Anatomy of axis contraction in seedlings from a fire prone habitat.

The hypocotyls or roots of many seed plants contract during seedling growth. Anatomical evidence is here reported for the first time that G-fibers (gelatinous or tension wood fibers) may cause contraction of roots and hypocotyls in dicotyledonous seedlings long after germination. To document repositioning of seedling buds, selected perennials (20 dicotyledons and one cycad) native to the fire-prone pine rocklands of subtropical South Florida were germinated and measured for 4-5 mo. The height of cotyledonary nodes above the soil decreased because of axis contraction or bending in eight species. Anatomy suggested that two mechanisms operate: (1) previously well-documented collapse of parenchyma cells in two species (Convolvulaceae and Zamiaceae) and (2) newly documented production of G-fibers in six species (all Fabaceae). Contraction or bending of the hypocotyl and/or taproot moved the cotyledonary and later buds of the seedling closer to the soil surface or buried them. Bud repositioning by these mechanisms may protect the lateral buds from injury by fire or other environmental stresses and allow resprouting.