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What is a Cactus?

Cacti are...
Five Common Fallacies

  Cacti are...

Cacti are succulent xerophytes.

Succulents are plants that store large quantities of water in their leaves, stems, or roots, giving them a fleshy (succulent) character. Many succulent plants grow in drought-prone, nearly arid climates or physiologically dry soil (e.g. frequently frozen or partially or periodically salty soils). This involves adaptations that enable them to make do with a minimal amount of available water, and to survive long periods without rainfall. In botanical ecology, plants with these qualities are called xerophytes.

Terrestrial plants have three organ systems: the shoots, the roots and the leaves. In cacti however, these systems have undergone extreme evolutionary modification as a means of adaptation to droughty conditions. Leaves have been sublimated to spines (to reduce water loss), stems evolved succulence to store water and nutrients; and they gained chlorophyll to supplant the leaves in photosynthesis. Typically, the root systems are shallow and wide-spreading to provide maximum access to water from dew, mists or sparse rainfall.

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Cacti are members of the family Cactaceae.

Cacti are perhaps the best known family of the succulents. Other succulents are often mistaken for cacti, based on their similarity of form, but have no direct relationship to cacti. These other succulents belong to any one of about 37 other families. The Cactaceae belong to the class Spermatophyta (seed-bearing plants) and the superfamily Angiospermae (flowering plants).

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Cacti are almost exclusively natives of the Americas. 

Their point of origin is thought to be Southern Mexico or Northern South America (Nobel, 1984) where they are most numerous, but cacti are also found in central and western Canada, from the Peace River district, Lat. 57N (Opuntia, Coryphantha) increasing in number of species and varieties of form southward through Central America and the Gulf Islands to Mexico. Here, they continue southward, declining in numbers and forms, finally petering out in Patagonia, at Lat. 49S (Opuntia). Only examples from the genus Opuntia can be found in all cactus habitats of the Americas. Only one species, a Pereskia, has been long known in West Africa. Almost certainly it was introduced there, presumably by ocean currents or more recently, perhaps, by mariners. Cacti are also endemic in the Galapagos Islands off the west coast of South America.

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Cacti share 7 defining traits.

If a plant lacks any one of the following traits, it is not a cactus.

  1. Cacti are dicotyledons. Their seeds always produce two cotyledons or seed leaves upon germination.  
  2. Cacti are perennial shrubs or herbs, which live on year after year. Annual species do not exist. 
  3. Cactus flowers usually have an indefinite number of sepals and petals and have numerous stamens
  4. Almost all cactus plants, except for the primitive Pereskia have an epigynous ovary
  5. The cactus fruit is a one-celled berry, smooth or spiny, with seeds scattered throughout. 
  6. Cacti are caulocarpic: that is, they do not die after flowering. 
  7. Cacti bear areoles, a unique cushion-like structure on the stems which bear spines or flowers. 

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  Five Common Fallacies
(Adapted from Chidamian, 1958)

  1. All succulents are cacti. 
    Nonsense! About 37 plant families contain succulent plants. Only true cacti form the unique family Cactaceae.

  2. All succulents grow in full, blazing sunlight.
    Not so! Very many xerophytes including some cacti require the shade of shrubs, grasses, rocks, etc. to avoid harmful scorch and dessication. Many cacti of the harsher habitats protect themselves with dense spination (Espostoa), very dark colours (some Lobivias), very low profiles above ground (Ariocarpus, Lophophora) and other strategies.

  3. All succulents grow in the desert. 
    Although the deserts of the world may contain the largest number and variety of cacti and other succulents, they occur also in alpine, jungle and shoreline habitats. Cacti thrive in the low-lying and warm valleys and plains of Central America, in the full blaze of tropical sun, or they can inhabit the rocky and arid slopes of the high mountain systems of the Andes in Ecuador, Peru and Bolivia, where extremes of sullen heat and bitter cold are common. Certain species of Opuntia and Mammillaria are covered with snow in winter, in the arid or semi-arid districts of Arizona, Utah and Colorado. Others are epiphytic, living on the mossy trunks of trees in tropical or sub-tropical forests or on ledges of rocks in deep ravines where they receive ample shade, warmth and a moist soil rich in humus. 

  4. All succulents can grow in pure sand.
    Nothing grows in pure sand! Cacti and other succulents are adapted to a great many different but nutritious environments and soils. Desert sands are often nutrient rich. 

  5. Succulents do not need water. 
    Water is absolutely essential to all life! Although most cacti and other succulents require much less water than other plant families, they must have a sufficient supply of water in order to support life. For example, succulent plants do not exist in very extreme deserts such as the Sahara or Australian outback, where conditions are especially arid. 

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