The  section  where  the  buoyancy  of  the  fruit  is  connected  with
unoccupied  space  in  the  fruit-cavity  is  extremely  heterogeneous  in
its  composition.  Every  fruit  has  a  method  of  its  own,  and  the
great  variety  of  causes  of  buoyancy  of  a  mechanical  character  is
here  exemplified.  For  instance,  with  Gyrocarpus  jacquini  and
Cassytha  filiformis  the  cause  of  buoyancy  is  in  the  main  the  same
as  that  described  in  the  case  of  the  Convolvulaceae.  The  origin  of
the  floating  power  of  the  pods  of  Derris  uliginosa  is  two-fold.  In
the  first  place  the  seed  or  seeds  but  partly  fill  the  pod,  and  in  the
second  place  the  seed  is  able  to  float  of  itself  by  reason  of  its
possessing,  as  in  the  seeds  of  Entada  scandens,  a  large  central
cavity  produced  by  the  arching  out  of  the  cotyledons  during  the


112  A   NATURALIST   IN   THE   PACIFIC  chap.
final  stage  of  maturation.  A  double  cause  is  also  to  be  assigned
to  the  buoyancy  of  the  fruits  of  Heritiera  littoralis  and  of  Smythea
pacifica,  where,  in  addition  to  the  unoccupied  space  produced  by
the  shrinking  of  the  seed,  the  fruit-case  itself  floats,  though  nothing
but  a  mechanical  explanation  is  to  be  given  of  the  floating  of
empty  ligneous  fruits.
One  of  the  most  suggestive  types  of  buoyancy  belonging  to  the
first  group  is  presented  by  those  cases,  which  are,  however,  not
very  frequent,  where  the  floating  power  is  to  be  attributed  to
empty  seed-cavities  produced  by  the  abortion  of  the  ovule  or
failure  of  the  development  of  the  seed.  A  significant  instance  of
this  is  afforded  by  the  fruits  of  Premna  taitensis,  a  coast  plant.
