name = "Pachypodium"
image_width = 250px
image_caption = The pachycaule trunk of "
divisio = Magnoliophyta
genus = "Pachypodium"
genus_authority = Lindl.
subdivision_ranks = Species
subdivision = "
"Pachypodium" is a
genusof succulent spine-bearing trees and shrubs, native to Africa. It belongs to the dogbane family, Apocynaceae. "Pachypodium" comes from Greek "pachy" (thick) and "podium" (foot), hence meaning "thick-footed".
All "Pachypodium" are
succulentplants that exhibit, to varying degrees, the morphological characteristics of pachycaule trunks and spinescence. These are the most general features of the genusand can be considered distinguishing characteristics.
The pachycaule trunk is a morphologically enlarged trunk that stores water so as to survive seasonal
droughtor intemitent 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 hardending. 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 sapis always clear.
:"For detailed description of the morphology of the genus, see
Morphology of Pachypodium."
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 inselbergjutting above the humid canopy of the forest.
Morpholigically, "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 40cm in diameter
* bottle- or oval-shaped shrubs to 4 m tall
* both branching and unbranched cigar- and cactus-like trees to 5m 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/abrasion can result is plants with few if any spines.
Variation among "Pachypodium" species is significant but all "Pachypodium" are succulent plants inhabiting seasonally or chronically dry landscapes. The genus employs two morphological
adaptationsto these xeric, isolated, habitats: Pachycaul trunks and spinescence.
"Pachypodium" trunks and branches are anormally thickened with water-storing tissue. Plants must relie 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 photosyntehtic 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.
"Pachypodium" make use of
spinescenceas an adaptive mechanism responding to the landscape. Adaptively this spinescence is employed to different degree in various species to collect moisturefrom fogs and dews. The spines point in all angles, are paired or in sets of threes where the third spine is unequal, and thus captures moisture that can drip directly down to the soilbeneath the plant from the branches and branchlets. The degree of spinescence demonstrates the degree to which species rely on spines as a means to collect moisturefrom microclimateconditions, such as localized dews or fogs within microenvironments, and drip to the soilimmediately below the spine on a branch or branchlet. The falling moistureimmediately around the plant saturates the soil. A superficial root system will often develop to take advantage of this source of moisture.
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 placeor location. The landscapeof 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 evolutionas it occurs within the xeric landscape and climate.
(1) 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.
(2) The geological formations of locally xeric landscapes break up
populationsof organisms, i.e. plants, into smaller groups, where each group can initially interbreed but, with time, develop new genotypesand cannot be bred with exception to natural hybridization. Localized geologybecomes harder to cross over for a given population to be "continuous" in a xeric geological landscape, because more demands are placed on the population. Therefore, populations are broken down into smaller units within this landscape. Groups of the original population become located to unique microenvironments within the landscape. Accordingly measures to adapt to these microenvironments become more singular to the isolated habitat. Adaptive mechanisms are employed so as to aid the survival of the plant group. This adaptation eventually, in part, leads to speciation in the habitat, or diverse species across the spectrum of the landscape.
(3) Taxa tend to develop specialized xeromorphoric structures at some architectural level in
arid, geological and topological landscapes, where a strategy of a "flexible" and "strict" architectural, organizational morphology at various levels of structure for "Pachypodium" becomes advantageous to succeeding in the isolated, specialized landscape. This strategy is seen in the manifest "flexible" variations of habit in species of "Pachypodium" while all the same they are "strictly" xeromorphic pachycaule trunks meant to conserve water for dry periods. At another level of structure, namely that of organs, we can see that dewand fogdripping spines are examples of a xeromorphic adaptive mechanized organ responding to microenvironments.
These newly created species from within the xeromorphic landscape take on different characters as responses to the habitat. For instance, there is an advantage to morphologically developing into bottle-shaped "shrubs" where the plants exist in open, sunny microenvironments on top of porous
sandstone. Little completion exists for height within the habitat. Likewise, where competition for resources is more competitive--"both in the number of species and the height of surrounding plants"--there are times when it is to the advantage of a plant to develop into arborescent, dendroid “trees.” This development is because these particular "Pachypodium" must compete with other plants for resources in a dry deciduous forest, composed of, perhaps, arborescent " Aloe", members of the Didiereaceaegenera"-- Alluaudia, Alluaudiopsis, Decaryia, and Didierea; all endemic to Madagascar--"and "Uncarina" species, for instance.
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.
hypothesisof 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 pachycaule and spinescence as well as strict and flexible structural organization at various levels of plant physiology.
:"See main article
Taxonomy of the Pachypodium genus."
Number of species
There are now 25 known species, of which 20 come from
Madagascar, where isolated landscapes and micro-environmental conditions have produced highly specializedspecies. The species count continues to grow as " Pachypodium menabeum" has been resurrected from invalid taxonomy and " Pachypodium makayense" added newly to the list. One can speculate that in regions such as Madagascar, there might still be unidentified species that are confined to a single rocky outcrop or an inselberg.
Affinities within the Apocynaceae
The family Apocynaceae before it included Asclepiadaceae had 3 genera that can be considered
succulent plants: "Adenium", "Pachypodium", and " Plumeria". The first two genera ("Pachypodium" and "Adenium") are generally assumed to have closed association with each other. Studies; however, of these two genera reveal that they are not as intimately close as once thought.
However, a study of key characteristics of the taxon and a
cladisticstudy of the subfamilyApocynoideae and the family Asclepiadaceae(before its merging with the Apocynaceae), demonstrates that this closed association is not warranted. True, both are succulent plants and pachycaule. According to Leeuwenberg however, "Adenium" is maintained in the subtribe Neriinae, placed underneath the tribeWrightieae whereas "Pachypodium" is placed beside the in the subtribe Pachypodiinae, within the tribe Echiteae. Though related, these taxa means that the two are not intimately related.
Distribution and habitats
"Pachypodium" are native to
Madagascarand continental Southern Africa, i.e. Angola, Botswana, Mozambique, Namibia, South Africa, Swazilandand Zimbabwe.
:"See main article
In elevation, "Pachypodium" in both mainland
Africaand Madagascargrow between an altitude of sea level, where some speciesgrow in sand dunes, such as " Pachypodium geayi", to 1600 m (5200 feet) for " Pachypodium lealii" in southern Africa and 1900 m (6200 feet) for " Pachypodium brevicaule" in Madagascar.
In continental southern Africa, the extreme
temperatures range from -10 °C (14 °F) in some locations to as much as 45 °C (113 °F). Whereas in Madagascar, with not such a great temperature amplitude, the temperature ranges from -6 °C (21 °F) to 40 °C (104 °F).
A generalization about precipitation regimes for both southern Africa and Madagascar does not have much meaning because the habitats of "Pachypodium" vary so greatly with a moisture regime. In some places, "Pachypodium" receive annually from as little as 75 mm (2.95 inches) from the southern part of Africa to a high level of 1985 mm (78.15 inches). A precipitation regime for a species of "Pachypodium", therefore, depends upon a habitat's location relative to the influences of the Atlantic and Indian Oceans and the various mountain ranges of southern continental Africa and of Madagascar.
The genus grows in areas where there are significant periods of dry months that range from five months to ten months. It would seem likely that the Atlantic and India Oceans pay a major role in the creation of weather conducive to
rainfall, not to mention mountain ranges. For example, the Madagascar dry deciduous forestswith their long dry season and severe limestone ridden soils provide one ideal setting for "pachypodium".
"Pachypodium" grows in various types of substrates. Some species only grow in one substrate whereas other will grow in several. The degree to which a taxon can grow in a given substrate seems to determine how specialized its habitat is within the
landscapeand climates. On outcrops, steep hills, and inselbergs, the plants are subjected to fluctuating moisture, high winds, and temperatureextremes. Only plants with special adaptations to exposure and extreme drought can survive, let alone thrive, on these exposed geological habitats. "Pachypodium" root in cleft, fissures, and crevices of those rocky formations. The non-succulent roots penetrate deeply into the acuminated soil, mineral, and humusin these crevices. Moisture is able to seep deep into these crevices. Very little transpirationoccurs. In this manner, rocky substrates provide moisture in the habitat. This saturation of crevices can only occur, however, if there is not a considerable runoff from the rock's surface and if there is abundant fine soil in the cracks that, in turn, retain water. The substrate, therefore, plays a critical role in the creation of micro-environmental "arid islands."
Sand readily store water because it is taken up easily and there is less evaporation except for the top layer. Very deep sand; however, has the problem of seepage. Yet in moderation shallow and deep sand substrates have water available to "Pachypodium". With shallow sand substrates, "Pachypodium" grow on sand dunes near the sea. Where water is in deep sandy substrate, "Pachypodium" grow on sand "over"
lateritered soil. Laterite soil is a largely impermeable soil that traps water for the use of the flora that include "Pachypodium".
Internationally "Pachypodium" are protected under the
CITEStreaty. According to it, members of this genus cannot be collected from endemic, native locations within the landscape. They are not easily, readily imported and exported between nations either. The protection afforded by the CITES treaty responses to two issues:
*The esteem the genus has within Collector's and Nursery Trade. As highly esteemed plants, succulent enthusiast desire to collect more and more species and
cultivars. In the case of "Pachypodium", seed, seedlings, and even mature, nursery-grown specimen plants are fortunately available readily in Nursery Trade.
*Destruction of the genus's endemic habitats, e.g. through
agriculture. Extinctionof identified species seems yet unlikely, as the collection of seed and the cultivation of the plant safeguard the genus.
History of the genus
:"See main article
History of the genus Pachypodium"
The early history of the
genus"Pachypodium" demonstrates the typical process of a taxon becoming a new genus. Initially debate occurred over if "Pachypodium" belonged to the genus " Echites" or if it constituted a separate genus. "Pachypodium "were first published as a unique genus, separate from "Echites", by Leandleyin 1830.
Then the debate centered on the nomenclature of
speciesuniquely found in continental Southern Africa. That changed when, in 1892, Baker contributed the first species accepted into the genus from Madagascar. The degree of speciationthen turned to Madagascar, where the count of species far exceeds those on the mainland.
Costantinand Boisconstructed the first monograph, of "Pachypodium", in which they enumerated 17 species, where ten were from Madagascarand seven were from continental southern Africa.
There is no
fossilrecords of "Pachypodium" known. Yet certain conclusions can be drawn from the geology of the landscape in Madagascar as to the past natural historyof "Pachypodium".
*Eggli, Urs. (1993) Glossary of botanical terms with special reference to Succulent Plants. with German Equivalents (British Cactus & Succulent Society: United Kingdom)
*Endress & Bruyns : "A revised classification of the Apocynaceae." "Botanical Review" 66: 1-56.
*Endress, Mary: "The unification of Asclepiadaceae and Apocynaceae." "Haseltonia: The Cactus and Succulent Society of America's Yearbook" Vol. 8.
*Lavranos, John, J. (2004) "Pachypodium makayense: A New Species From Madagascar". "Cactus and Succulent Journal": United States 76 (2) 85-88.
*Lüthy, Jonas M. "Another look at the pachypodiums of Madagascar." Bradleya: The British Cactus and Succulent Society Yearbook. (22/2004) ISBN 0-902099-74-4
*Mays, Harry. [European Union Honorary Representative] "The Huntington Botanical Gardens' 2005 offering of International Succulent Introductions for the European Union." [A Posting] (Woodsleigh, Moss Lane, St. Michaels on Wyre, Preston, PR3 0TY, UK: 2005)
*Rapanarivo, S.H.J.V., Lavranos, J.J., Leeuwenberg, A.J.M., and Röösli, W. Pachypodium (Apocynaceae): Taxonomy, habitats and cultivation "Taxonomic revision of the genus Pachypodium," S.H.J.V. Rapanarivo and J.J. Lavranos; "The habitats of Pachypodium species" S.H.J.V. Rapanarivo; "Cultivation" W. Röösli. (A.A. Balkema: Rotterdam, Brookfield, 1999) [Rapanarivo et al.]
*Rowley, Gordon, D. Cactus Handbook 5: Pachypodium and Adenium (British Cactus and Succulent Society, (1983) 1999)
*Rowley, Gordon. Didiereaceae: "Cacti of the Old World" (The British Cactus and Succulent Society [BSCS] : 1992)
*Rowley, G.D. "The Pachypodium rosulatum aggregate (Apocynaceae) - one species or several?" Bradleya: The British Cactus and Succulent Society Yearbook. (16/1998)
*Rapanarivo, S.H.J.V., Lavranos, J.J., Leeuwenberg, A.J.M., and Röösli, W. Pachypodium (Apocynaceae): Taxonomy, habitats and cultivation "Taxonomic revision of the genus Pachypodium," S.H.J.V. Rapanarivo and J.J. Lavranos; "The habitats of Pachypodium species" S.H.J.V. Rapanarivo; "Cultivation" W. Röösli. (A.A. Balkema: Rotterdam, Brookfield, 1999, p.5) [The rest of the list is based on Rapanarivo et al.(1999)]
*Rapanarivo et al. (1999) p. 5.
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