ErioBio

Eriospermum Biology

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.

Enations.

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. Mesembryanthemum, 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.