Systematic and functional anatomy of seedlings in mangrove Rhizophoraceae: vivipary explained?

The hypocotyl of viviparous seedlings of mangrove Rhizophoraceae (tribe Rhizophoreae) has a common suite of anatomical features but a range of anatomical structures that may be diagnostic at die generic or even specific level. This information is summarized in the form of a dichotomous key. A common and previously unreported anatomical feature is the presence of tension wood fibres developed eccentrically in secondary xylem at the distal pole of seedlings grown in both artificial and natural conditions. These fibres are apparently the source of the mechanism whereby an initially horizontal seedling becomes erected rapidly once it is rooted distally, even though this appears to be achieved at a considerable mechanical disadvantage. The morphological result is a distinct 'hook' at the distal end of the established seedling. This mechanism allows rapid erection of a horizontally stranded seedling, such that the plumule is raised, snorkel-like, above immediate tidal influence. This may be necessary, since gas exchange through the surface of the seedling is not possible via the thick, cutinized and stomata-free epidermis of the hypocotyl. We suggest that the need to produce an elongated propagule, with this self-erecting capability favours the evolution of vivipary in these plants because it permits the earliest development of the plumule in air rather than water. The suitability of mangrove seedlings for further experimental and anatomical study is emphasized. The study also exemplifies how anatomy can be placed in a dynamic context. (C) 2000 The Linnean Society of London.