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Very long tubercle of
Astrophytum caput-medusae |
A tubercle (or tubercule) is an enlarged
modified specialized
leaf
base, or petiole, and adjacent
stem
tissues fused together (podarium), the corresponding leaf of which no longer
existing. Thus, the leaf base functions as the
photosynthetic or
food-producing part. The remaining leaves are the ones that are
modified into
spines. The spines radiate from a
type of reduced bud peculiar to cacti
known as an areole usually located
at the tip of each tubercle like in
Mammillaria.
The tubercles in most of the globose
cactus are spirally-arranged. Otherwise the tubercles in some
species can coalesce end to end in
a continuous line forming ribs, (
example of ribbed cactus are
Echinocactus and
Cereus )
There is also a number of intermediates in between where tubercles may
line up and form rows, or conversely ribs may discontinue to form
distinct tubercles at the extreme examples with many combinations
between the extreme examples.
Tubercles range from long and leaf-like - the extreme being
Leuchtembergia and
Astrophytum caput-medusae which looks more like an Agave
than a cactus and to broad and
shallow like
Turbinicarpus and
Lophophora. In some species
the tubercles are provided with an
adaxial furrow which
extends nearly to the base
(for
example
Coryphantha spp.,
Ariocarpus kotschoubeanus).
The tubercle, and the rib in columnar
cacti, are expandable to allow the cactus to
store
water during times of plentiful
rainfall. In times of
drought, these organs contract. |
