may, in fact, be more like the rungs of a ladder. Thus, among
the “climbing animals,” probably only raccoons or bear cubs might be
deterred (though Garber  claims that “squirrels never climb
these thorned trees”). However, mere climbing by those animals
would not cause an evolutionary pressure strong enough to translate
into the evolution of such structures.
To narrow the discussion to large herbivores, students were asked
which animal might open its mouth wide enough that the thorns
would deter a bite, and which animal might want to come so close
to the tree trunk that the thorns might be an effective deterrent. Typically, I had to draw attention to the location of the thorns and ask
which part of the plant these are protecting and what animals might
harm bark and cambium, before students said “elephants.” In one
case, though, a student suggested that the thorns might be useful for
elephants to scratch their itchy back, which does not quite follow a
line of evolutionary thinking. Once the discussion had taken a proboscidean turn, I first drew a comparison to the defense systems of
plants like umbrella thorn acacia (Vachellia tortilis) of the African
savannas that show similarly large thorns. But then I pointed out that
there was a lamentable dearth of elephants in the wild in America,
which may make such an adaptation appear rather pointless.
I reminded students that the emergence of a “useless” feature,
such as a defense structure against elephants where there are
none, would be incongruous with Darwinian evolutionary theory.