RHIZOPHORACEAE 499 



the parent tree, or are uprooted, they are carried by water and find a resting- 

 place in the mud, eventually establishing themselves in an upright position 

 through the positively geotropic nature of the lower extremity and the 

 negatively geotrophic nature of the upper extremity (shoot). 



It will thus be seen that the term sowing or dibbling of mangrove seed is 

 strictly speaking incorrect ; it is the embryo or young seedling which is planted 

 in the ground. 



In the other species of the mangrove formation vivipary is also exhibited 

 by Aegiceras majus and Avicennia officinalis, and although it does not occur 

 in other species, the embryos of some, particularly those of Acanthus ilicifolius, 

 are always further developed than is usual in inland plants. 



Local occurrence of species. The requirements of the various species 

 of the mangrove swamp differ to some extent, particularly as regards water, 

 and their local distribution is influenced accordingly. The species of Rhizo- 

 phora grow typically on the outer and seaward fringe of the swamp, wher'^ 

 the water is most salty and exposure to wind and wave is greatest. R. mucro- 

 nata can apparently live in pure salt water, for Schimper notes that he has 

 seen it thriving on the rocky ground of the coral islands of the Java Sea, where 

 there is no fresh water. ^ Geriops spp. grow well out in the swamp. Bruguiera 

 gymnorhiza and B. parviflora require a greater admixture of fresh water, and 

 grow farther inland behind the RhizopJiora, the first named, the largest of the 

 mangroves, overtopping the surrounding vegetation. Sonneratia spp. and 

 Avicennia, officinalis not only grow in deep mud in the mangrove swamp, but 

 also extend some distance inland into the tidal forest, and are found in the 

 upper stretches of the tidal streams. Carapa spp. occur mainly in the drier 

 parts of the swamp, often near or just beyond high- water mark, and extend 

 into the tidal forest in areas remote from the mangrove swamp. Avicennia 

 officinalis may occur near the outer edge of the swamp or at some distance 

 up tidal streams on wet ground : it sometimes comes up in abundance on 

 cleared areas. Acanthus ilicifolius sometimes forms a rather dense prickly 

 undergrowth, growing near the sea. The palm Nipa fruiicans is at times very 

 abundant, not only in the mangrove swamp but also in the drier tidal forest 

 beyond high tide, forming fringes along the banks of creeks. 



Interference with the free flow of tidal water and of fresh water from the 

 landward side may alter the character of or destroy the normal vegetation 

 of the mangrove swamp. Thus at Port Blair in the Andamans attempted 

 reclamation by means of embankments has rendered large areas unfit for the 

 growth of mangroves, and marshy blanks have resulted. 



Method of working mangrove forest. The working of mangrove 

 forest is carried out under difterent methods. The coppicing and pollarding 

 capacity of the different species have not been studied in detail. In unregulated 

 fellings rough pollarding is often carried out. Working plans for mangrove 

 areas in the Madras Presidency prescribe coppice fellings under a rotation of 

 five years in one case and ten years in another. Larger trees which have been 

 pollarded are said to have thrown out vigorous pollard-shoots. In the Anda- 

 mans a considerable area has been clear-felled and replanted. In the Federated 

 Malay States the selection system is in operation, the felling cycle being twenty 



1 Plant Geography, p. 396. .; 



M 2 



