Cytological aspects of the hypocotyl correlated to the behavior of the embryo radicle of Tillandsia atmospheric species

Generally in Bromeliaceae, as in all monocotyledons, upon germination of the seed, the embryo radicle grows very little. In the atmospheric species of Tillandsia, not even this minimal growth occurs. In the adult stage, these species use special hairs as their sole absorbing organ. The present work analyses the morphological and cytochemical events that are responsible for the lack of growth in the embryo radicle. Ultrastructural analysis and specific stainings for light microscopy highlighted a special area (that we have called the «constriction zone») in the median region of the hypocotyl-root axis of embryos of T. flabellata Back. and T. schiedeana Steud., both atmospheric species. During the final phase of seed maturation, the cells of the aforesaid zone thickened their walls by means of materials that, at least in part, consisted of acid polysaccharydes and neutral lipids. These materials were derived from the fused contents of characteristic cytoplasmic vesicles. Due to the presence of hydrophobic components in the walls, the cytoplasm degenerated into an amorphous mass in which polysaccharides and neutral lipids were also present. All the modifications observed seemed to resemble certain aspects of the suberization and abscission processes; consequently, the distal region of the hypocotyl-root axis remained isolated from the proximal region. The embryo radicle did not grow at all and the isolated region remained within the integuments. Both of these latter conditions are favorable to the life of the Tillandsia atmospheric species, plants which must avoid any loss of trophic resources.