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This whole central
island is populated by cacti. There are three tribes in this
family:
Pereskieae: leaves broad and flat (Pereskia
)
Opuntieae: leaves small, terete, deciduous (Opuntia,
Nopalea )
Cacteae (Cereeae): leaves rudimentary (columnar cacti, epiphytes,
(Cereus,
Mammillaria )
By extreme adaptation to the desert environment they have
become much modified, with a variety of growth forms. The leaves,
if present at all, are very small and quickly fall off. The areole
is a modified bud with long spines and even more dangerous tiny,
hairlike glochids. In some species, the areole is borne on a
podarium, the modified petiole of the obsolete, subtending leaf.
In the tall columnar forms, the podaria are fused into the
long ribs. The cacti are entirely New World except for the genus
Rhipsalis, a few species of which are in Africa,
but whether they are truly native there is much in dispute. Note
the resemblance of the flattenedstems of Rhipsalis
mesembryanthemoides to the leaves of the smaller
orchids. The genus Rhipsalis contains some of the
most specialized of all the Cereoid cacti. The species epithet
refers either to the resemblance of the succulent leaves or the
flowers to the Old World genus of desert plants, Mesembryanthemum.
Some species of cacti, such as Opuntia ficus-indica
(Indian Fig) with flat stem segments (pads) is
widely grown in tropical and subtropical countries for the abundant
edible fruit, while the stem of some spineless forms is used
for forage. Pereskia spp. are the most primitive
cacti and look like "normal" plants in that they still
have leaves and the stem is only slightly succulent.
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