GOLA, EDYTA M. Institute of Botany, Wroclaw University, Kanonia 6/8, 50-328 Wroclaw, Poland. - Formation of phyllotactic patterns in cactus seedlings.
In phyllotaxis of the majority of cactus genera, especially
Mammillaria, Notocactus, Gymnocalycium, the main
Fibonacci pattern predominates. In some genera, to which
Rebutia and Aylostera belong, a broad phyllotactic
spectrum with the accessory patterns prevailing, was discovered. The
analysis of the sequential initiation of areoles in seedlings of both
groups shows that in Rebutia, Aylostera,
Pseudolobivia, Gymnocalycium areoles are initiated in
pairs. The first pair is circumferentially positioned exactly as
cotyledons, slightly above them. The subsequent pair arises at the
right angle to the first pair. Later either the decussate arrangement
of areoles develops or, after a small torsion of successively
initiated pairs - the main bijugy. In some cases, due to the initial
tricotyly, areoles similarly form either tricussate pattern or
initiate the main trijugy. In Rebutia occasionally areoles of
one pair differ in timing of their appearance or deviate from expected
circumferential position indicating the change of the pattern into
simple spiral. In contrast to these situations, areoles in
Mammillaria are initiated not in whorls but separately, the
first one between cotyledons. Successive areoles appear in a spiral
sequence, quickly establishing low expressions of the main Fibonacci
pattern. Despite the different modes of areole initiation, the reasons
for the phyllotaxis diversity in such genera as in Rebutia are
not completely elucidated since in seedlings only the most common
patterns have been found. It is suggested that the diversity of
patterns in cacti is due to subsequent ontogenetic transformations of
phyllotaxis. These may involve qualitative change of patterns as
commonly in Rebutia and ontogenetic increase of contact
parastichy numbers of the main Fibonacci pattern as in
Mammillaria.
Key words: Cactaceae, morphogenesis, pattern ontogenetic transformations, phyllotaxis