Pelargonium ceratophyllum L'Hérit.
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Hortus Kewensis 2 (1789) 422.
Section Otidia

Habit
Succulent, branched shrub <20 cm, usually smaller, with highly succulent stems, brownish grey, 20-30 mm thick
already in juvenile plants, smooth, but with persistens remains of leaf bases, apparently glabrous but initially covered with microscopically short, adpressed, white hairs. Roots simple, without tubers.


Leaves
Pinnatisect to bipinnatisect, 12-)20-40(-55) x 12-20(-35) mm, petiole 15-30(-40) mm long, succulent, leflets slender and cylindrical, almost glabrous but with microscopical glandular hairs, green. Stipules triangular.

Inflorescence
Sparsely branched, with 3-5 reduced pseudo-umbels of 1-3(-4) flowers each, on peduncles 20-65 mm long. Pedicel 7-13 (-20) mm, often longer than hypanthium. Flowers not scented.


Sepals
5, lanceolate. Hypanthium 4-10(-15) mm.

Petals
Subequal, up to 12 mm, auricled at base, with feather-like purple markings, longer than sepals, obovate, anterior slightly narrower, recurved slightly. Colour white. 

Stamens
7 fertile.

Distribution


Habitat

The lunar landscape of the Lüderitz peninsula in southern Namibia is an experience that cannot be easily forgotten. Much of the peninsula is so frequently and intensively swept by winds that plants only survive as miniatures with low, almost horizontal branches, thick as the result of water storage and slow growth. This is the landscape of P. crassicaule that here develops extremely thick branches of up to 30 mm, of 
horizontal spineless Sarcocaulon patersonii, which otherwise develops into upright shrubs, and of scattered but substantial populations of P. sibthorpiifolium and P. cortusifolium.


A leafless, extremely thick-stemmed P. ceratophyllum from the 
Lüderitz peninsula, with Othonna furcata in the back. The plants are quite frequent on the peninsula, and occupy sandy, as well as rocky habitats.

Illustrations

P. ceratophyllum, Curtis's Botanical Magazine
1795, Tab. 315.


P. dasycaule, Curtis's Botanical Magazine 1818, Tab. 2029. 

Synonyms
dasycaule Haw. 
dasycaulon (Haw.) Sims

Notes
The recent publication of P. albersii has left the definition of P. ceratophyllum somewhat unclear. There appears to be some degree of overlap in the distribution, though this remains unexplored: P. ceratophyllum is found near Lüderitz in southern Namibia, while the type population of P. albersii is east of Beauvallon near Orange River in the Northern Cape Province. The 250-km coastal belt between these two populations could be populated with either and whether natural hybrids form is unknown.

Many important characters that seem to separate
the two species may be all but lost in cultivation, which quite likely contributes to the confusion that surrounds them (cf. illustration of P. ceratophyllum above, with atypically thin upright stems). On the basis of POSA2 (which contains an illustration of P. albersii and the description of what was then considered to be a single species), Becker M., Albers F. (2008), and M. Becker's PhD Thesis (2006), the following key can be developed:

1a. Stems up to 20 cm tall and up to 30 mm thick. Roots simple. Leaves almost green, the indumentum consisting of ~10 um long glandular hairs. The inflorescence has 3-5 pseudo-umbels only. Petals white, longer than sepals, flowers not scented.

Pelargonium ceratophyllum 
L'Hérit.
Hortus Kewensis 2 (1789) 422.

1b. Stems up to 5 cm tall and up to 10 mm thick. Roots tuberous. Leaves greyish due to the indumentum consisting of numerous adpressed ~50 um long hairs interspersed with ~10 um long glandular hairs. The inflorescence has up to 8 pseudo-umbels flowering in succession. Petals cream, of similar length to sepals, flowers lemon-scented.

Pelargonium albersii 
M. Becker
Schummania 5 (2008) 157.

With thanks to David Victor for useful discussions.

The status of P. dasycaulon (Haw.) Sims remains unclear. T
he characters that could be seen as typical of P. dasycaulon (Haw.) Sims are: bipinnatisect leaves, short pedicels, which are shorter than hypanthia (5-8 mm), as well as dichotomously branched inflorescences. There appears to be only one collection that is still in cultivation, by Piet Drijfhout from the late 1970s, and that could tentatively be traced to southern Namibia (exact location unknown). On the basis of DNA analyses of these specimens it has recently been claimed (M. Becker, PhD Thesis, 2006) that they represent a separate taxon, thus validating P. dasycaulon (Haw.) Sims.

However, in the absence of a well-identified
natural population, it would appear to be impossible to reliably ascertain the identity of P. dasycaulon for the following reasons: (i) greenhouse-grown plants hybridise easily (which could also have been reflected in the illustration of P. dasycaule above); (ii) the natural variability of this taxon cannot be established; and (iii) vegetative characters that are essential for the separation of species in section Otidia are often not reliable in cultivated plants. Presently, it would appear best to keep its former status, i.e. synonymy with P. ceratophyllum, which, if grown in a greenhouse, develops the same thin and upright stems (cf. illustrations above) that seem to be characteristic of P. dasycaulon (Haw.) Sims.

Literature
Becker M., Albers F. (2008).
POSA2
M. Becker, Revision der Pelargonium - Sektion Otidia (Geraniaceae) aus dem Winterregengebiet des südlichen Afrikas und Bewertung evolutiver Strategien der Pelargonien aus der Capensis, PhD Thesis, University of Münster, 2006.


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