Madagascar palm (Pachypodium rutenbergianum)

Description

Pachypodium rutenbergianum is a species of Pachypodium native to Madagascar. The plant can reach 3 to 8 m high, and its trunk up to 60 cm in diameter at base. The plant has short branches and 1-cm long spines. Leaves are green and 10 to 15 cm long, 4 cm wide. Its flowers are white. Pachypodium is a genus of succulent spine-bearing trees and shrubs, native to Madagascar and Africa. It belongs to the family Apocynaceae. All Pachypodium are succulent plants that exhibit, to varying degrees, the morphological characteristics of pachycaul trunks and spinescence. These are the most general features of the genus and can be considered distinguishing characteristics. The pachycaul trunk is a morphologically enlarged trunk that stores water so as to survive seasonal drought or intermittent periods of root desiccation in exposed, dry, and rocky conditions. Whereas there is great variation in the habit of the plant body, all Pachypodium exhibit pachycaul growth. Variation in habit can range from dwarf flattened plants to bottle shaped shrubs to dendroid-shaped trees. The second general characteristic of Pachypodium is spinescence, or having spines. The spines come clustered in either pairs or triplets with these clusters often arranged in rings or whorls around the trunk. Spines emerge with leaves, and like leaves grow for a short period before stopping growth and hardening. Spines do not regenerate so weathering and abrasion can wear away all but the youngest spines from older specimens - leaving smooth trunks and branches. To some extent, branches are a characteristic of the genus. Some caution is warranted in over-generalizing this characteristic. Pachypodium namaquanum is often branchless. Pachypodium brevicaule has no clear branches, and indeed may have evolved an alternative to branching in the form of nodes from which leaves, spines, and inflorescences emerge. In general Pachypodium have few branches. Since the environmental stresses and factors that contribute to branching can vary widely even in small areas, individual plants of the same species exhibit wide variation in branching morphology. Unlike many members of the Apocynaceae, including some members of the superficially similar Adenium, Pachypodium species do not exude a milky latex. Rather, the sap is always clear. The morphology of the genus Pachypodium varies significantly both within and between species and is highly responsive to its immediate surrounding microenvironment. Pachypodium do not overly respond morphologically to larger vegetative zones. For example, Pachypodium can sometimes occur in prehumid vegetative zones where a taxon might find a suitable habitat on a rocky, sunny inselberg jutting above the humid canopy of the forest. Morphologically, Pachypodium can be highly flexible in organization. Branching, if present at all, can be from either the base of the plant or at the crown. Freeform branching is a morphological adaptation to factors of the immediate microenvironment which, by their diversity, account for the wide range of habits: * flattened dwarf species less than 8 cm tall but reaching 40 cm in diameter * bottle- or oval-shaped shrubs to 4 m tall * both branching and unbranched cigar- and cactus-like trees to 5 m tall. Despite microenvironmental variation, Pachypodium are always succulent and always exhibit pachycaul trunks. Pachypodium are usually spinescent, but individual variation in spinescence as well as weathering and abrasion can result in plants with few if any spines. Pachypodium trunks and branches are thickened with water-storing tissue. Plants must rely on the food and water stored in their thickened trunks during seasonal or intermittent drought when leaves have been shed and no water is available from the substrate. In addition to the lower surface-to-volume ratio which aides in water retention, the thickened trunks and branches can also possess photosynthetic surface tissue to allow nutrient synthesis even when leaves are not present. Some species of pachypodium have developed geophytic pachycaul trunks, or trunks that are beneath the soil's surface. These geophytic trunks are caudexes, enlarged stems or trunks that store water. They should not be mistaken for roots, because the enlargement occurs above the point where the roots branch off the main axis of the trunk. The various species of Pachypodium are more or less heavily spined. Species from more arid regions have evolved denser and longer spines. Fog condenses on their spines in the form of dew, which drips down to the ground and increases the amount of moisture that's available to their often shallow roots. The concept of "micro-endemism" plays an important role in this relationship between adaptation mechanisms and speciation. It suggests a certain small scale "nativeness" by virtue of originating or occurring naturally in a particular place or location. The landscape of Madagascar is a perfect example of "micro-endemism" for species of Pachypodium and other taxa. Three factors can be seen to attribute speciation, or the occurrence of species diversity, via adaptive mechanisms to accelerated evolution as it occurs within the xeric landscape and climate. The variation of geology and topology in dry climates is thought to have a greater effect upon plants than in areas with high rainfall. Xeric environments are thus more demanding of adaptive mechanisms to aid in the plant's survival than in places where rainfall is plentiful. The more the demanding, generally the more "mechanized" or "mechanisms" are needed to aid the plants' survival. The adaptive mechanism in a morphological form and an ecological response to habitats are typically manifested together at once for the genus Pachypodium. Examining Pachypodium reveals characteristics of various organs that adapt to the microenvironment. These adaptations, variations on habit, trunks, branches, branchlets, spines, leaves, or flowers, are plentiful in demonstrating how Pachypodium as a genus fosters greater variation in its speciation. The manner in which speciation occurs in Pachypodium, therefore, is apparent: adaptive mechanisms on a morphological level respond to the microenvironment of Pachypodium habitat. The genus' unique organizational, architectural morphology shapes plants that are highly, adaptively responsive to their immediate, surrounding, microenvironments. The duplicity of an adaptive mechanism that is at once "strict" and "flexible" at differing levels of plant physiology, or structure, has granted Pachypodium the ability to evolve within the landscape into variations that fulfill an ecological niche as various species. The hypothesis of micro-endemism, therefore, states that speciation occurs in small specific habitats as aided by adaptive mechanism occurring in geological, topographical, and climatic isolation. Geologically and topographically, plant populations in xeric climates are broken down into smaller groups. The microclimate responds to the given location transforming it into a habitat. Isolated, the duplicity of organization in Pachypodium form through geology and location significant variation where over evolutionary time a new species might develop, if not have developed. The development of new species is through, in part, the adaptive mechanisms of pachycaul and spinescence as well as strict and flexible structural organization at various levels of plant physiology.