456 INTRODUCTION TO PLANT GEOGRAPHY [CHAP • 



(J. Wyatt-Smith voce). Frequently, however, the interior tallness 

 situation is reversed in that the fringe of the mangrove, at least 

 where it does not consist of young pioneer plants, is made up of tall 

 trees, the interior being of much lower or bush-like plants. This 

 is due to the loss of true mangrove conditions and an approach to 

 those of the hinterland forest (J. Wyatt-Smith voce). 



The abundant strut-like and often arching prop-roots of the 

 mangrove trees or lower dominants, among other features, cause 

 deposition of silt and building up of the surface ; often, new mud- 

 flats are formed and the forest may extend year by year. These 

 prop-roots are well seen in the illustrations, while Fig. 158 shows 

 also numerous slender conical aerating roots growing vertically out 

 of the mud. Both types of roots have ' breathing-pores ' and con- 

 tain numerous air-spaces that serve for the conduction of oxygen 

 to the underground parts of the system. This function is rendered 

 vitally important by the nature of the substratum and by the 

 usually frequent inundation, and is performed in some cases by 

 knee-like or keeled projections of roots above the surface of the mud. 

 Another function of the ' pneumatophores ', as the aerating roots 

 are called, appears to be to help keep pace with the tendency of the 

 surface level to rise through deposition, for their underground parts 

 frequently bear the fine rootlets on which the tree is dependent 

 for absorption. Different types of mangrove plants bear these 

 different kinds of roots ; and the species may be mixed together or, al- 

 ternatively, segregated in more or less pure stands. In mangroves 

 in general, and particularly in those of the Indo-Malayan region, 

 there is often to be found a fairly definite succession. The stages 

 of this are usually characterized by different species, and range from 

 the pioneers growing on almost continually submerged surfaces to a 

 mature mangrove forest of often tall trees whose bases may be 

 inundated by only the highest spring tides or, in some instances, 

 scarcely ever reached at all. 



Many of the characteristic dominants of mangroves have another 

 feature in common, namely, the ' viviparous ' development of the 

 seeds — that is, their germination while still within the fruits and 

 attached to the parent plant. The typical arrangement, exhibited 

 for example by the Red Mangrove {Rhizophora mangle), is for the 

 primary root of the seedling to burst through the hanging fruit and, 

 with adjacent tissues, to grow down as a long and dart-like, slender 

 but bottom-heavy structure {cf. Fig. 26, B). Later on the seedling 

 drops — root downwards, so that the tip may be driven into the 



