GOLA, EDYTA M. Institute of Botany, Wroclaw University, Kanonia 6/8, 50-328 Wroclaw, Poland. - Lycopod vasculature - unique system in plants.
Lycopod specific features including appearance of microphylls, wide
spectrum of phyllotaxis, high frequency of phyllotactic
transformations, shoot dichotomy significantly affect their
vasculature and make it exceptional in plants. Partitioning of the
stele into vertically oriented protoxylary ribs (PXRs)affects the
pattern of the vascular connections with microphylls. The number of
PXRs in the stele is not correlated to the type of phyllotactic
pattern or its expression. In shoots with whorled phyllotaxis
microphylls from one to three phyllotactic ortostichies connect to one
PXR. In shoots with helical phyllotaxis the vertical course of PXRs
and the regular microphyll pattern cause the characteristic
"zig-zag" course of stelar parastichies: each PXR supplies
microphylls from different foliar parastichies - traced on the surface
of the stem. As each trace connects to the nearest vertical PXR, the
microphyll traces must vary in length, unlike as in seed plants where
megaphyll traces are of rather equal length. The regulating role of
the apex is expressed by efficient packing of primordia and its
control of the stelar diameter. During dichotomy of the shoot,
circumferential discontinuity of the microphyllous phyllotaxis is
paralleled by the reconstruction of the stelar system: the change of a
stele symmetry and architecture. Changing number of PXRs is associated
with appearance of endarchic protoxylem elements inside of the stele.
Parallelism of the stem and stele surfaces, with regularly distributed
elements, results in spatial adjustment of microphyll and PXR
patterns. In this light lycopod vasculature seems to be the first,
maybe imperfect yet, supplying system for superficial elements.
Key words: dichotomy, Lycopodium, morphogenesis, phyllotaxis, stele, vascular system