Fleshy, succulent stems have evolved in several distantly related desert plant families, including cacti, certain species of Euphorbia and two genera of the family Asclepiadaceae, Hoodia and Stapelia.

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, the succulents, we find some extraordinary cases of convergent adaptation. A classic example is the convergence between so-called ‘stem succulent’ cacti in the Americas and cactus-like Euphorbia species in Africa and South Asia. In addition to the well-known cacti and euphorbs, a few members of the milkweed family Asclepiadaceae, namely Hoodia and Stapelia, have evolved remarkably cactus-like stem succulent features as an adaptation to life in an arid environment. The blue diagram (or phylogeny) to the right shows the relationships between these stem succulents, highlighting their evolutionary separation and the fact that shared features have arisen by independent innovation.

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.