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| 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.
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| 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.
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| P.cotyledonis |
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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