This month's succulent pin-up
a succulent plant native to high elevations of the Drakensberg mountains of South Africa where it may experience snow, seen growing outside in the rockery at Royal Botanic Gardens, Kew. The fleshy, gel-filled leaves store water when it is available for seasons when it is not.
This is one of the few Aloes that is sufficiently hardy to withstand a cold wet Winter.
What is a succulent plant ?
Succulent (Latin: succos = juice, sap) plants from more than 60 families and 300 genera have evolved special water-storage tissues in thickened or swollen leaves, stems or roots as an adaptation to the arid climates of deserts and semi-deserts. Many of these habitats are associated with high day-time temperatures and special mechanisms have evolved to collect and conserve the limited moisture that is available, sometimes only from dews, mists and fogs. By making the most of scarce available moisture, succulents can survive in habitats that are far too dry for most other plants.
Convergent evolution has often found similar solutions to the problems of living in a harsh environment. It may be obvious that similar looking succulent plants belong to different families only when they are flowering. Some plant families are mostly non-succulent and include just a few succulent members that are adapted to particular niches in challenging environments.
Stem Succulents: Fleshy stems contain water storage cells overlaid by photosynthetic tissue. Leaves are almost or entirely absent, reducing surface area to prevent evaporative loss of water.
Examples: most cacti, Euphorbia obesa, Stapelia.
Root Succulents: Swollen fleshy roots store water underground away from the heat of the sun and hungry animals. Stems and leaves are often deciduous and shed during prolonged dry seasons.
Examples: Fockea capensis, Pterocactus kunzei, Peniocereus viperinus.
Caudiciform Succulents store water in both roots and swollen stems, with either deciduous or long-lived fleshy succulent leaves.
Examples: Calibanus hookeri, Ceraria pygmaea, Pachypodium, Tylecodon, Cyphostemma juttae.
Halophyte Succulents are able to survive in salty desert or marine environments by biochemical resistance to salt, sequestering salt from the cytoplasm in special vacuoles or by excreting salt. Salts often accumulate in arid soils so salt tolerance is more common among succulents than one might imagine.
Examples: Salicornia, Sarcocornia.
The above mechanisms for coping with dry conditions may occur in combination, with more than one organ used to store water.
Sadly, like many other flora and fauna, succulent plants are under pressure throughout the world from encroaching urbanisation, agriculture and the depredations of widespread non-indigenous livestock such as goats.
While these pressures may be inevitable, there is much that succulent plant enthusiasts can do to promote the conservation and survival of this interesting group of plants.