Abstract:
Sunflower (Helianthus annuus) is annual plants native to Mexico and
is valuable from economic, as well as from ornamental point of view. In this
paper the edification of the primary and the secondary structure of the
vegetative organs were followed during the ontogenesis. The root with
primary structure is tetrarch, with four strands each of xylem and phloem
that alternate with one another. In the secondary structure a massive of
xylem could be observed in the central cylinder. The young stem has primary
structure and circular shape in cross-section. The central cylinder is
delimited by an endodermoid layer with starch grains. The vascular bundles
are of collateral type, with an intense morphogenetic activity. The secondary
structure is formed by only by cambium activity. It produces new vascular
bundles, only with secondary structure, between the initial one. At the plant
stem basis, a continuous ring o secondary xylem is present. Both in the stem
with primary and secondary structure, secretory canals could be observed in
cortex, pith and medullar rays. The petiole has semicircular shape, with
small and large vascular bundles distributed on an arch. A continuous band
of angular collenchyma is visible under the epidermis. The mesophyll of the
leaf is formed only from by palisade parenchyma
