236 HALOPHYTES sect, vii 



ADAPTATIONS 



1. Fixation. The degree of softness of the soil and the variety in 

 depth of water evoke differences, and cause the species to assume a zonal 

 distribution into associations. 



Distinctions in sahnity of the different zones probably also play a part. 



On the landward side mangrove-plants vanish, and are replaced by 

 other shrubs and trees. 



Furthest out towards the sea are those that are best capable of fixing 

 themselves in deep water, namely, species of Rhizophora (rhizophoretum) ; 

 within these, in shallower water or on drier ground, succeed those species 

 that have smaller capabilities in this respect, namely, species of Avicennia, 

 Bruguiera, Aegiceras, Carapa, and others. 



The species of Rhizophora are fixed by prop-roots, that is, by aerial 

 roots which spring from the trunk and often emit radiating branches, 

 which curve down to the ground in an arching fashion. These bow-like 

 roots upon which the tree depends are very numerous ; the foundation 

 of the tree is firmer and its power of resistance to bending, which may 

 be caused by wind or movements of the water, is greater than if the 

 stem were the sole support. The anatomical construction of these roots 

 agrees with the unusual nature of the demands to which they are exposed 

 as supporting structures, and deviates from that of most other roots in 

 that the mechanical tissue is made to assume a tubular arrangement round 

 a large pith.^ Similar prop-roots are possessed by Ceriops and Acanthus 

 ilicifolius. The lower parts of these prop-roots are often beset with 

 algae .2 



Inasmuch as Rhizophorae form the outposts of the mangrove they 

 entangle mud between their roots and thus add to the land. 



2. Respiratory roots. Respiration is a matter of difficulty in the soil, 

 which is water-logged, rich in organic bodies, and poor in oxygen. For 

 this reason all the mangrove-plants have a strongly developed system of 

 aeriferous spaces ; the submerged parts are very spongy and soft in 

 structure ; stomata and unusually large lenticels on parts projecting 

 above water place the atmosphere in communication with the inter- 

 cellular spaces. The prop-roots of Rhizophora serve as respiratory roots. 

 Other species possess quite peculiar respiratory roots. Avicennia has 

 erect, unbranched ' asparagoid ' roots a foot in height ; these stand in 

 very long rows, which radiate from the tree and indicate the position of 

 the horizontal roots from which they spring.^ Similar respiratory roots 

 are owned by Sonneratia acida * and Laguncularia. Bent knee-like roots, 

 with the knee projecting above water, occur in Bruguiera and to a less 

 extent in Lumnitzera ; while comb-like prolongations in connexion with 

 the root occur in Carapa. Karsten's ^ experiments confirm the view 

 that these peculiar structures are respiratory roots. And their anatomical 

 construction harmonizes with this function.^ 



3. Germination and vivipary. Several species of mangrove-plants 



* Warming, 1883. Tansley and Fritsch, 1905. 

 ' Figured by Warming in Borgesen and Paulsen, 1900. 



* Gobel, 1886. * Karsten, 1891. 



' For similar respiratory roots in plants occupying fresh-water swamps, see 

 p. 186; Kearney, 1901 ; Koorders, 1907. 



