Thursday, 27 April 2017

In Flower: Pelargonium aridum

Pelargonium aridum
One of the most familiar of garden and house plants in much of the world are the various forms of what are often called Geraniums or storksbills, when they are not referred to as their proper generic name of Pelargonium.

In cultivation there are a wide variety of cultivars, all derived from a comparatively small number of wild species, which have been hybridized in numerous combinations to generate the vast array now available as plants or seeds. In the wild there are even more to choose from, as there are around 250 species currently described.

The centre of distribution is south west Africa, with additional species extending north and east as far as Iraq and outside Africa to St Helena in the Atlantic and across to Madagascar, Australia and even New Zealand. They tend to be shrubby species and many are adapted to withstand drought, which is why the cultivars are often successfully grown in tubs and hanging baskets, where water stress is common.
P.aridum flowering in a 7cm pot April 2017
The species shown in this post is P. aridum, which originates from the border of Western and Eastern Cape Provinces extending into Lesotho, usually at above 1100m. In this part of the world rain falls and plants grow and flower during the winter, then go dormant for the dry summer months. To survive they have large tuberous roots to retain water.

Surviving a long dry period is a common necessity for plants in warm climates, and there are various means used by plants to get through the enforced dormancy. The commonest is probably by seed as an annual, but pelargoniums are perennial plants. For a plant to survive many months of drought it must have several adaptations to conserve water until rainfall enables leaves and flowers to be produced. This succulent habit usually takes the form of one or more parts of the plant body being developed to retain water. Most pelargoniums have modified stems which are thickened and have thick bark to cut down water loss. In some cases the water-retaining stems are underground, forming an Underground Storage Organ (USO) or caudex.

Many are deciduous during the dry season, especially those from drier areas. Leaves also have adaptations to reduce water loss in the form of leaves with a waxy cuticle or covered with fine hairs, both of which reduce transpiration losses of water from the plant. Another adaptation, which is used to various degrees by different Pelargonium species, is Crassulacean Acid Metabolism or CAM. In this, carbon dioxide is taken in at night and stored as a simple acid. During the day the stomata in the leaves close thus preventing water loss, and the acid is used as the source of carbon in photosynthesis.  In pelargoniums the CAM process appears to be mainly optional, with plants using it or not depending on how water stressed they are.

The radiation in species in South Africa is not only in body form but also in flower type and colour, the result of different species using different pollinators. Several species use long-tongued flies such as specialised hoverflies and bee flies. Others use birds such as sunbirds, and as with most bird-pollinated flowers these flowers are red and long lasting, qualities which have been passed on to their cultivated descendants. The species shown at the head of this post has a pale yellow flower and is probably pollinated by moths, which are attracted to paler shades.

Protection of their vital foliage is a problem that many plants solve by loading them with deterrent chemicals. This works on insects, but not always on humans, and as a result many pelargoniums are cultivated for the oils that can be distilled from them. In addition, many are edible and leaves and flowers of the scented leaved species are used for a variety of culinary purposes. Local peoples such as the San in in South Africa may also seek out USO’s of some species as a source of carbohydrates.
Given the large range of the genus it is not surprising that some species are considered threatened. One of the most endangered is probably P.insularis from the island of Samha in the Socotran Archipelago. This species is only known in the wild from an area of limestone pavement of around 5 square kilometres which is threatened by goat grazing, and is classed as Critically Endangered. In a similar situation is P. cotyledonis from St Helena in the south Atlantic, although in this case the threat is an introduced invasive grass.

P.aridum photos are mine, P.cotyledonis from wikipedia

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