﻿102 
  The 
  Vegetative 
  Morphology 
  0/ 
  Pistia 
  and 
  the 
  Lemnacece. 
  

  

  conspicuous 
  axillary 
  structure, 
  as 
  in 
  Pistia, 
  but 
  the 
  two 
  small 
  free 
  flaps, 
  /and 
  ' 
  

   /', 
  may 
  be 
  interpreted 
  as 
  reduced 
  ligules. 
  The 
  basal 
  regions 
  of 
  the 
  two 
  

   sheath- 
  wings 
  each 
  form 
  a 
  bud-containing 
  pocket, 
  p 
  and 
  p', 
  equivalent 
  to 
  the 
  

   one 
  sheath-pocket 
  accompanying 
  each 
  leaf 
  in 
  Pistia. 
  These 
  pockets 
  are 
  

   ultimately 
  open 
  in 
  Spirodela, 
  but 
  in 
  younger 
  stages 
  they 
  are 
  closed 
  in 
  the 
  

   basal 
  region 
  as 
  in 
  Pistia. 
  The 
  development 
  of 
  the 
  buds 
  in 
  pockets 
  between 
  

   the 
  sheath 
  and 
  axis, 
  below 
  the 
  limb 
  and 
  occupying 
  a 
  lateral 
  position 
  with 
  

   regard 
  to 
  it, 
  is 
  thus 
  identical 
  in 
  Pistia 
  and 
  the 
  Lemnaceae. 
  

  

  Summary. 
  

  

  Anatomical 
  examination 
  of 
  the 
  " 
  limb 
  " 
  of 
  the 
  leaf 
  of 
  Pistia 
  Stratiotes, 
  L., 
  

   the 
  river 
  lettuce, 
  shows 
  that, 
  in 
  addition 
  to 
  normally 
  orientated 
  vascular 
  

   bundles, 
  there 
  is 
  a 
  series 
  of 
  inverted 
  bundles 
  towards 
  the 
  upper 
  surface. 
  This 
  

   fact 
  is 
  regarded 
  as 
  indicating 
  that 
  the 
  leaf 
  is 
  of 
  the 
  nature 
  of 
  a 
  petiolar 
  

   phyllode. 
  This 
  interpretation 
  is 
  extended 
  to 
  the 
  distal 
  part 
  of 
  the 
  frond 
  of 
  

   the 
  Lemnaceae 
  (duckweeds). 
  

  

  The 
  general 
  view, 
  put 
  forward 
  by 
  Engler 
  40 
  years 
  ago, 
  as 
  to 
  the 
  morpho- 
  

   logical 
  relation 
  of 
  the 
  Araeese 
  — 
  through 
  Pistia 
  — 
  to 
  the 
  Lemnaceae, 
  is 
  accepted 
  

   in 
  the 
  present 
  paper, 
  and 
  it 
  is 
  shown 
  that 
  more 
  detailed 
  investigation 
  by 
  

   modern 
  methods 
  makes 
  it 
  possible 
  to 
  carry 
  the 
  comparison 
  considerably 
  

   further. 
  Serial 
  sections 
  through 
  a 
  developing 
  shoot 
  of 
  Pistia 
  reveal 
  the 
  

   presence 
  of 
  a 
  " 
  pocket 
  " 
  in 
  connection 
  with 
  each 
  leaf, 
  occurring 
  below 
  the 
  

   level 
  of 
  the 
  free 
  part 
  of 
  the 
  limb 
  ; 
  this 
  pocket 
  is 
  formed 
  on 
  one 
  side 
  by 
  the 
  

   leaf-sheath, 
  and 
  on 
  the 
  other 
  side 
  by 
  the 
  axis, 
  and 
  encloses 
  a 
  bud 
  occupying 
  a 
  

   lateral 
  position 
  in 
  relation 
  to 
  the 
  limb 
  of 
  the 
  leaf. 
  It 
  is 
  shown 
  that 
  these 
  

   pockets 
  are 
  exactly 
  equivalent 
  to 
  the 
  pockets 
  at 
  the 
  base 
  of 
  the 
  frond 
  in 
  the 
  

   Lemnaceae, 
  which,, 
  in 
  the 
  case 
  of 
  Spirodela, 
  are 
  easily 
  seen 
  to 
  be 
  formed, 
  on 
  the 
  

   lower 
  side, 
  by 
  the 
  wings 
  of 
  the 
  leaf-sheath, 
  terminating 
  in 
  two 
  minute 
  ligular 
  

   flaps, 
  and, 
  on 
  the 
  upper 
  side, 
  by 
  the 
  axis. 
  

  

  A 
  cknowledgm 
  ents. 
  

  

  I 
  have 
  to 
  acknowledge 
  a 
  grant 
  from 
  the 
  Dixon 
  Fund 
  of 
  the 
  University 
  of 
  

   London 
  in 
  aid 
  of 
  this 
  and 
  other 
  researches 
  carried 
  out 
  at 
  the 
  Balfour 
  

   Laboratory, 
  Cambridge. 
  

  

  LIST 
  OF 
  MEMOIES 
  CITED. 
  

   Arber, 
  A. 
  (1918). 
  "The 
  Phyllode 
  Theory 
  of 
  the 
  Monocotyledonous 
  Leaf, 
  with 
  Special 
  

   Eeference 
  to 
  Anatomical 
  Evidence," 
  'Ann. 
  Bot.,' 
  vol. 
  32, 
  pp. 
  466-501, 
  32 
  text- 
  

   figs. 
  (1918). 
  

  

  Domin, 
  K. 
  (1911). 
  " 
  Morphologische 
  und 
  phylogenetische 
  Studien 
  uber 
  die 
  Stipular- 
  

   bildungen," 
  'Ann. 
  Jardin 
  Bot. 
  de 
  Buitenzorg,' 
  vol. 
  24 
  (2nd 
  Ser., 
  vol. 
  9), 
  

   pp. 
  117-326, 
  11 
  plates 
  (1911). 
  

  

  