Structural Comparison between Free and Anchored Roots in Tillandsia (Bromeliaceae) Species

SUMMARY

The loss of the absorbing function by aerial roots observed in Tillandsia (Bromeliaceae) is a display of extremely specialized epiphytism. The roots of the «atmospheric» Tillandsia usually anchor the plant to the support: however, some of these roots grow free. The structure of both free and anchoring roots is compared in this paper. Analyses by SEM, TEM and LM were carried out on the atmospheric species: T. latifolia Meyen, T. macdougalli Smith. The following features emerged: in the rootcap of both adhering and free roots the dead and degenerating cells remain to form a protective cover; no ultrastructural differences can be noted. In contrast, the underlying living rootcap cells show ultrastructural differences. In the free roots and in the non contact side of the anchored roots the rootcap cells appear to be involved in a utilization process of the cytoplasmic content of the outer degenerating cells; in the adhering side of the anchored roots they show large cytoplasmic zones which appear to be modified into amorphous material. Cytochemical reactions indicate that this material has a protein-polysaccharidic nature, therefore, since it is hydrophilic, contact with the support can be facilitated by means of changes in turgor pressure. Some epidermal cells of the adhering side of the anchored roots form unicellular hairs. The ultrastructural features of these are consistent with those found in cells which secrete lipo-polysaccharidic material. In fact, some similar material is observed on the outer side of the hairs. All of the above characteristics of the anchoring roots may be regarded as morphologic features which contribute to their adhering function.