﻿98 
  Dr. 
  A. 
  Arber. 
  

  

  indicate 
  that 
  this 
  organ 
  is 
  of 
  the 
  nature 
  of 
  a 
  petiolar 
  phyllode, 
  flattened 
  in 
  

   rthe 
  horizontal 
  plane. 
  

  

  i.b. 
  

  

  Fig. 
  2. 
  

  

  [Figs. 
  1 
  and 
  2. 
  — 
  Pistia 
  Stratiotes, 
  L. 
  Transverse 
  sections 
  of 
  leaf 
  ( 
  x 
  21). 
  Fig. 
  1, 
  complete 
  

   section 
  near 
  base 
  of 
  leaf. 
  Fig. 
  2, 
  two 
  of 
  the 
  ribs 
  in 
  the 
  median 
  region 
  of 
  the 
  fan- 
  

   shaped 
  limb 
  ; 
  n.b., 
  normally 
  orientated 
  vascular 
  .bundle 
  ; 
  i.b., 
  inversely 
  orientated 
  

   bundle 
  ; 
  lac, 
  lacuna. 
  

  

  The 
  distal 
  region 
  of 
  the 
  frond 
  of 
  Spiroclela 
  polyrrhiza, 
  Schleid., 
  the 
  greater 
  

   duckweed, 
  bears 
  a 
  strong 
  general 
  resemblance 
  to 
  that 
  of 
  Pistia, 
  both 
  in 
  form 
  , 
  

   venation, 
  and 
  development 
  of 
  air-tissue. 
  The 
  vascular 
  system 
  is, 
  however, 
  

   much 
  simplified, 
  and 
  only 
  one 
  series 
  of 
  bundles 
  remains. 
  These 
  are 
  normally 
  

   •orientated, 
  but 
  in 
  such 
  a 
  reduced 
  type 
  of 
  leaf, 
  in 
  which, 
  as 
  compared 
  with 
  

   Pistia, 
  several 
  rows 
  of 
  bundles 
  may 
  be 
  supposed 
  to 
  have 
  been 
  lost, 
  it 
  is 
  not 
  

   surprising 
  that 
  the 
  inversely 
  orientated 
  series 
  has 
  failed 
  to 
  survive. 
  There 
  

   seems 
  every 
  reason 
  to 
  interpret 
  the 
  distal 
  part 
  of 
  the 
  frond 
  of 
  the 
  Letnnaceee 
  

   as 
  a 
  petiolar 
  phyllode 
  — 
  exactly 
  equivalent 
  to 
  that 
  of 
  Pistia 
  — 
  especially 
  if, 
  as 
  

   we 
  hope 
  to 
  show, 
  both 
  sheath 
  and 
  ligule 
  of 
  the 
  river 
  lettuce 
  are 
  represented 
  

   in 
  the 
  proximal 
  region 
  of 
  the 
  frond 
  of 
  the 
  greater 
  duckweed. 
  

  

  