Pachypodium
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Pachypodium ambongense |
Pachypodium is a plant genus that belongs to the recently modified dogbane family (Apocynaceae). The inclusion of "Milkweed" family Asclepiadaceae in 2000 has great significance for botanists and horticulturalists interested in the alliance succullents. Pachypodium comes from Greek pachy (thick) and podium (foot), hence meaning thick-footed.
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Genus characteristics
In all cases of genus, Pachypodium are succulent plants that depend upon two morphological organs, characters, to adapt to the microenvironment, not the larger scale vegetative zone, but the immediate surrounding of the plant. These two characters are the presence of a pachycaule trunks and spinescent. They are the most general characteristic features of the genus among all the breadth of the species, and therefore the heart of the genus definition.
The pachycaule trunk means that Pachypodium employ a morphologically enlarged trunk that stores water so as to survive extended periods of drought or exposed sunny, xeric rupicolous (inhibiting or thriving among rocks) conditions. Whereas there is great variation in the habit of plant body, all of forms of Pachypodium bodies are pachycaule. Variation in habit can range from dwarf flatten plants to bottle shaped shrubs to oval shaped shrubs to "cigar"-shaped trees to "cactus"-shaped tress to denroid-shaped trees. All exhibit pachycaule characters.
The second most important general characteristic of Pachypodium is spinescent, or having spines. The spines of Pachypodium come in either pairs or in sets of threes with the third being unequal to the pair for the species. They all point at different angles depending on the species as a means to collect moisture from microclimate conditions, such as localized dews or fogs within microenvironments, and drip to the soil immediately below the spine on a branch or branchlet. The falling moisture immediately around the plant saturates the soil. A superficial root system will often develop to take advantage of this source of moisture.
To lesser extent, branches are a general character of defining the genus. But some cautious is naturally warranted in overly generalizing this characteristic. Pachypodium brevicaule never branches, for instance. It is often the genus' anomaly as is Pachypodium namaquanum. Pachypodium brevicale forms "nodes" from which leaf and inflorescence form. Spines occur densely on the protrusion of the node directly underneath the leaves. There is no current knowledge about whether these "nodes" functioning in the same manner as branches, or branchlets, do in all the other species. It would be hasty to conclude that they do function like branches or branchlets. So any discussion of branching as general character of the genus is not actually completely accurate. It precludes a genus definition, or a meaningful account of general characteristics of the genus.
That said, we can talk on a qualified basis about the qualified significance of general characteristics of branching to the genus. In most species, Pachypodium have few branches. The branch is an organ that responds to some micro-environmental conditions, resulting from either a topological, geological, and climatic feature of the landscape. Given these tendencies to branch according to habitat conditions, Pachypodium show stereotypy, little regular pattern organization, or no pattern organization whatsoever. Because Pachypodium take advantage of the microenvironment through branching, the branching of the taxa is like maps bespeaking the very responses that the taxa have made to the habitat within its microenvironment.
A common misunderstanding about Pachypodium is the type of sap it has. Unlike many members of the Apocynaceae, including some members of Adenium, Pachypodium does not exude milky latex. Rather, Pachypodium sap is always clear.
Morphology
- For detailed description of the morphology of the genus, see Morphology of Pachypodium.
The morphology of the genus Pachypodium varies significantly, with exception to the Pachypodium rosulatum aggregates. It is highly responsive to its immediate surrounding microenvironment, or "ecological coat" worn by an organism, like a Pachypodium taxon. Pachypodium do not overly respond morphologically to larger vegetative, biotic zones. Some of these zones are condusive to Pachypodium, whereas others are not. They are not too important. Pachypodium, for example, can sometimes occur, in of all places, in prehumid vegetative zones where a taxon might find a suitable habitat on a rupicolous, sunny inselberg--a mountinous level rock formation that resists erosion--jutting, xeric and exposed, above the humid canopy of the forest. Significantly; however, it is the morphological consequences to microenvironments over evolutionary time that has created responses that are at the level of species. At this level, we can see the variation of the genus in morphological characters through its speciation. Pachypodium morphology, therefore, reveals varied adaptations to dry microenvironments within the greater geological landscape and climate.
Morphologically Pachypodium demonstrate a duplicity in organization and architectural, structural morphology that varies from strict to flexible responses to microenviroments.
On the one hand, Pachypodium is a highly flexible genus at various levels of architectural morphology. This flexibility is evident in the plant as a whole demonstrating a certain flexibility of branching free-style from either the base of the plant or at the very top of trees and shrubs--the crown--for instance. The freeform of branching in all species but Pachypodium brevicaule is a morphological adaptation to manipulating the singularities of an immediate microenvironment by adapting an organ--i.e a branch or branchlet--morphology. Also this flexibility of morphology is evident in the sheer variation of habits of Pachypodium, for instance, that range from :
- dwarf species no more than 8 cm (3.15 inches) high yet 40 cm (15.75 inches) in diameter,
- shrubs that are bottle shape to oval shaped that stay within 4 m (13.12 feet)
- trees that form candelabra branching, little branching upon a cigar-like and cactus-like tree that all grow to about 5 m (c. 16-17 feet).
On the other hand, Pachypodium are also strict in their architectural morphology. Overall a species will strictly retain the habit that is associated with it, even though the branching will be constructed in a flexible manner. Pachypodium ambogense takes the form of a bottle-shaped shrub. Pachypodium namaquanum will always have the habit of a cactus-like tree. Furthermore, overall, despite variation from flattened dwarf plants to bottle-shaped shrubs, oval shaped shrubs, cigar-like trees or cactus-like trees, the species of Pachypodium adhere to a pachycaule trunk shaped in some various manner. Its succulence is its strict constant character.
Adaptive mechanisms
To talk about adaptive mechanisms of Pachypodium, one evidently invokes a discussion about speciation, or variation in species. Pachypodium have two general succulent characteristics to respond to the xeric landscape: "pacycaule trunks" and "spinescent." Yet it is also uniquely "flexible" and "strict" in its organizational, structural morphology, which has aided it in responding to the challenges of geological, xeric landscapes. Variation among the species is so significant, with exception to the Pachypodium rosulatum aggregates, among their morphology that the discussion of adaptive mechanisms leads to a question: "How do taxa became species?" Pachypodium are always succulent plants. Hence the genus employs two types of adaptive means to xeric, isolated, geological conditions of the landscape, especially in Madagascar:
Pachycaule. The plant develops a pachycaule trunk that retains water. The trunk, as well as branch, is abnormally thickened into various shapes. Within this genus, these shapes range from dwarf plants (Pachypodium brevicaule)), to bottle-shaped shrubs (Pachypodium windsorii), and cigar-like trunk for a tree (Pachypodium lamerei). With the two species Pachypodium bispinosum and Pachypodium succulentum, the adaptation to develop geophytic trunks, or trunks that are beneath the soil's surface can be observed. But morphologically, these geophytic trunks are caudexes, enlarged stems or trunks that stores water. They should never be misunderstood for roots because the enlargement occurs above the point where the roots branch off the main axis (the crown) or trunk. This evolutionary adaptation of a geophytic trunk is a response to the habitat of a species given to its typical landscape conditions. For instance, Pachypodium succulentum grows in areas of Southern Africa where winter temperatures reach -10°C (14°F) and snowfall is possible over the grasslands. A geophytic trunk might possibly be an adaptive mechanism to these cold climate conditions. Also in Southern Africa, the opposite might be said of Pachypodium bispinosum where it experiences temperatures up to 45°C (113°F). A subterranean, geophytic trunk would be a cooler trunk than one exposed above ground, although both ‘’Pachypodium namaquanum’’ and ‘’Pachypodium lealii’’ share the same environment and have tree sized trunks.
Spinescent. Pachypodium make use of spinescent as an adaptive mechanism responding to the landscape. Adaptively this spinescence is employed to different degree in various species to collect moisture from 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 soil beneath the plant from the branches and branchlets. The degree of spinescent, ranging from ‘‘Pachypodium namaquanum’’ to ‘‘Pachypodium decaryi,’’ demonstrates the degree to which species rely on moisture-collecting spines. Pachypodium decaryi has few spines on its branches and branchlets, if not any at all; whereas Pachypodium namaquanum has the largest spines in the genus and is reliant upon fogs and dews to sustain its moisture regime.
Thus, the best example of this succulent mechanism of spinescent is found in Pachypodium namaquanum, with its very long spines, is located in the coastal fog deserts of Namibia. The area's annual rainfall is only 75 mm (2.95 inches). It can be observed that the immediate soil around the taxon is always considerably moist, when fogs roll in off the Atlantic Ocean. Pachypodium namaquanum develops a superficial root system in response to these rather large spines collecting moisture and supplementing its moisture regime.
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.
(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 populations of organisms, i.e. plants, into smaller groups, where each group can initially interbreed but, with time, develop new genotypes and cannot be breed with exception to natural hybridization. Localized geology becomes 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 dew and fog dripping spines are examples of a xeromorphic adaptive mechanized organ responding to microenvironments.
These new species 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, as is the case for Pachypodium ambongense. 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,” such as Pachypodium rutenbergainum. 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 Didiereaceae genera--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.
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 pachycaule and spinescent as well as strict and flexible structural organization at various levels of plant physiology.
Taxonomy
- See main articles Invalid species and varieties of Pachypodium, Valid species of Pachypodium, Unrecognized species of Pachypodium and Pachypodium key to species. Those 4 articles are to be merged into one, to be renamed Taxonomy of Pachypodium.
Number of species
There are now 25 known species, of which 20 come from Madagascar, where isolated broken landscapes and micro-environmental conditions have produced highly specialized species. The species account 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.
However, a study of key characteristics of the taxon and a cladistic study of the subfamily Apocynoideae and the family Asclepiadaceae (before its merging with the Apocynaceae), demonstrate 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 tribe Wrightieae 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
Distribution
Pachypodium are endemic to Madagascar and continental Southern Africa, i.e. Angola, Botswana, Mozambique, Namibia, South Africa, Swaziland and Zimbabwe.
Habitat
- See main articles Pachypodium habitats, Habitats of Pachypodium of Southern Continental Africa and Madagascar and Larger context to habitats of Pachypodium. Those 3 articles are to be merged into one, to be renamed Habitats of Pachypodium.
In elevation, Pachypodium in both mainland Africa and Madagascar grow between an altitude of sea level, where some species grow in sand dunes, such as Pachypodium geayi, to 1600 m (5249 feet) for Pachypodium lealii in southern Africa and 1900 m (6234 feet) for Pachypodium brevicaule in Madagascar.
In continental southern Africa, the extreme temperatures range from -10°C (14°F) for Pachypodium succulentum in some locations to as much as 45°C (113°F) for Pachypodium bispinosum, Pachypodium lealii, and Pachypodium namaquanum. Whereas in Madagascar, with not such a great temperature amplitude, the temperature ranges from -6.3°C (21°F) to 40°C (104°).
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, such as Pachypodium namaquanum, to a high level of 1985 mm (78.15 inches) for Pachypodium baronii, Pachypodium rosulatum, and Pachypodium rutenbergianum. 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.
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 landscape and climates. On outcrops, steep hills, and inselbergs, the plants are subjected to fluctuating moisture, high winds, and temperature extremes. 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 humus in these crevices. Moisture is able seep deep into these crevices. Very little transpiration occurs. In this manner, rocky substrates provide moisture in the habitat. This saturation of crevices can only occur, however, if there is not an 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" laterite red soil. Laterite soil is a largely impermeable soil that traps water for the use of the flora that include Pachypodium.
Protection status
Internationally Pachypodium are protected under the CITES treaty. 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.
Extinction of 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 Leandley in 1830, with Pachypodium tuberosum as the holotype species.
Then the debate centered on the nomenclature of species uniquely found in continental Southern Africa. That changed when, in 1892, Baker contributed the first species accepted into the genus from Madagascar, Pachypodium rosulatum. The degree of speciation then turned to Madagascar, where the count of species far exceeds those on the mainland.
In 1907, Costantin and Bois constructed the first monograph, of Pachypodium, in which they enumerated 17 species, where ten were from Madagascar and seven were from continental southern Africa.
Natural history
There is no fossil records of Pachypodium known. Yet certain conclusions can be drawn from the geology of the landscape in Madagascar as to the past natural history of Pachypodium.
References
- Eggli, Urs. (1993) Glossary of botanical terms with special reference to Succulent Plants. with German Equivalents (British Cactus & Succulent Society: United Kingdom)
- Endress & Bruyn : "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. "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: 0902099744
- 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)
- Endress & Bruyn : "A revised classification of the Apocynaceae." Botanical Review 66: 1-56.
- 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.