Mangrove Rhizophoraceae (tribe Rhizophoreae) are unusual in their viviparous condition. Most commonly establishment is from seedlings that are stranded in a horizontal position. How do they become erect? Seedlings are erected by a 90º bend at the radicular end of the hypocotyl, once anchoring adventitious roots have developed. What is the structural mechanism that induces this remarkable change? The xylem on the upper side of the hook region develops tension wood fibers (“gelatinous fibers”) whereas the lower side develops normal lignified fibers. If fibers do produce tension, erection comes about by a pull along the upper side of the hypocotyl base, inducing hook formation. It is suggested that since the seedling is hermetically sealed (no stomata, so flooding is prevented in the dispersal phase), it is necessary to raise the plumular end as high as and as early as possible to permit gas exchange to begin and flooding at high tide to be minimized. Thus vivipary in mangrove plants can be better understood. This is one further example in which reaction fibers, other than in woody stems, is important in an ecological context because it involves secondary changes that have direct adaptive significance. They provide model systems in which the development and function of reaction fibers may be studied comparatively and by means of manipulative experiments.

Key words: gelatinous fibers, gravitropism, reaction wood, Rhizophora, tension fibers