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This month's succulent pin-up

Matucana madisoniorum

Matucana madisoniorum
A cactus native to the edge of the rainforest in the Amazonas region of Peru, an example of a stem-succulent.


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.
Leaf Succulents: Leaves are almost entirely composed of water storage cells covered by a thin layer of photosynthetic tissue.
Examples: Aloe, Haworthia, Lithops, Sempervivum.  
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: Calibanus hookeri, 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: Ceraria pygmaea, 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. Examples: Salicornia, Sarcocornia  
The above types may occur in combination, with more than one organ used to store water.  
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 succulents that are adapted to particular niches in challenging environments.  
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.  
The Succulent Plant Page is a resource for all those interested in collecting, growing, propagating, showing and the conservation of succulent plants including cacti.