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Opuntia aciculata
Opuntia compressa |
Opuntias are distinguished from other cacti by
4
characteristics:
-
The stems grow in
distinctly jointed
segments (cladodes).
The elongation
of joints is
permanently terminated at the reach of a certain size or by onset
of the dry
season (determinate
growth); subsequent
growth of the
plant occurs by the initiation of new joints by
branching from
the ends of older
segments. That is segments are
genetically
programmed to
expand for a limited time, then stop
growing. This
determinate growth characteristic of segmented plant is in stark
contrast to the indeterminate (virtually unlimited) growth of
stems typical of other
cacti which has
not a precisely determined or established limit of growth fixed in
advance (indeterminate
growth) subsequent growth of the plant occurs by the
initiation of new joints by branching from the
areoles. (Other
cacti, like saguaro have indeterminate growth).
-
Whether the stems
have spines or
not, Opuntia stems always have
glochids in
the areoles.
-
Rudimentary
and ephemeral
leaves are present on new joints.
-
Seeds have a pale
hard covering called an
aril; most other
cacti have black, shiny seeds.
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