Topic: Succulent desert plants
Classic examples of convergence in desert plants include the so-called 'stem succulent' cacti in the Americas and cactus-like Euphorbia species in Africa and South Asia, and also the striking similarity between 'leaf succulent' Agave and Yucca of the Americas and Aloe and its close relatives 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. Classic examples include convergence between the so-called ‘stem succulent’ cacti in the Americas and cactus-like Euphorbia species in Africa and South Asia, and also the striking similarity between ‘leaf succulent’ Agavaceans (e.g. Agave, Yucca) of the Americas and Aloe and its close relatives (e.g. Haworthia, Gasteria) in Africa. In addition to these well-known cases, several other plants have converged on succulent features as an adaptation to life in an arid environment, for example the cactus-like stem succulents Hoodia and Stapelia (of the family Asclepiadaceae), the enchanting ‘stone plants’ of the family Aizoaceae and leaf-succulents such as Dasylirion and Nolina (family Nolinaceae). The following diagram (or phylogeny) shows the relationships between these stem and leaf succulent groups, highlighting their evolutionary separation and the fact that shared features have arisen by independent innovation.
Fleshy, succulent stems have evolved in several distantly related desert plant families, including cacti (family Cactaceae), certain species of Euphorbia (Euphorbiaceae) and two genera of the family Asclepiadaceae, Hoodia and Stapelia. Shared features of all these plants include a succulent stem adapted for water storage and photosynthesis, protective spines, highly reduced or absent leaves and a photosynthetic mechanism termed ‘Crassulacean Acid Metabolism’, or CAM photosynthesis which conserves water in a process that entails the opening of stomata and fixing of CO2 almost exclusively at night. CAM photosynthesis is itself convergent, having evolved independently many times in plants and even certain algae. Desert stem succulents are a case example of this convergence: CAM photosynthetic cacti, Euphorbia, Hoodia and Stapelia are all more closely related to plants that use typical day-time (or ‘C3‘) photosynthesis than they are to each other or in fact any other CAM plants, indicating independent biochemical evolution of CAM photosynthesis as a marvelous adaptation to conserve water in their disparate desert habitats. Further convergent adaptations are best illustrated through specific examples, as detailed in the following paragraphs:
Convergence within the cacti
Cacti comprise around 1600 species primarily native to arid parts of the Americas. The family Cactaceae is extremely diverse, having originated from ancestors resembling Rhodocactus and Pereskia, which have leaves but also possess stem stomata and are CAM-inducible. Leaf loss and specialisation of a very succulent photosynthetic stem appears to have occurred independently in the two most well-known core cactus groups: the Opuntioidea (e.g. the prickly pear cactus Opuntia cochenillifera with its pad-like stem) and more derived Cactoidea (e.g the barrel cactus Ferocactus acanthodes), whose extreme ecological success is attributed to the evolution of transport vessels (termed ‘vascular bundles’) in the stem cortex. Stem succulence and leaflessness are therefore convergent features within the cacti and yet we can look to completely unrelated plant families for even more striking parallels: firstly, in the Euphorbiaceae, and secondly in the Asclepiadaceae
Convergence between cacti and Euphorbia
The family Euphorbiaceae is estimated to comprise 300 genera and more than 7500 species of herbs, shrubs and trees, but it is to one genus in particular – Euphorbia, that we turn our attention. There are around 850 succulent Euphorbia species (also termed ‘euphorbs’ or ‘spurges’), mostly originating from East and Southern Africa. Unlike cacti, which usually have a watery sap, euphorbs have a toxic milky-coloured sap (this too is convergent: see latex), and they also possess unique floral structures called cyathia (made up of one female and several male flowers) as distinct from the cactus’s typically large, single flowers. However, in the nature of their succulent and CAM-photosynthetic stems, several Euphorbia species bear an incredible resemblance to cacti, and present classic examples of convergence.
Many cacti and desert euphorbs have folded or ‘plicated’ stems, with longitudinal ribs or horizontal tubercules providing a degree of shade from UV insolation and physical ‘valleys’ in which stomata are localized, serving to minimise evaporative water loss. Critically, folding also makes the stem an excellent water storage device, allowing efficient stem expansion during rapid water uptake after rain and contraction during periods of drought. Perhaps the most famous of the cacti is Carnegiea gigantea, the ‘Giant Saguaro’ of Mexico, with its characteristic tall, branched stem folded into longitudinal ribs. As is typical for cactoids, the stem of C. gigantea also has clusters or ‘areoles’ of spines, derived from modification of a group of proto-scales in the axillary leaf buds, along its ridges. The branching stems of C. gigantea, as for smaller cacti such as the ‘Peruvian Apple Cactus’ Cereus peruvianus, are very similar in form to the ribbed columnar stems of certain Euphorbia (such as E. cactus, E. coelurulescens and E. confinalis). Of note, spines in cactus-like Euphorbia occur in pairs rather than areoles, as they derive from two structures at the leaf base called ‘stipules': natural selection acting to produce spines happened to work on the axillary buds of cacti and stipules of euphorbs, emphasising again the power of convergent evolution to shape adaptive features regardless of relatedness of the organisms involved. A further example of cactus-euphorb convergence is between the rare cactus Peniocereus striatus (also known as ‘Cordoncillo, the ‘Gearstem cactus’ or ‘Dahlia-rooted cactus’) of Arizona and the Sonora desert and Euphorbia cryptospinosa of East Africa. P. striatus and E. cryptospinosa have slender, branching succulent stems covered in longitudinal ribs that are able to contract tightly during times of extreme drought, shielding stomata hidden deep within their furrows. Both species possess only very short spines, which have been found to help restrict airflow and water loss, and both deposit a red pigment within their stems that makes them look dead, as an anti-herbivore strategy. As an aside, Euphorbia cryptospinosa closely resembles E. erlangeri, so it too may be considered convergent upon Peniocereus striatus. As a final example from among the more humble cacti, Astrophytum asterias (the ‘Sand Dollar Cactus’ or ‘Star Peyote’) is small and globular, growing only a few inches above the ground and not bearing spines on its stem. Astrophytum has a parallel in Euphorbia obesa, which is a globular, cactus-like species identical in form except for their distinct inflorescence. Euphorbia meloformis also closely resembles Astrophytum save for in the males unusual projections from otherwise bare stem ridges.
Convergence between cacti, Hoodia and Stapelia
Among the ‘eudicots’ (i.e. eudicotyledonous plants, or those with two leaves on the initial shoot) is a lineage termed the asterids, within which we find the family Apocynaceae. This family holds two genera of desert-adapted plants,
Hoodia and Stapelia that are native to Southern Africa. They display remarkable cactus-like or ‘cactiform’ features and both belong to the sub-family Asclepiadaceae. Hoodia, or ‘milkweed’ is represented by 14 species of stem succulents that resemble columnar cacti and possess characteristic large flowers that smell and look like rotting flesh (a convergent feature shared with other flowering plants, e.g. Rafflesia). For example, Hoodia gordonii, or the ‘Bushman’s Hat’ of South Africa and Namibia is entirely leafless and has a spine-covered stem folded into tubercles. Hoodia ruschi, or ‘Queen of the Namib’ is endemic to dry rocky areas of Central Namibia to South Angola, and its succulent cactiform stem may grow up to 1m tall. Stapelia are known commonly as ‘carrion flowers’ or ‘starfish flowers’ for their characteristic star-shaped infloresence which, similar to those of Hoodia, are large, hairy and often disguised with colour and scent as rotten flesh in order to attract fly pollinators. Around 40 species of Stapelia are known, mainly endemic to South Africa, and they grow as relatively short succulent stems that branch upwards from the base of the plant. The carrion flowers Stapelia gigantea and Stapelia variegata for example, have succulent stems with four pronounced ribs and rows of reduced spines or fleshy hooks along the rib margins. Stapelia thus closely resembles cacti such as the San Pedro cactus Echinopsis pachanoi with its 4-9 ribbed branched stem and short spines, as well as euphorbs such as E. cactus and the entirely spineless E. attastoma.
As illustrated in the summary phylogeny diagram, the Cactaceae is an advanced family within the large eudicot Order Caryophyllales, only distantly related to the family Euphorbiaceae of the rosid lineage and family Asclepidiaceae of the equally distinct asterid line. The occurrence of highly specialized stem succulence in all these groups is therefore explained only by convergent evolution, as similar selective forces acting upon unrelated plants in the world’s harshest deserts have independently favoured the development of cactus-type morphology and physiology.
In response to the demands of arid environments leaf succulence has evolved independently in numerous plant families (including the Agavaceae, Nolinaceae, Asphodelaceae, Aizoaceae, Crassullaceae and Portulaceae). 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.
1. 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.
2. 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 - "Succulent desert plants"
December 3, 2020