Stilt roots are generally known for support. However, in mangrove plant like Avicennia marina these roots are highly porous through which oxygen enters into the roots. Thus besides support stilt roots play important role in respiration.

Stilt roots are common in Rhizophora and Bruguiera species, these stilt roots are meant for support  not for respiration. In  species of Avicennia and Sonneratia  small and finger like structures are present around the plants  and grows up to 4 to 10 cm only. These structures are called pneumatophores. These species  are regularly inundated with water, so some finger like structures are  produced by the root system of the plant aerially to get sufficient oxygen from the atmosphere. I am attaching the picture of Rhizophora with stilt roots and Avicennia with pneumatophores. 

Since mangrove plants grow on wet soil hence they produce negatively geotropic roots pneumatophores for respiration. No doubt stilt roots are produced for support (e.g. Ficus benghalensis) but in mangrove plants during course of evolution stilt roots have developed dual function. Besides their main function of support they play important role in respiration in mangrove plants.

Since mangrove plants grow on wet soil hence they produce negatively geotropic roots pneumatophores for respiration. No doubt stilt roots are produced for support (e.g. Ficus benghalensis) but in mangrove plants during course of evolution stilt roots have developed dual function. Besides their main function of support they play important role in respiration in mangrove plants.

Thank you Dr Arvind for the information,  I am working on the mangroves since 1985, but  so far I could not observed or studied about the dual  function of the stlit roots

Stilt roots are the main organs for breathing in mangrove plants especially during high tide. Stilt roots are very common in many species of Rhizophora and two species of Avicennia (Avicennia marina & Avicennia officinalis). In several species of Rhizophora ( for example Rhizophora mucronata) the stilt roots extend more than a meter above the soil surface and contain many pores called as lenticels. These lenticels allow oxygen to diffuse into plant and down to underground roots by means of open passages called aerenchyma. The lenticels are highly hydrophobic and prevent water penetration into the aerenchyma system during high tide.

I didn't observe any stilt roots in Avicennia species , if you have that information kindly give a picture of the Avicennia along with stilt roots, I have published more 40 publications along with a book chapter on mangrove ecosystems and completed one DST project  but I did not notice this interesting phenomenon.