Second in the series on South African biomes and plants, again written by Taisha:
Pachypodium namaquanum is commonly known as halfmens (Afrikaans for "semi-human"), elephant's trunk, or elephant plant. This species is found in the Succulent Karoo. Drew Avery@Flickr took this photo back in 2009 at the Denver Botanic Gardens and shared it via the Botany Photo of the Day Flickr Pool. Thanks, Drew!
The Succulent Karoo biome extends down the western coast of Namibia and further inland and comprises two domains--the Namaqualand-Namib domain along coastal plain, and the Southern Karoo domain to its east. The altitude of this region is mostly below 800 meters, but may reach up to 1500 m. The temperature in the Succulent Karoo ranges from -4°C in the coldest month and can reach over 40°C at other times. Annual rainfall ranges from 20-400 mm and falls mainly in the winter months (summers are arid), with heavy fog supplementing the precipitation. Berg winds may also occur throughout the year. Soils of the succulent Karoo are much like the Nama Karoo--lime-rich, weakly developed soils on rock.
The vegetation in the Succulent Karoo is, as you may have deduced, dominated by succulents (particularly dwarf shrubby species)! There are many leaf succulents present in the Succulent Karoo, and annual displays of asters occur in the spring. Endemism is extremely high, with around 67 genera and over 1,900 species occurring nowhere else. Included amongst these endemics is Pachypodium namquanum, the species in today's photograph.
Pachypodium namquanum is a stem-succulent species of the Apocynaceae that can live up to 300 years or more. This species grows slowly and regularly, eventually attaining a tree-like appearance when mature at 1.5 to 2.5 m in height (see photos linked from the Wikipedia page: Pachypodium namaquanam). The stems are also covered in tubercles, from which spines protrude. The leaves are borne in rosettes and are distinctively wavy. Yellow-green flowers with red interiors appear from July through September and later develop into horn-like, dehiscent capsules enclosing wind-dispersed seeds.
The unusual appearance of the halfmens has made it one of the most famous and sough-after succulents in the world. One of the most intriguing characteristics is the bowing of the stem to the north. There are folklore interpretations of this phenomenon, however researchers attribute this nodding of ~45-65° to the low solar zenith angle during the winter months. They explain that during the winter, where conditions are more favourable for growth over summer, the low solar zenith angle may pose a potential limiting factor on plant growth. The 55° north incline, they explain, would maximize the solar irradiance on the plane of whorled leaves, of which most are also on an inclination to maximize irradiance. The researchers suggest that the slant northward may benefit the species by increasing net primary production, as well as increase tissue temperatures midwinter when the flowers and later fruits are developing thus minimizing the effects of late-season drought on reproductive output (see: Rundel, P., et al. (1995). Winter growth phenology and leaf orientation in Pachypodium namquanum (Apocynaceae) in the Succulent Karoo of the Richtersveld, South Africa. Oecolgia. 10(4):472-477).