Southern Africa is home to the
richest source of succulents in the world, with 40 percent of the world’s
plants occurring in South Africa alone; the Lithops
genus in particular, comprising of a massive 37 species and 93 varieties (Kellner et al., 2011).
Kellner et al. (2011) hypothesised
that the Lithops are such a large and
widespread genus because of their unique mimicry technique. Lithops mimic the geological patterns
around them, resulting in their appearance largely resembling a stone, giving
them the nickname of “Living stones” or the “Pebble plant”.
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| Variation of leaf colour within Lithops karasmontana (A = L. karasmontana ssp. bella, B, C = L. karasmontana ssp. eberlanzii, D, E, F = L. karasmontana ssp. karasmontana). Adapted from Kellner et al. (2011) |
The plant structure consists of
two opposite succulent leaves, with the leaf tips almost entirely sunken into
the ground. This compact growth form is thought to have developed to protect
the leaves against evaporation, and to be hidden from herbivores such as the
cape hare (Lepus capensis) and the armored
ground cricket (Hetrodes sp.) (Kellner et al., 2011).
Because nearly all of the plants light capture and photosynthetic tissue is
located underground, the exposed parts of the plant are translucent, which
likely allows light penetration deep into the leaves where the chlorophyllous tissues
are situated. Despite a seemingly perfect design, Lithops frequently experience overheating from the large amounts of
solar radiation allowed though the translucent leaves, which is reported to
reduce photosynthetic activity (Martin et al., 2013).
While many plants have
adaptations to minimise overheating, such as high transpiration rates,
reflective waxes or convective heat exchange, this is generally lacking in most
African succulents, including the Lithops
(Martin et al., 2013).
Although the Lithops incur operational problems, this has not hindered their
highly effective ability to adapt to and mimic their local surroundings. The extremely
variable geological patterns of southern Africa has been suggested as the cause
of such a large variety of colours, patterns and surface structures in the
genus. Species of Lithops can be found
on every soil formation in southern Africa, ranging from limestone to granite,
with each species flawlessly blending in with the local geology (Kellner et al., 2011).
It has been theorised that it is
the Lithops’ mimicry capability that
has triggered such a large variety of species and their widespread occurrence across
southern Africa, and although Lithops experience
lowered photosynthetic activity, it seems that the skill of mimicry is enough
to overcome the genus’ photosynthetic constraints (Kellner et al., 2011).
Lithops are one of my favourite plants! They are not easy to spot in nature, although I was lucky enough to see several different types while travelling in the north-western parts of South Africa. Their mimicry, as with many other species of plant, is not perfect, so I was wondering if you have any idea of the amount of herbivory that occurs to these plants? Very nice blog!
ReplyDeleteI did not find any studies that specified a level of herbivory, in fact studies dedicated to Lithops were surprisingly hard to find! Martin et al. (2013) hypothesise that initially the Lithops developed a rounded succulent form to protect them from the climate, and that colour and texture mimicry was a sort of 'byproduct'. So assuming mimicry to minimise herbivory was not the initial intent of the plants morphology, I would guess that herbivory was a lesser threat than the climate and perhaps isn't particularly high.
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