498 XXVI. RHIZOPHORACEAE 



ascended by boats at high tide. The ground is a soft mud which is often 

 knee-deep, but can be traversed on foot, with some difhcidty and discomfort, 

 at low tide. On the seaward side there is usually a stretch of shallow water 

 into which the mangrove is spreading, and approach by boat from that side is 

 often difficult or impossible ; the mangrove swamp can, however, be penetrated 

 by boat and often by launch, along the creeks. 



Root-system. The root-system of the mangroves is highly specialized. 

 In the case of Bhizophora the lower part of the stem dies early, and the stem 

 is supported by numerous stilt-like roots which raise it above the mud, while 

 aerial roots are sent down from the stem and branches and anchor themselves 

 in the ground. These stilt -roots are covered by water at high tide and exposed 

 at low tide. Bhizophora is usually characteristic of the outer edge of the 

 mangrove swamp, and the mass of stilt-roots is a conspicuous sight on approach- 

 ing the shore. These peculiar stilt -roots are not conspicuously developed in 

 other species of the mangrove formation except in Acanthus ilicifolius, a thistle- 

 like herbaceous or shrubby plant with pricldy leaves, which sometimes forms 

 a dense undergrowth. In other species the roots are superficial, twisting 

 about on the surface of the mud, sometimes ribbon-shaped as in Carapa 

 ohovata, sometimes bending out of the mud in the form of knees (i. e. knee- 

 rooted), as in Bruguiera, Kandelia, and Lumnitzera. Some species produce 

 pneumatophores which rise out of the mud from the superficial roots and 

 resemble inverted tent-pegs. The ribbon-roots, knee-roots, and vertical 

 pneumatophores, some mere knobs or finger-like outgrowths and others, as 

 in Sonneratia apetala and Avicennia officinalis, of considerable size, are all 

 adaptations for supplying the roots with oxygen, and are covered with lenticels 

 or exhibit other devices for breathing purposes, such as the shedding of cortex. 



Leaf-structure. The habitat of the mangroves, namely swamp^^ 

 ground impregnated with salt, is a physiologically dry one, and the leaves of 

 the trees therefore possess a marked xerophilous structure, ' with a thick 

 cuticle, large mucilage-cells, protected stomata, and especially a large-celled 

 thin-walled, aqueous tissue, the dimensions of which increase with the age of 

 the leaf and v/ith the corresponding rise in the amount of salt contained. Old 

 leaves serve essentially as water-reservoirs for the younger leaves ' (Schimper). 



Germination. One of the most interesting characteristics of the Rhizo- 

 phoraceae is that they exhibit vivipary. The fruit is indehiscent, and there is 

 no resting stage for the embryo as in the case of normal seeds. As soon as the 

 fruit is fully developed the embryo commences to grow inside it ; the radicle 

 soon pierces its apex, and the hypocotyl elongates and protrudes, hanging 

 vertically from the fruit. After it has reached a length varying from a few 

 inches in some species to 1^-2 ft. or more in the case of Bhizophora mucronata , 

 the embryo plant falls, leaving the cotyledons inside the fruit, which remains 

 on the tree. The lower part of the hypocotyl is tliicker than the upper part, 

 and in some cases the lower extremity (radicle) comes to a sharp point ; when 

 the embryo falls into the mud, therefore, it becomes firmly planted in a more 

 or less vertical position. Within a short time of falling the young seedling- 

 produces rootlets from its lower extremity, thus fin-ther establishing itself, 

 and before long the first pair of foliage leaves are produced at its apex. Tlie 

 embryos are buoj'ant. and if they do not obtain an immediate footing under 



