|Alexgeorgea - male vegetative plant with inflorescences (left), a female vegetative|
plant (center), and the above ground portion of the female flower (right), positioned
for the comparison but would normally be shorter. Source: Sherwin Carlquist.
Most dictionaries and encyclopedias will tell you that geocarpy is a process by which a plant stem or inflorescence elongates and deposits the developing fruits into the ground. And they will probably mention the famous example of the peanut being buried up to 10 cm below ground. But that's not entirely accurate. Geocarpy is a broad term that encompasses any subterranean ripening of the fruit. Technically, Spigelia genuflexa and peanuts are perfect examples of a type of geocarpy known as hysterocarpy, "in which the fertilized ovary penetrates into the soil by means of a long peduncle," according to the book Dispersal Biology of Desert Plants by Karen van Rheede van Oudtshoorn and Dr. Margaretha W. van Rooyen. In other words, these flowers were fertilized above ground, then the developing fruits were deposited into the soil later. In another twist, only some of the fruits of Spigelia genuflexa were deposited, which the authors of Dispersal Biology would further demarcate as an amphicarpic habit.
|The subterranean fruit of Alexgeorgea,|
about the size and shape of an acorn.
Source: Sherwin Carlquist.
The male plants (yes, the plants in this genus are dioecious, meaning there are separate male and female plants) of Alexgeorgea nitens appear like many other Australian restiads, so much so that it was first described by the German botanist Christian Gottfried Daniel Nees von Esenbeck in 1846 as Restio nitens. von Esenbeck had mistaken deformed portions of plants above ground, possibly the result of a smut fungus, as the fruit and included this information in his description when placing it in the large genus Restio. Overlooked for the next 130 years were the subterranean fruits of this species until Sherwin Carlquist, an American botanist and one of my botanical heroes, noticed the ephemeral thread-like stigmas of the female flowers emerging from the sand. He admitted that had he not been present during the short period in which the plants produce the female flowers, the peculiar subterranean flowers and fruits would have likely remained unknown to science. Carlquist first came upon these plants in 1974 and later described two species in a new genus named in honor of the Australian botanist Alex George in 1976. Consider how fortuitous this discovery was in not only being in the right place at the right time, but also being able to recognize that the minute above ground portion of the female flower no more 3 cm tall was worthy of further investigation.
|Male flowers in the background;|
emergent female flower parts seen
in front as red or purple threads.
Source: Sherwin Carlquist.
But why? What conditions lead to the evolution of geocarpy? What are the advantages? Carlquist hypothesized that in Alexgeorgea the evolution of a single-seeded subterranean fruit was an adaptation in response to a predominant fire ecology in Western Australia. Fruits at or below ground are protected from the extreme heat of fire. Indeed, many other plants in Western Australia have adapted to frequent fires by producing their fruits at ground level (basicarpy). More widely, geocarpy is assumed to have evolved in several plant lineages in response to the harsh environments in which the species grows. In Dispersal Biology of Desert Plants, it is noted that plants may develop a geocarpic habit to ensure close proximity of the offspring in favorable micro-habitats. For example, Alexgeorgea persists in seasonally wet peaty sand and if it instead relied on wind dispersal of its fruit, it is assumed that a larger percentage of offspring would fail to germinate in favorable soils by being blown far from the small region in which it can thrive. The disadvantages are obvious: restriction of seed dispersal, limitation of population genetic structure, and the possibility of small stochastic events wiping out entire colonies.
Regardless, this is a fascinating plant habit and an equally fascinating genus. Leave it to Australia to bring us yet another botanical oddity!
Carlquist, S. (1976). Alexgeorgea, a bizarre new genus of Restionaceae from Western Australia.
Australian Journal of Botany, 24 (2), 281-295 DOI: 10.1071/BT9760281
Briggs, B., Johnson, L., & Krauss, S. (1990). The species of Alexgeorgea, a Western Australian genus of the Restionaceae. Australian Systematic Botany, 3 (4), 751-758 DOI: 10.1071/SB9900751