TROPICAL ZONE, MANGROVES, ETC, 191 
Of the various associations which grow upon tropical 
coasts the chief is the mangrove , found covering the muddy 
swamps at the mouths of rivers and elsewhere, over which 
the tide flows daily, leaving the mud bare at low water. 
The plants found here present a great similarity in habit and 
other features, though belonging to different families; the 
association appears to be a very old one, and most of its 
members are taxonomically much isolated. In the Old World 
about 22 species occur, in America four. The chief genus 
is Rhizophora ; Bruguiera (Rhizophoraceae), Avicennia (Ver- 
benaceae), Aegiceras (Myrsinaceae), Sonneratia (Blattiaceae), 
and Acanthus ilicifolius are other important members of the 
group. In general these plants are much branched, with a 
great development of aerial roots, both ‘ flying-buttress ’ 
roots from the main stem and supporting 4 pillar ’ roots from 
the branches. In Sonneratia, Avicennia, and Bruguiera, 
peculiar erect ‘aerating’ roots rise out of the mud (p. 161). 
The seeds of many mangroves germinate in the fruit (so- 
called viviparous germination) while still attached to the 
tree, and develope long radicles, which hang down, often to 
the length of a foot. When the seedling falls the root 
sticks into the mud or floats tip downwards so that it catches 
in holes, and thus the danger of being carried away by the 
tide is lessened. The mangroves are xerophytic in structure 
with thick fleshy or leathery leaves, thick cuticle, water- 
tissue, &c. 
In Asia and Australia there often occurs, further inland 
than the mangroves, a vegetation composed chiefly of the 
‘ stemless ’ palm, Nipa fruticans\ in America a somewhat 
similar part is played by Bactris. 
Another association in the Old World is the beach-jungle , 
characterised by Barringtonia sp., and by numerous shrubby 
forms, e.g. species of Pandanus, Pemphis, Scaevola, Thes- 
pesia, Sophora, Premna, &c. There are also herbaceous 
associations, mostly on sandy beaches, characterised by 
Ipomoea biloba ( pes-caprae ), Spinifex squarrosus , many 
grasses, &c. 
V. Southern temperate zone with hot summers 
This includes all the land south of zone IV., except southern 
1 Schimper, &c., op. cit. ; Hooker, Introductory Essays , see p. 140, 
note ; and cf. Xerophytes, &c. 
