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Horticulturally the term succulent is often used to exclude plants that botanists would regard as succulents, such as cacti.
Succulents can be distinguished by abnormally fleshy parts, where the cells retain additional water. It may be the roots that hold this water like Eurphorbias or it could be their stem as demonstrated by Pachypodiums and Stapliads.
Succulents have modified their stems, roots and leaves to retain moisture in response to their arid surrounding where water conservation is key to survival. Geophytes that survive unfavourable periods by dying back to underground storage organs may be regarded as succulents.
The storage of water often gives succulent plants a more swollen or fleshy appearance than other plants (a succulence) and other water-saving features. These include:
- Modified leaves
- Reduced number of stomata
- Reduced growth form
- Waxy, hairy or spiny outer surface
- Impervious outer cuticle
- Roots very near the surface of the plant
- Remain plump and full of water even with high internal temperatures
- Mucilaginous substances which retain water
Succulent species include Agaves, Aloes, Asparagus, Beschorneria, Cordyline, Euphorbia, Furcraea, Gasteria, Hesperaloe, Lithops, Pachypodium and Yucca.
Cacti belong to the plant family Cactaceae. Succulents are the overarching category and cacti form a sub-group of that category (cacti also hold water in their cells, hence why they are succulents).
They come in a wide variety of shapes and sizes and most live in regions that are subject to some drought. Unlike succulents which use their roots, stems, and leaves to hold water cacti only use the stem.
Cacti do not photosynthesis during the day like other plants. Instead they store the carbon dioxide as malic acid and wait for the cooler night to transpire in order to transpire hence significantly reducing water loss.
Although many cacti have lost their true leaves (which are now modified as spines) not all cacti have spines. In addition some succulents such as Eurphobias and Agaves have spines, so the presence of spines can be misleading and is is not a defining characteristic of a cacti.
Cacti can be distinguished from succulents by their areoles, a highly reduced branch. These appear as small, white, fluffy cotton like lumps from which spines, flowers, branches sprout from.
Most cacti have a long dormancy period which means that they can make the most of the infrequent rainfall. They are also ribbed or fluted, which means that they can expand and contract significantly to adapt to infrequent rainfall. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 170 gallons of water during a rainstorm.
Popular cacti species include Astrophytum, Austrocylindropuntia, Cereus, Echinocactus, Echinopsis, Epiphyllum, Ferocactus, Mammillaria, Opuntia, Pilosocereus, Rebutia and Stenocereus.
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