BANASIAK, ALICJA, S.* AND BEATA ZAGóRSKA-MAREK. Institute of Botany, University of Wroclaw, Kanonia 6/8, 50-328 Wroclaw, Poland. - Torreya case indicates autonomy of shoot apical meristem generating phyllotaxy.
In shoots with spiral phyllotaxy axial homodromy occurs when
ontogenetic helix continues its course in successive flushes of the
axis. The opposite case is axial antidromy when the helix course
alters every year. Initially, we thought that in conifers axial
homodromy occurs without exception, suggesting that the apical
meristem is not autonomous in a process of primordia formation but is
influenced by the older, well-developed parts of the shoot. Recently,
however, we have discovered that in Torreya nucifera
characterized by bijugate phyllotaxis the direction of ontogenetic
helix is every year either the same or the opposite to that of the
previous year. Discontinuity of organogenetic process in the region of
bud scales, resulting perhaps from changing primordia identity, was
considered to be one of the causes. The comparative studies showed
that in genera with axial homodromy, such as Abies,
Picea, Cephalotaxus and Taxus, phyllotaxy in a
region of bud scales is as regular as in the area of normal leaves. In
Torreya, however, the divergence angle between successive pairs
of scales increases gradually leading to the decussate arrangement.
Its symmetry guarantees the random selection of a new configuration of
ontogenetic helix in a subsequent growing season. Another possible
reason for the special behavior of Torreya is system of
internal signaling. If the initiation of leaves is governed by the
vascular system, then the axial homodromy should result from the
system mature and close enough to this area where primordia are
initiated. Torreya case might result from isolation of the
meristem from the signals sent by vascular system. It appeared that
indeed in Torreya, the topology of vascular strands and
collenchymatous plate is such, that neither differentiated, nor the
procambial, vascular strands penetrate the plate. In other conifers
they clearly pass the plate reaching out for the level of lowest
primordia.
Key words: chirality, morphogenesis, phyllotaxy, shoot apical meristem, Torreya, vascular system