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The areolar groove on the tubercles of
Ariocarpus kotschoubeanus
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Areolar grooves are found on
tubercles
of
sexually mature
cacti
belonging to many genera (e.g
Coryphantha, Ariocarpus, Thelocactus, Escobaria,
Glandulicactus, Ancistrocactus etc) This
plants
have grooved and groove-less tubercles, the latter occur on
juvenile
plants and emergent
branches and
may occur mixed with grooved tubercles on
adult
shoots. The
areolar meristem
in the groove has a
vegetative
portions, usually
spiny
at tubercle apices
and a
reproductive portion (flowering
or
fruiting) portions in the
axils of the
tubercles but the 2 parts remain connected by a band of
trichomes
on the
adaxial side of the tubercle (e.g., Coryphantha).
Grooved tubercles have
monomorphic
areoles and are the only ones which bear
flower.
Several genera have a short groove reaching only part way to the
axil of the
tubercle, with the flower produced at the
adaxial
end of the extension. On the contrary in some genera (e.g.,
Mammillaria) the connecting groove is absent; the
axillary
reproductive regions are separate areoles distinct from the
spiniferous
vegetative
areoles. (dimorphic
areoles on grooveless tubercles) |
In some species
nectary glands
occur in the groove of most tubercles and may occur at any
position along an areole groove. The glands initiate by the
areole meristem
in the same manner as
spine, with
which they are
homologous.
Areolar groove are also present in Ariocarpus:
A. fissuratus and
kotschoubeyanus has
monomorphic
areoles with both
floriferous
and
spiniferous
areolar areas developing within the same
meristem
and never splitting off.
A. retusus and
A. agavoides, on the other hand, has areole
dimorphism
with the original single growing point becoming elongated and
then dividing into two portions. However, A. trigonus,
differs from both the above groups as the spiniferous growing
point never develops from the original meristem. All species
begin with an original
meristematic
area, with
differentiation occurring during
elongation
and maturation
of the tubercle. It is believed that this species have areolar
meristematic areas with potential floral and spiniferous areas,
but that growth and maturation can occur in one of three places
in relation to them. |
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