CHAP, LX LITTORAL SWAMP-FOREST. iMANGROVE 237 



show the rare phenomenon of vivipary : that is to say, the embryo-plant, 

 while still attached and nourished by the parent tree, grows into a more 

 or less developed plant without undergoing any resting period : this 

 behaviour, abnormal to other plants, is normal to these. The following 

 series of stages may be noted : 



a. In Aegiceras the embryo emerges from the seed but remains 

 inside the fruit ; it has a large stem and is green. 



b. In Avicennia the endosperm and subsequentl}- the embryo emerge 

 from the seed and lie uncovered within the chamber of the ovary. The 

 embryo is green and obtains food from the parent-plant by means of a 

 long, repeatedly branched, hypha-like haustorial cell, which traverses 

 the placenta. 



c. In Rhizophora and allied genera (Bruguiera and Ceriops) the 

 embryo grows not only out of the seed, but also out of the fruit, and 

 projects from the latter in the form of a green seedling displaying the 

 hypocotyl and root, which in some species exceed one-third of a metre in 

 length ; ' like long, green pods the full-formed seedlings hang dowTi from 

 the branches '. The cotyledons serve as a haustorium, sucking food from 

 the parent-plant. Finally, the seedling breaks loose from the cotyledons 

 (Rhizophora has only one cotyledon), which remain behind inside the fruit 

 and shrivel with it ; it falls into the water or mud ; its club-shape and 

 pointed root-end adapt it to pierce the mud, into which it rapidly thrusts 

 lateral roots that had previously been initiated.^ If the seedhng does not 

 succeed in fixing itself, it floats and may strike root on some distant shore ; 

 in this way the species is provided with means of dispersal by water. Vivi- 

 pary is most pronounced in those Rhizophoraceae that grow in very deep 

 water and very soft soil, and is obviously of great advantage to the species. 



We must regard as adaptations to the environment the green nature 

 of the infant-plant, and the presence of anchoring organs which take the 

 form of upwardly-directed bristles on the seedling, or (in Avicennia, 

 Aegiceras, Sonneratia and Rhizophora) of lateral roots which are already 

 prepared and can rapidly burst out. 



4. Means of migration. All littoral plants show very wide areas of 

 distribution. The mangrove includes approximately the same species 

 along all the tropical shores from Australia to East Africa. This is due 

 in part to the fact that the medium and temperature remain uniform 

 throughout, and in part to the efficient means of dispersal. Thanks to 

 air-containing spaces in the integument or in other parts, and to the 

 consequent decrease in specific gravity, fruits, seeds, and seedlings of 

 mangrove-plants can float for a very long time, and in doing so are not 

 robbed of their germinative power.- The strongest likeness exists between 

 the mangrove of the West Indies and of West Africa, and, on the other 

 hand, between those of East Africa and Asia. 



5. Xerophytic structure. The constituent species of the mangrove, 

 with one exception,^ are shrubs and trees. Despite the circumstance 

 that these plants grow in inundated muddy soil, their vegetative shoots " 

 display a number of structural features that occur in plants adapted to 

 withstand drought. The features in question are the following : 



' Warming, 1883 ; Karsten, 1891. 



' Hemsley, 1885 ; Schimper, 1891 ; Guppy, 1906. * Seep. 235. 



* Regarding their morphology, see J. Schmidt, 1003- 



