Ecology and Conservation.
I am personally familiar with eriospermums in
habitat in western South Africa (Namaqualand, Bushmanland, and the Richtersveld),
so I'll confine my discussion to that part of the world. While this
area is only a fraction of the range of the genus, it represents the center
of diversity, harboring a significant portion of the total number or species,
and including numerous endemic taxa. The habitat preferences of eriospermums
vary greatly from species to species, though many occur on open flats,
in deep sandy-loam soil. One can, however, find taxa that are confined
to rocky, well-shaded slopes, or blindingly bright quartz flats, or seasonally
wet, muddy seeps. Eriospermums are seldom prominent components of
the vegetation, but a careful search of virtually any habitat in Namaqualand
will turn up one or two species.
It is difficult to say how rare or threatened
the various Eriospermum species are; no systematic studies of their
population sizes exist. Moreover, such studies would be difficult,
as eriospermum tubers have the odd habit of sometimes taking a year off,
and not producing any flowers or leaves. During a year with good
rains, however, most plants are probably active above ground, and one can
get a reasonable idea of population sizes and distributions. Some
species, like E. paradoxum, are frequently encountered over a vast
range, and are in no danger of extinction from anything short of catastrophic
climate change. Others, like
E. titanopsoides, are narrow
endemics with exacting habitat requirements, and could be severely impacted
by an unfortunately placed quartz mine, or a morally deficient succulent
plant collector. It is fair to say that, in general, eriospermums
are not common plants: I have on many occasions come across two or
three plants of a species, and been unable to find any more despite a thorough
search of the surrounding vegetation. Such species would be vulnerable
to all sorts of acute, localized disasters, such as plowing for winter
wheat, road building and off-road vehicle activity, and the above mentioned
mining and depredations by human vermin.
On a positive note, eriospermums seem to be
quite resistant to overgrazing, one of the chief threats to desert plants
in South Africa. Their tubers are well protected underground, and
the leaves are rarely grazed, perhaps being toxic. On land near settlements
that has been wrecked by goats, the surviving vegetation may consist mostly
of geophytes, including eriospermums. Interestingly, eriospermums
can persist in deep, soft soils that harbor mole-rats (voracious bulbivores
that keep some geophytes confined to rockier situations) -- someone really
ought to check to see what sort of secondary compounds these plants use
to deter herbivores.
Below: E. bowieanum, leaf with enations,
about 10 cm tall.
One of the most peculiar features of eriospermums is the enations that
grow from the leaf surface in about 20% of the described species.
Enations are protuberances of green tissue that arise from the adaxial
(upper) surface of the leaf. Enations are often branched, and can
form such a mop of bifurcating axes that the leaf looks like a miniature
tree. Larger, more elaborate enations are vascularized, and can dwarf
the scale-like leaves which produce them. Almost nothing is known
about the details of the morphology and development of eriospermum enations;
they might result from the activity of the adaxial meristem, which typically
is involved only in the thickening of the leaf.
Enation-type eriospermum leaves end up
having much the same structure as dissected (compound) leaves in dicots
such as the succulent, winter-growing pelargoniums. What sorts of
advantages follow from having a mop of dichotomizing outgrowths, as opposed
to a simple, undissected leaf? It has been proposed that dissected
leaves are resistant to wind damage (and it can be very windy in the deserts
of South Africa), while still presenting a large surface area to catch
sunlight. The three-dimensional branching structure of eriospermum
enations might be especially good at catching weak light from a wintertime
sun low in the sky -- all of the eriospermums with enations are winter-growers.
If enations do indeed function like dissected leaves
in dicots, they form an interesting instance of convergent evolution.
On the whole, monocots are unable to form dissected leaves at all because
of developmental constraints (their leaves grow from the base, making it
hard to produce anything but simple, strap shapes). Most monocots
that have something like dissected leaves develop them from initially simple
leaves, which are modified in anomalous ways: programmed cell death
creates the lobes in the leaves of some aroids; palm leaves become dissected
by mechanically splitting. The enations of Eriospermum represent
another class of alternate path to leaf dissection, one whose developmental
mechanisms are wholly unknown.
Eriospermum titanopsoides - misfit among misfits.
Eriospermum titanopsoides is a recently discovered dwarf species
from the white, quartz-pebble covered hills of the Knersvlakte. With
its sparkling, undulate blue-green leaves, barely a centimeter across in
mature plants, E. titanopsoides could hardly be mistaken for any
other species in the genus. It does, however, bear an uncanny resemblance
to a disembodied leaf from one of the South African "ice plants" (e.g.
Dorotheanthus) in the family Mesembryanthemaceae. Both E.
titanopsoides and the ice plants get their crystalline appearance from
an epidermis covered with clear, spherical cells. The function of
these structures, called bladder cells, is uncertain, but they may act
like tiny lenses to focus and control the sunlight entering the leaf.
As far as I have been able to discover, E. titanopsoides is not
only the sole species in its genus with bladder cells, but also the only
monocot with bladder cells. The presence of bladder cells in the
ice plants, which are dicots, as well as in one species of monocot, is
another striking example of convergent evolution from the xerophytic flora
of South Africa.
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copyright Matt Opel, 2002