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| Ariocarpus fissuratus |
The cactus family
With about 200 genera and 2,500 species, the cactus family constitutes
the bigger group of succulent plants. Cactus are xerofitic plants,
showing reduction of the surfaces that could loss water. These plants
appeared in the Earth 600,000 years ago, so they are a group of recent
plants, in comparison to others, which arose 115 million years ago.
The predecessors of the cactus show very similar shapes to ordinary
plants. Some primitive cactus had normal stems and leaves. Nevertheless,
when the climate of many zones of the Earth changed, living beings
had to evolve to structures and shapes that allowed them to survive
in the new conditions of the environment. The first representative
of this family who evolved was Pereskia, primitive sort that still
exists, that adapted to the new conditions with moderate changes.
From this one plant, by evolution, the Pereskiopsis sort arose. Like
its predecessor, it have leaves, spines and glochids.
Glochids developed further more and increased importance in the Opuntia.
The evolutionary trunk get to Quiabentia and reached a stage with
heavy stems. Soon, in Nopalea, segments of squashed shape appeared.
The reduction of the foliar size is another aspect of the evolution.
From Pereskia, through the narrow leaves of Pereskiopsis, to the remainders
of leaves in Opuntia. Besides, even when the leaves of Pereskia are
not perennial, those of Pereskiopsis are lost when cold arrives, and
in Opuntia the small atrophic foliar remainders, only last while the
bud is young. Cactus are still today in evolution towards more perfect
and adapted forms. |
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