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grow in isolated patches around our coastline. Also, few species are present, and so a list for Hong Kong (Table 1) would contain only 8 species (Hodgkiss, 1986).
Hodgkiss, Thrower and Man (1981) give colour illustrations of these species together with some mangrove associates and they are described by Hodgkiss in a WWF (undated) pamphlet. Hodgkiss (1986) also provides a key for their identification.
Adaptations of mangrove plants
Because of their location at the fringes of land and sea, mangrove plants face three major problems: an unstable substratum, anaerobic conditions and high salinity.
Mud is a shifting substrate and to overcome this problem some mangrove plants have root systems which either spread out horizontally just beneath the substrate surface (cable roots) as in Avicennia (Fig 1), or they produce supporting roots from the main stem (prop or stilt roots) as in Kandelia (Fig 1) and Bruguiera, or they produce upwardly projected ridges from their main lateral roots (buttress roots) as in Heritiera (Plate 1).
Some mangroves (Kandelia, Aegiceras, Bruguiera, Avicennia) produce seeds which develop while the fruit is still attached to the plant and so do not undergo the normal period of seed dormancy before germination - a state known as vivipary. Kandelia (Fig 2a) and Bruguiera (Fig 2b) show true vivipary in that the developing hypocotyl ruptures the pericarp forming the so-called 'dropper', whereas the other 2 species, Aegiceras (Fig 2c) and Avicennia (Fig 2d), exhibit cryptovivipary since no rupture of the pericarp is involved during initial embryo development.
Vivipary allows rapid growth and establishment of the young plants when they are finally detached from the parent plant and come into contact with the substratum. In the case of those which produce 'droppers' this can be almost instant in that, as soon as the dropper touches the substrate, adventitious roots are produced from the tip of the hypocotyl and the plant becomes established before it can be washed away (Plate 2). Alternatively, if the dropper lands in water, it floats,