Topic: Desert plants with succulent leaves
Perhaps the most striking case of convergence among leaf succulents occurs between Agave and its relatives Yucca and Hesperaloe in the Americas and Aloe and its relatives (e.g. Haworthia and Gasteria) in Africa.
A surprising diversity of plants inhabit the arid and semi-arid ecosystems of the world, displaying adaptations to prolonged drought conditions and extremes of temperature. Among the most charismatic of these plants, namely those having fleshy succulent stems and/or leaves, we find some extraordinary cases of convergent adaptation. Leaf succulent desert plants have evolved independently in numerous families (including the Agavaceae, Nolinaceae, Asphodelaceae, Aizoaceae, Crassullaceae and Portulaceae) in response to the demands of arid environments. Two illuminating examples are documented here: the convergence between Agave-related and Aloe-related leaf succulents, and convergence between leaf succulent ‘stone plants’ (Aizoacaens such as Lithops) and the derived cactus genus Ariocarpus. The diagram (or phylogeny) to the right shows the relationships between these leaf succulents, highlighting their evolutionary separation and the fact that shared features have arisen by independent innovation.
Convergence between Agave-related and Aloe-related plants
Perhaps the most striking case of convergence among leaf succulents occurs between Agave (Agavaceae) and its relatives Yucca and Hesperaloe in the Americas and Aloe (Asphodelaceae) and its relatives (e.g. Haworthia and Gasteria) in Africa. All of these plants have a generally similar form of succulent leaf rosette and have evolved CAM photosynthesis (itself convergent), a process that entails the opening of their stomata and subsequent fixing of CO2 only at night when the air is cooler and holds less water.
Both Agave and Aloe display a rosette of large, thick, succulent leaves with spiny margins that taper to a sharp apex and bear a rigid needle-like spine. The leaf rosette typically emerges from a shoot at ground level (a few Aloe species are on short stems or tree-like in form), and flowering involves production of a tall stem (or ‘raceme’) from the centre of the rosette, bearing many tubular flowers. The similarity in form between certain species of Agave and Aloe is illustrated in the common name for Agave americana, which is ‘American aloe’ (also known as ‘Century plant’ and ‘Maguey’), after its resemblance to ground-rosette aloes such as Aloe parvula, A. rankii and A. sophie! Aloe is very similar in form to its close relatives in the Asphodelaceae, Haworthia and Gasteria, and indeed convergence between Agave-type plants and Aloe must by association also include these genera. Gasteria is native to South Africa and has long, tapering, strap-like succulent leaves that are arranged as opposing rows or in an Aloe-like rosette, emerging either prostrate, that is flat to the ground (e.g. G. croucheri), or inclined from it on a very reduced or absent basal stem (e.g. G. brachyphylla, G. acinacifolia). Haworthia is endemic to South Africa and comprises 68 species with firm but relatively short succulent leaves in a stemless, basal rosette (e.g. H. emelyae).
The Agave-related genus Yucca comprises about 50 species native to arid parts of North and Central America. Like Agave, Yucca has rosettes of tough, succulent leaves, but it has distinct clusters of white flowers held on a vertical stem and is pollinated exclusively by four species of ‘yucca moth’ (Tegeticula or Parategeticula) in a classic co-dependent, mutualistic relationship. Yucca and yucca moths depend completely on one another for survival, and yet it appears that even this very specific relationship has evolved independently several times (see pollination and mutualism for more intriguing cases of convergence involving plants, insects, hummingbirds and even bats). Basal or ground-level rosette species include the ‘Mojave yucca’, Y. schidigera and Y. yucatana of Mexico; these resemble basal rosette plants such as Agave chiapensis, Aloe parvula and Gasteria acinacifolia. Tree or ‘arborescent’ Yucca have leaf rosettes at the end of branches, and include the well-known ‘Joshua tree’ (Y. brevifolia) of the Mojave and Sonora deserts in Mexico and ‘Soaptree yucca’ (Y. elata). Arborescent Yucca closely resemble tree-like aloes of Africa, most notably the famous ‘Quiver tree’ Aloe dichotoma. Another, more minor Agave-related genus Hesperaloe is native to arid Texas and Mexico, has basal rosettes of long narrow leaves, and is distinguished by its flowers, which are brightly coloured and borne on a raceme several feet above the main leaf rosette. Hesperaloe shares characteristics with its close relatives Agave and Yucca, with members of the family Nolinaceae, which are native to arid regions of the Southern USA and Mexico, and is convergent with certain species of Aloe in Africa. Hesperaloe parviflora is commonly called ‘Red yukka’ due to its Yucca-like leaves, and it closely resembles Nolina microcarpa or ‘Bear grass’ of the Nolinaceae, having a basal rosette of narrow leaves with spiny margins and hairs at the leaf tips. Nolina is directly related to another Hesperaloe/Aloe-like genus Dasylirion, which has a basal rosette of flat blade-like leaves with either saw-toothed (D. wheeleri) or gritty (D. longissima) margins and a spoon shaped petiole base – hence the common name ‘Desert Spoon’ for D. wheeleri and ‘Toothless Spoon’ for D. longissima.
As shown in the summary phylogeny, Agave, Yucca and Hesperaloe belong to the family Agavaceae, comprising an estimated 23 genera within the larger grouping (order) ‘Higher Asparagales’, whereas Aloe, Haworthia and Gasteria are from the family Asphodelaceae, comprising around 15 genera and belonging to the ‘Lower Asparagales’. Molecular, cytological and morphological evidence places the Hesperaloe-like leaf succulents Nolina and Dasylirion in the family Nolinaceae, distinct from but very closely related to Agavaceae within the Higher Asparagales. As such it is clear that agavacean and asphodelacean leaf succulents evolved independently, their shared features representing a striking case of convergence upon similar solutions to life in arid habitats
Convergence between stone plants (Aizoaceae) and Ariocarpus cacti
Within the dicotyledonous plants we find two separate lineages of succulent plants, the Aizoaceae and the Cactaceae, within which a few arid habitat species have independently evolved to resemble stones. The Aizoaceae are a family of leaf succulents almost exclusively endemic to arid and semi-arid parts of Southern Africa. They are divided into ‘stone plants’ or ‘mesembs’ (e.g. Lithops vallis-mariae) and ‘carpet weeds’ or ‘ice plants’ (e.g. Carpobrotus edulis), and members of the former group provide an interesting case of convergent evolution with the cactus genus Ariocarpus. Stone plants superficially resemble pebbles on the ground (e.g. ‘stone face’ Lithops of Namibia), and have been called ‘living stones’ (e.g. Dinteranthus, Lapidaria, Titanopsis). The plant is mostly buried below ground, with only the camouflaged upper surface of two succulent, opposed leaves (termed ‘window’ leaves) exposed for photosynthesis. New leaves and flowers arise from a meristem between the two main leaves. The leaf pair can be bulbous, flat or retracted beneath the surface, depending on water availability or drought severity, and as well as serving as a drought adaptation the pebble-like leaves serve to deter herbivores. Cacti from the genus Ariocarpus are commonly called ‘living rocks’ (e.g. the ‘Living Rock Cactus’ A. fissuratus and ‘Tamaulipas Living Rock Cactus’ A. agavoides), and are endemic to arid rocky limestone areas of North and Central Mexico. Ariocarpus is a stem rather than leaf succulent, but the stem is modified such that the only part showing above ground is a flattened stem apex bearing a rosette of protruding tubercles. Just as in stone plants such as Lithops, Ariocarpus tubercles are camouflaged in both colour and texture to look like inedible rock, and can shrink and retreat below ground during extreme drought.
The great evolutionary separation between Aizoaceans such as Lithops and the derived cactus genus Ariocarpus means that these two plant groups independently evolved a strategy of only exposing a few succulent ‘leaves’ above ground, camouflaged as small stones and able to respond to water availability and deter herbivores. Therefore, the ‘living stones’ and ‘living rocks’ provide us with another truly surprising and elegant example of convergent evolution in desert-adapted plants.
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Map of Life - "Desert plants with succulent leaves"
December 3, 2020