28 
The Queensland Naturalist. 
May, 1932. 
Bruguiera Rheedii, B. gymnorrhiza, Ceriops candolleana, 
Sonneratia alba, Avicemna officinalis, Aegialitis annulata, 
Exoecaria agalloclia, and Aegiceras majus. The last men- 
tioned plant is not common, and does not form an im- 
portant part of the general picture. It is found along the 
tidal creeks, generally on the outskirts of the formation. 
The dominant species are Khizophora mucronata, Bru- 
guiera Rheedii, Avicennia officinalis, and Bruguiera gym- 
norrhiza, At the northern end of Challenger Bay on a 
silting up coral flat the first stages of mangrove coloniza- 
tion may be seen. Here on the muddy sand small trees of 
Khizophora mucronata are growing rapidly. Though no 
measurements were made the increase in size in the period 
which elapsed between the visits of 1925 and 1926 were 
very noticeable. Mud was accumulating round the prop 
roots of the trees and new plants had become established 
in it. The various aerial organs which are developed by 
all these mangroves for the purpose of aerating their roots 
have this secondary function of accumulating mud. Gradu- 
ally the mud deposit becomes thicker and the area is re- 
claimed from the sea by degrees, and is able to support 
strand trees which are unable to invade the area below high 
tide mark. These trees do not immediately suppress the 
mangroves, and thus we get a mixed formation about the 
high tide mark. Gradations from the original coloniza- 
tion to the final strand-mangrove mixed formation may be 
observed within a distance of a mile or two in Challenger 
Bay. On the eastern shore of Eclipse Island the reclama- 
tion of the tidal strip has been effected in a remarkable 
way. Here immense quantities of coral debris have been 
thrown up on the shore amongst the mangroves. W ithout 
doubt these trees have assisted in the holding of the shingle, 
which has-been colonized by a peculiar mixture of strand 
and monsoon forest types. A few mangroves, particularly 
Avicennia, persist in this mixture and indicate the past 
ecological history of the belt. 
The adaptations of the various species of mangrove 
for root aeration vary considerably. Khizophora mucron- 
ata, the Black Mangrove, is the only species of its genus on 
Palm Island (or in Queensland, for that matter), and can 
be identified at a glance by its prop roots. In a dense 
stand these are so interlaced that stepping from one to 
the other is the only way of penetrating the forest. The. 
mangrove crabs cause a certain amount of damage b\ 
destroying the tips of the roots as they enter the ground. 
Bruguiera Rheedii has no prop roots, but the crabs cause 
a high mortality to its viviparous seedlings by nipping off 
the ends after they have fallen to the ground. In this 
species, which, like Khizophora mucronata, attains a height 
