(1) Stilt roots are characteristic of Rhizophora but also occur in Brugeria and Ceriops. They may also occur in Avicennia alba and A.officinalis. They grow downwards from the trunk and branches, providing gas exchange in the exposed sections and water uptake in the sections below the sediment. They also provide support in older trees.
(2) KNEE/BLADE ROOTS
Knee roots are raised loop sections of cable roots, with thickening on the upper exposed side, forming 'knees'. These are characteristic of species of Brugueira and Ceriops. Some species of Xylocarpus have knee roots that grow from the upper surface of horizontal roots, forming extended blade-like structures. Lumnitzera develops knee roots without the thickening of secondary growth, which are structurally intermediate between kneeroots and pneumatophores.
(3) PENCIL/PEG ROOTS (aka pneumatophores)
Pneumatophores project through the sediment surface from underlying cable roots and function primarily in gas exchange. They are characteristic of Avicennia spp and Sonneratia spp. Pneumatophores in Avicennia usually project less than 30cm above the sediment surface. Those of Sonneratia species are bark-covered and may reach heights of 3 metres. Some other genera may also develop pneumatophores. In some circumstances, Laguncularia may develop pneumatophores, which are often branched, unlike those typical of Avicennia and Sonneratia.
(4) BUTTRESS/PLANK ROOTS
In Xylocarpus granatum, horizontal cable roots develop vertically blade growth, forming extended blade or plank roots above sediment surface. Similar development from the base of the trunk in Heriteira species results in development of buttress roots.
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