FAUNA AND FLORA O F MANGROVE SWAMPS
139
surface, they ensure that nutritive roots can always be pushed out into
these rich layers. These layers of soil also contain some oxygen as the
result of the activities of burrowing animals. I have frequently found
bivalves in cocoons formed by interlacing roots and byssus threads.
The bivalves extend their siphons to the surface and circulate sea
water through their mantle cavities. This activity helps oxygenate the
mud around them which is always paler than that only a little farther
away.
Troll suggested that the respiratory function of the pneumatophores
is less important than the nutritive function. But this is not necessarily
so. Emould (1921) suggested and Scholander et al. (1955) have shown
that, in both Avicennia nitida and Rhizophora mangle from Florida, the
air capillary system of the roots is in communication with the lenticels
of the pneumatophores or prop roots. I n the case of Avicennia when the
tide covered the lenticels on the air roots the pressure in the root
system began to drop and continued to do so until the tide began to
fall. Then air was rapidly sucked in. At low tide oxygen concentration
was around lO-lS%, when submerged the oxygen content fell, to
return again to this level when the lenticels were exposed to air as the
tide ebbed. This rhythm was not present in trees growing a t a high
level where the roots were rarely submerged. When the lenticels were
closed by greasing them, the oxygen in the roots fell reaching in one or
two days 1 % or less, proving that the pneumatophores are chimneys
serving as ventilators of the root system in the anaerobic mud. In the
case of Rhizophora the lenticels on the prop roots acted as ventilators
and the oxygen concentration of the underground roots displayed a
similar variation to that shown by the cable roots of Avicennia.
The deeper layers of the mud are not only anoxic but are also
saturated with hydrogen sulphide, so i t is essential that there should
be no communication between the anchoring roots and the soil around
them.
But some hydrogen sulphide may be present in the upper layers of
the soil in which the absorptive roots lie. When one realizes that those
polychaetes which live in such muds are relatively insensitive to
hydrogen sulphide (and to cyanide) one wonders why no work appears
to have been done on the sensitivity of mangrove trees to such
poisoning.
The form of the pneumatophores varies from slender pencil-like
structures in Lumnitzera racernosa and Avicennia spp. (Figs. 14 and 25)
to stout knobbly structures in Xonneratia spp. and Xylocarpus moluccensis. Geriops spp. and Bruguiera spp. possess variants of knee roots
which rise above the surface and go down again; these are connected to,
139
surface, they ensure that nutritive roots can always be pushed out into
these rich layers. These layers of soil also contain some oxygen as the
result of the activities of burrowing animals. I have frequently found
bivalves in cocoons formed by interlacing roots and byssus threads.
The bivalves extend their siphons to the surface and circulate sea
water through their mantle cavities. This activity helps oxygenate the
mud around them which is always paler than that only a little farther
away.
Troll suggested that the respiratory function of the pneumatophores
is less important than the nutritive function. But this is not necessarily
so. Emould (1921) suggested and Scholander et al. (1955) have shown
that, in both Avicennia nitida and Rhizophora mangle from Florida, the
air capillary system of the roots is in communication with the lenticels
of the pneumatophores or prop roots. I n the case of Avicennia when the
tide covered the lenticels on the air roots the pressure in the root
system began to drop and continued to do so until the tide began to
fall. Then air was rapidly sucked in. At low tide oxygen concentration
was around lO-lS%, when submerged the oxygen content fell, to
return again to this level when the lenticels were exposed to air as the
tide ebbed. This rhythm was not present in trees growing a t a high
level where the roots were rarely submerged. When the lenticels were
closed by greasing them, the oxygen in the roots fell reaching in one or
two days 1 % or less, proving that the pneumatophores are chimneys
serving as ventilators of the root system in the anaerobic mud. In the
case of Rhizophora the lenticels on the prop roots acted as ventilators
and the oxygen concentration of the underground roots displayed a
similar variation to that shown by the cable roots of Avicennia.
The deeper layers of the mud are not only anoxic but are also
saturated with hydrogen sulphide, so i t is essential that there should
be no communication between the anchoring roots and the soil around
them.
But some hydrogen sulphide may be present in the upper layers of
the soil in which the absorptive roots lie. When one realizes that those
polychaetes which live in such muds are relatively insensitive to
hydrogen sulphide (and to cyanide) one wonders why no work appears
to have been done on the sensitivity of mangrove trees to such
poisoning.
The form of the pneumatophores varies from slender pencil-like
structures in Lumnitzera racernosa and Avicennia spp. (Figs. 14 and 25)
to stout knobbly structures in Xonneratia spp. and Xylocarpus moluccensis. Geriops spp. and Bruguiera spp. possess variants of knee roots
which rise above the surface and go down again; these are connected to,