Plant 160

Pachypodium lamerei Drake (Apocynaceae)


Pachypodium lamerei comes from the species-rich, dry, succulent forests of western Madagascar. Eighteen of the 23 known Pachypodium species are endemic to the island; the other five species are endemic to southern Africa and Namibia.

Pachypodium means 'thick-footed' and is derived from the Greek pachus 'thick' and podion 'foot' on account of the shape of its swollen stems. These extraordinary shaped stems are thickened with water storage tissue. All species are stem succulents. This is an adaptation to either the seasonally dry habitats in which the plants grow or to enable them to survive prolonged periods of drought. When damaged, pachypodium stems will bleed copious amounts of white sap.

The stems of Pachypodium lamerei are covered in sharp spines, up to five centimetres long and grouped in threes, which emerge almost at right angles. The spines perform two functions, protecting the plant from grazers and helping with water capture. Pachypodium lamerei grows at elevations up to 1,200 metres, where sea fog from the Indian Ocean condenses on the spines and drips onto the roots at the surface of the soil.

Pachypodiums are deciduous but when leaf fall has occurred photosynthesis continues through the bark tissue on the stems and branches. Pachypodiums use two methods of photosynthesis. The leaves use typical photosynthetic chemistry. In contrast, the stems use CAM, a special adaptation to harsh environmental conditions used by some plants when the risk of excessive water loss is high. Stomata (holes in plant surfaces surrounded by guard cells) are closed during the day but they open at night so carbon dioxide can be acquired and stored. During the day, carbon dioxide is released inside the plant and used in photosynthesis.

Apocynaceae leaves are usually arranged opposite one another, but in Pachypodium they are alternately arranged and tightly packed into clusters at the end of the stems and branches. The white, fleshy, tubular flowers are up to seven centimetres long and borne in clusters. The dry fruits are produced in pairs, each containing small, parachute-like seeds; the hairs aid long-distance, wind dispersal.

Threats to pachypodiums come from habitat destruction because of over grazing and the illegal collection of plants for the horticultural trade. All species of Pachypodium are listed on either Appendix I or Appendix II of The Convention on International Trade in Endangered Species (CITES), an international agreement that governs the trade in plants to ensure their survival is not threatened.

Further reading

Huxley A, Griffiths M, Levy M 1999. The new RHS dictionary of gardening. Macmillian.

Kate Pritchard

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