Pelargonium

Pelargonium is a genus of flowering plants commonly called geraniums, pelargoniums, or storksbills. It includes about 280 species of perennials, succulents, and shrubs. Taxonomist Carl Linnaeus originally included all the species of Pelargonium and Geranium under the latter name. In 1789, Charles Louis L'Héritier de Brutelle separated them into two genera.
While Geranium species are mostly temperate herbaceous plants, dying down in winter, Pelargonium species are evergreen perennials indigenous to warm temperate and tropical regions of the world, with many species in southern Africa. They are drought and heat tolerant but can tolerate only minor frosts. Some species are extremely popular garden plants, grown as houseplants and bedding plants in temperate regions. They have a long flowering period, with flowers mostly in red, orange, or white, but intensive breeding has produced a huge array of cultivars with great variety in size, flower colour, leaf form and aromatic foliage.

Description

Pelargonium occurs in a large number of growth forms, including herbaceous annuals, shrubs, subshrubs, stem succulents and geophytes. The erect stems bear five-petaled flowers in umbel-like clusters, which are occasionally branched. Because not all flowers appear simultaneously, but open from the centre outwards, this is a form of inflorescence is referred to as pseudoumbels.
The flower has a single symmetry plane, which distinguishes it from the Geranium flower, which has radial symmetry. Thus the lower three petals are differentiated from the upper two petals. The posterior sepal is fused with the pedicel to form a hypanthium. The nectary tube varies from only a few millimeters, up to several centimeters, and is an important floral characteristic in morphological classification. Stamens vary from 2 to 7, and their number, position relative to staminodes, and curvature are used to identify individual species. There are five stigmata in the style. For the considerable diversity in flower morphology, see figure 1 of Röschenbleck et al..
Leaves are usually alternate, and palmately lobed or pinnate, often on long stalks, and sometimes with light or dark patterns. The leaves of Pelargonium peltatum, have a thick cuticle better adapting them for drought tolerance.

Taxonomy

Pelargonium is the second largest genus within the family Geraniaceae, within which it is sister to the remaining genera of the family in its strict sense, Erodium, Geranium, and Monsonia including Sarcocaulon. The Geraniaceae have a number of genetic features unique amongst angiosperms, including highly rearranged plastid genomes differing in gene content, order and expansion of the inverted repeat.

Genus history

The name Pelargonium was first proposed by Dillenius in 1732, who described and illustrated seven species of geraniums from South Africa that are now classified as Pelargonium. Dillenius, who referred to these seven species with apparent unique characteristics as Geranium Africanum suggested "Possent ergo ii, quibus novi generis cupido est, ea, quorum flores inaequales vel et irrregulares sunt, Pelargonia vocare". The name was then formally introduced by Johannes Burman in 1738. However Carl Linnaeus who first formally described these plants in 1753 did not recognise Pelargonium and grouped together in the same genus the three similar genera Erodium, Geranium, and Pelargonium. Linnaeus' reputation prevented further differentiation for forty years. The eventual distinction between them was made by Charles L’Héritier based on the number of stamens or anthers, seven in the case of Pelargonium. In 1774, P. cordatum, P. crispum, P. quercifolium and P. radula were introduced, followed by P. capitatum in 1790.

Circumscription

Pelargonium is distinguished from the other genera in the family Geraniaceae by the presence of a hypanthium, which consists of an adnate nectar spur with one nectary, as well as a generally zygomorphic floral symmetry.

Subdivision

first proposed dividing the genus into 12 sections in 1824, based on the diversity of growth forms. Traditionally the large number of Pelargonium species have been treated as sixteen sections, based on the classification of Knuth who described 15 sections, as modified by van der Walt et al. who added Chorisma, Reniformia and Subsucculentia.
These are as follows;

section Campylia de Candolle
section Chorisma de Candolle
section Ciconium Harvey
section Cortusina Harvey
section Glaucophyllum Harvey
section Hoarea de Candolle
section Isopetalum de Candolle
section Jenkinsonia de Candolle
section Ligularia Harvey
section Myrrhidium de Candolle
section Otidia de Candolle
section Pelargonium Harvey
section Peristera de Candolle
section Polyactium de Candolle
section Reniformia Dreyer
section Subsucculentia J.J.A. van der Walt
Phylogenetic analyses

All subdivision classifications had depended primarily on morphological differences till the era of phylogenetic analyses. However phylogenetic analysis shows only three distinct clades, labelled A, B and C. In this analysis not all sections were monophyletic, although some were strongly supported including Chorisma, Myrrhidium and Jenkinsonia, while other sections were more paraphyletic. This in turn has led to a proposal, informal at this stage, of a reformulation of the infrageneric subdivision of Pelargonium.
In the proposed scheme of Weng et al. there would be two subgenera, based on clades A+B, and C respectively and seven sections based on subclades. Subsequent analysis with an expanded taxa set confirmed this infrageneric subdivision into two groups which also correspond to chromosome length, but also two subclades within each major clade, suggesting the presence of four subgenera, these correspond to clades A, B, C1 and C2 of the earlier analysis, A being by far the largest clade with 141 taxa. As before the internal structure of the clades supported monophyly of some sections but paraphyly in others. A distinct clade could be identified within the paraphyletic Polyactium, designated section Magnistipulacea. As a result, Polyactium has been split up to provide this new section, which in itself contains two subsections, Magnistipulacea and Schizopetala, following Knuth's original treatment of Polyactium as having four subsections.
Image:Pelargonium crithmifolium3.jpg|thumb|upright|Subgenus Pelargonium section Otidia: P. crithmifolium
Thus Röschenbleck et al. provide a complete revision of the subgeneric classification of Pelargonium based on four subgenera corresponding to their major clades ;

subgenus Magnipetala Roeschenbl. & F. Albers Type: Pelargonium praemorsum F Dietrich
subgenus Parvulipetala Roeschenbl. & F. Albers Type: Pelargonium hypoleucum Turczaninow
subgenus Paucisignata Roeschenbl. & F. Albers Type: Pelargonium zonale L'Hér. in Aiton
subgenus Pelargonium L'Hér. Type: Pelargonium cucullatum Aiton

Sixteen sections were then assigned to the new subgenera as follows, although many species remained only assigned to subgenera at this stage

subgenus Magnipetala 3 sections
* section Chorisma de Candolle – 4 species
* section Jenkinsonia de Candolle – 11 species
* section Myrrhidium de Candolle – 8 species
subgenus Parvulipetala 3 sections
* section Isopetalum de Candolle – 1 species
* section Peristera de Candolle – 30 species
* section Reniformia Dreyer – 8 species
subgenus Paucisignata 2 sections
* section Ciconium Harvey – 16 species
* section Subsucculentia J.J.A. van der Walt – 3 species
subgenus Pelargonium 8 sections
* section Campylia de Candolle – 9 species
* section Cortusina Harvey – 7 species
* section Hoarea de Candolle – 72 species
* section Ligularia Harvey – 10 species
* section Magnistipulacea Roeschenbl. & F. Albers Type: Pelargonium schlecteri Knuth – 2 subsections
** subsection Magnistipulacea Roeschenbl. & F. Albers Type: Pelargonium schlecteri Knuth – 2 species
** subsection Schizopetala Roeschenbl. & F. Albers Type: Pelargonium afrum Steudel – 3 species
* section Otidia de Candolle – 14 species
* section Pelargonium L'Hér. – 34 species
* section Polyactium de Candolle – 2 subsections
** subsection Caulescentia Knuth – 1 species
** subsection Polyactium de Candolle – 7 species
Subgenera

Subgenus Magnipetala: Corresponds to clade C1, with 24 species. Perennial to short lived, spreading subshrubs, rarely herbaceous annuals. Petals five, but may be four, colour mainly white. Mainly winter rainfall region of South Africa, spreading into summer rainfall region. One species in northern Namibia and Botswana. Two species in East Africa and Ethiopia. Chromosomes x=11 and 9.
Subgenus Parvulipetala: Corresponds to clade B, with 39-42 species. Perennials, partly annuals. Petals five and equal, colour white or pink to deep purplish red. Mainly South Africa, but also other southern hemisphere except South America. a few species in East Africa and Ethiopia. Chromosomes x=7-19.
Subgenus Paucisignata: Corresponds to clade C2, with 25-27 species. Erect sometimes trailing shrubs or subshrubs, rarely geophytes or semi-geophytes. Petals five and equal, colour pink to red sometimes white. Summer rainfall region of South Africa, spreading into winter rainfall region and northern Namibia, with a few species in tropical Africa, Ethiopia, Somalia, Madagascar, the Arabian Peninsula and Asia Minor. Chromosomes x=mainly 9 or 10, but from 4–18.
Subgenus Pelargonium: Corresponds to clade A, with 167 species. Frequently xerophytic deciduous perennials with many geophytes and succulent subshrubs, less frequently woody evergreen shrubs or annual herbs. Petals five, colour shades of pink to purple or yellow. Winter rainfall region of South Africa and adjacent Namibia, spreading to summer rainfall area, and two species in tropical Africa. Chromosomes x=11, may be 8–10.