[“Areole”
is an English adaptation of the Latin “Areola” meaning a
"tiny area" or an "open space"]
It is a distinguishing organ
universal to the entire
cacti
family.
Through evolution the
cacti ancestors begin to shorten the
interleaf spaces.
Subtended in the
axil of each
leaf there is a
lateral meristem. In
cacti, this lateral meristem (Areolar meristem) became the place from which
branches,
flowers,
leaf and
spines arise. It became the areole that is to say a condensed
lateral
shoot. Generally circular or oval, looks like a raised
cushion-disc, often covered by small hairs or by dangerous tiny,
hair-like
glochids. In some species, the areole is borne on a
podarium, the modified petiole of the obsolete,
subtending leaf.
The leaves usually are not present, if present , they are very small
and quickly fall off, only a few of the less ancestral species
still produce wide leaves. They are found along stems and
cladodes of the plant and sits on the
ribs or on the top of tubercles
(or at their base). There are two types of areole: radial,
(e.g. Epiphyllum) with spines spreading star-like around the
growth centre, and unilateral where the growth bud is above
the spine cluster this can be seen in Thelocactus, and in
Coryphantha in which the flower is set on the upper side of the
tubercle, sometimes on old plants with a visible
groove running on
to the spines. Mammillaria completes the separation by
bearing flowers in the usually woolly
bases of the spine-tipped
tubercles.
In some species areole proliferation leads to the lateral
cephalia of several genera of columnar cacti, and to the big
terminal cephalium of Melocactus. |
