ON  THE  STRUCTURE  OF  GALLS. 
453 
5.  Protecting  layer  disappears,  the  sub-epidermal  cellular  tissue 
only  remains.     (Spherical  galls  on  briar.) 
Compound  or  multilocular  external  galls  are  due — 
L  To  cohesion  of  simple  tumors,  allied  to  the  fifth  group  described 
above.  (Briar-gall.) 
2.  To  the  development  of  a  hollowed  cellular  mass.  They  may 
be  compared  with  different  groups  of  the  unilocular,  being  some- 
times hard  (tumors  on  roots  of  oak)  or  spongy  (oak-apples.) 
In  all  the  external  galls,  whether  simple  or  compound,  the  fibro- 
vascular  bundles  are  placed  on  the  outside  of  the  protecting  layer. 
Internal  galls  are  true  or  false. 
True  galls  contain  the  insect  in  the  interior  of  their  tissues. 
They  are  hypertrophies,  and  have  their  seat  upon  all  parts  of  the 
plant — on  the  parenchyma,  the  nerves,  the  petioles,  the  cellular 
tissues,  cortical  fibres,  medullary  rays,  and  the  pith. 
False  galls  are  hypertrophies,  deforming  the  organs  and  affording 
the  insects  protection  and  nourishment ;  but  the  parasites  are 
always  on  the  outside  of  the  tissues  of  the  plant.  To  this  division 
belong  the  egg-masses  of  aphides,  found  on  the  leaves  of  the 
poplar,  lime,  elm,  &c,  and  the  nodosities  of  the  trunk  of  the 
apple-tree. 
The  vegetable  hypertrophy,  in  whatever  form  of  gall  it  develops 
itself,  does  not  cause  the  disappearance  of  any  of  the  organic  ele- 
ments ;  it  increases  their  number  and  volume,  and  modifies  their 
form. 
The  cause  of  external  galls  is  the  deposite  of  a  liquid  venom 
with  specific  properties,  a  true  morbid  poison,  secreted  by  the  in- 
sect, which  deposits  it  in  the  plant  with  its  eggs.  The  form,  con- 
sistence, &c,  of  the  tumors,  vary  with  the  specific  properties  of 
the  virus  of  which  they  are  the  consequences.  Internal  galls,  and 
more  especially  the  false,  appear  to  owe  their  formation,  as  Reau- 
mur has  shown,  to  the  abstraction  of  the  liquids  of  the  plant,  by 
the  suction  of  the  aphides.  This  abstraction,  in  augmenting  the 
vitality  of  the  part,  determines  also  its  hypertrophied  growth. 
We  could  make  a  third  general  division,  presenting  at  the  same 
time  the  characters  of  external  and  internal  productions  ;  e.  g. 
artichoke  galls. — Transactions  of  Phytological  Club,  in  Pharm. 
Jour.,  July  1,  1853. 
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