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ON  THE  STRUCTURE  OF  GALLS. 
enter  into  the  composition  of  plants.  They  may  be  divided  into 
external  and  internal  galls,  from  their  relations  to  the  vegetable 
which  bears  them.  The  first  project,  outwards,  and  are  only  con- 
nected with  the  plant  by  a  very  small  peduncle  ;  the  second  kind 
are  developed  within  the  tissues  and  organs  they  deform. 
External  galls  are  sufficiently  naturally  divided  into  unilocular 
and  multilocular,  from  the  number  of  cells  which  they  contain. 
The  unilocular  class  may  be  divided  into  live  groups,  whose 
structure  is  more  and  more  simple  as  it  recedes  from  the  first  type 
This  type  is  represented  by  the  large  gall  of  commerce  and  the 
French  gall.  If  taken  Avhen  fully  developed,  they  exhibit  from 
the  surface  to  the  centre, 
1.  Epidermis  without  stomata. 
2.  Cellular  sub-epidermal  layer,  analogous  to  the  cellular  tissue  of 
vegetables,  containing  coloring  matter. 
3.  Zone  of  irregular  ramose  cells,  with  large  cavities :  the 
spongy  layer. 
4.  Layer  of  hard,  prismatic,  dotted  cells. 
5.  Layer  of  very  thick,  polyhedral  cells,  very  hard,  much  dotted, 
and  forming,  the  protecting  layer  to  the  nucleus. 
6.  Central  alimentary  mass  of  soft  cells  filled  with  liquid,  the 
external  part  containing  starch-granules  colorable  by  iodine,  the 
internal  not  producing  this  reaction. 
The  central  amylaceous  mass  disappears  gradually  during  the 
developement  of  the  larva,  which  does  not  commence  its  metamor- 
phoses until  it  has  consumed  all  the  alimentary  portion.  May  the 
most  internal  portion  be  regarded  as  fecula,  modified  by  a  process 
analogous  to  the  commencement  of  digestion  ;  or  rather  as  matter 
more  specially  azotized,  serving  for  the  first  phases  of  embryotic 
development  ?  The  French  gall,  like  that  of  commerce,  contains 
fibro-vascular  bundles,  which  pass  from  the  point  of  insertion  to- 
wards the  centre,  and  ramify  in  the  interior  of  the  parenchyma. 
We  find  in  these  bundles  fibres,  branching  and  dotted  vessels,  and 
true  spiral  vessels.  These  disappear  successively  and  give  the 
.five  groups  of  external  unilocular  galls  before  mentioned. 
1.  Hard  and  spongy.    (French  gall  and  gall  of  commerce.) 
2.  Hard.    (Spherical  galls  on  oak-leaves.) 
3.  Spongy.    (Cellular  oak-galls  with  regular  tubercles.) 
4.  Cellular.    (Lenticular  galls  on  oak-leaves.) 
