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Colloquially a "pad" is
the flattened stem or
clatode of
some cacti belonging to several species (especially Opuntia) |
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A " Pad" (more properly called cladode) is a stem
modified for photosynthesis that looks like a leaf. It is flat for
increasing the surface area, thick for storing water and green for
photosynthesis.
Cladodes
are not leaves but swollen water-storing stem segments.
In cladodes the leaves are often
modified into
spines,
the
function of spines is
protection as well as to reduce transpiration.
Axillary buds are modified into thin short and greenish structures.
and cuticle is an outer covering which is present for preventing
transpiration.
To identify and distinguish a phylloclade from a true
leaf it is
necessary to the observe the vegetative shoot, on which leaves are still
present small or scale-like, and the leaves of these
species (if
present) are ephemeral and soon
abscise (e.g. Opuntia).
Most genera of cladode-producing plants occur in the cactus family (Cactaceae).
More than 200 in subfamily
Opuntioideae. These are comparatively thick cladodes, with pads often
exceeding one centimetre in thickness, and the flattened pads, also
called joints, have
nodes on the two flat surfaces as well as along the
edges, arranged in a helical pattern. A narrow leaf forms at each node,
and from the axillary bud forms a
determinate short shoot (the
areole) on which are produced
modified leaf primordia
that are the spines. The foliar leaves soon
abscise,
leaving the spine-covered cladode to perform
photosynthesis.
Flattened green stems also appear in more than a dozen genera of
epiphytes and
hemiepiphytes cactus. Also referred
to as 2-ribbed, most of these flattened stems have areoles arranged only
along the two edges, not on the broad surfaces, although in the rhipsalid epiphytes this is not always true. In general the 2-ribbed
epiphytic stems have evolved from species having three or four
ribs on
the stem, hence they formed via a different
developmental and
evolutionary sequence than did the platyopuntias. Some cactus epiphytes
exhibit two, three, and four ribs on a single individual, and there
commonly are differences in stem morphology from
juvenile versus
adult
types.
(e.g. Zygocactus = christmas cactus)
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