﻿The 
  Vegetative 
  Morphology 
  o/" 
  Pistia 
  and 
  the 
  Lemnacea. 
  97 
  

  

  far 
  reduced 
  from 
  the 
  Pistia 
  type 
  as 
  to 
  bear 
  only 
  one 
  leaf 
  in 
  a 
  pseudo- 
  

   terminal 
  position, 
  with 
  two 
  lateral 
  buds 
  ; 
  while 
  in 
  Pistia 
  the 
  stem 
  bears 
  a 
  

   rosette 
  of 
  numerous 
  closely-placed 
  leaves 
  accompanied 
  by 
  a 
  series 
  of 
  lateral 
  

   shoots. 
  

  

  The 
  soundness 
  of 
  Engler's 
  general 
  position 
  in 
  regard 
  to 
  Pistia 
  and 
  the 
  

   Lemnaceae 
  can 
  scarcely 
  be 
  doubted 
  by 
  any 
  botanist 
  who 
  studies 
  his 
  memoir 
  

   on 
  the 
  Araceae. 
  There 
  remain, 
  however, 
  two 
  questions, 
  to 
  which 
  he 
  does 
  not 
  

   allude, 
  but 
  which 
  seem 
  to 
  me 
  to 
  require 
  answers. 
  These 
  questions 
  are 
  : 
  — 
  

  

  (1) 
  What 
  region 
  of 
  the 
  leaf 
  do 
  the 
  " 
  limb 
  " 
  of 
  Pistia 
  and 
  the 
  distal 
  part 
  of 
  

   the 
  " 
  frond 
  " 
  of 
  the 
  Lemnaceae 
  represent 
  ? 
  

  

  (2) 
  What 
  is 
  the 
  exact 
  morphological 
  nature 
  of 
  the 
  " 
  pockets 
  " 
  of 
  the- 
  

   Lemnaceae, 
  and 
  what 
  is 
  their 
  equivalent 
  — 
  if 
  any 
  — 
  in 
  the 
  Pistia 
  shoot 
  ? 
  

  

  The 
  present 
  paper 
  is 
  an 
  attempt 
  to 
  find 
  some 
  solution 
  of 
  these 
  two 
  

   problems. 
  

  

  The 
  Vegetative 
  Morphology 
  of 
  Pistia 
  Stratiotes, 
  L. 
  

  

  The 
  main 
  part 
  of 
  the 
  leaf 
  of 
  Pistia 
  consists 
  of 
  a 
  limb 
  which 
  is 
  ob-cuneate 
  

  

  or 
  fan-shaped, 
  with 
  parallel 
  veins, 
  deeply 
  grooved, 
  and, 
  especially 
  in 
  young 
  

  

  stages, 
  densely 
  hairy. 
  It 
  is 
  often 
  much 
  swollen 
  — 
  particularly 
  in 
  the 
  median 
  

  

  basal 
  region 
  — 
  with 
  air-containing 
  tissue. 
  A 
  ligule, 
  which 
  is 
  sheathing 
  in 
  its 
  

  

  upper 
  portion, 
  occurs 
  between 
  the 
  " 
  limb 
  " 
  and 
  the 
  axis 
  at 
  the 
  extreme 
  base 
  of 
  

  

  the 
  former.* 
  This 
  makes 
  it 
  obvious 
  that 
  the 
  " 
  limb 
  " 
  cannot 
  be 
  interpreted 
  

  

  as 
  of 
  the 
  nature 
  of 
  a 
  leaf 
  -sheath, 
  and 
  we 
  are 
  left 
  with 
  the 
  alternatives 
  that 
  it 
  

  

  may 
  represent 
  petiole, 
  or 
  lamina, 
  or 
  both. 
  On 
  the 
  phyllode 
  theory 
  of 
  the 
  

  

  Monocotyledonous 
  leaf, 
  which 
  I 
  have 
  advocated 
  in 
  a 
  previous 
  paper,f 
  we 
  

  

  should 
  expect 
  the 
  lamina 
  to 
  be 
  absent, 
  while 
  the 
  limb 
  would 
  be 
  of 
  the 
  nature 
  

  

  of 
  a 
  flattened 
  petiole 
  — 
  and 
  there 
  is 
  nothing 
  in 
  its 
  form 
  and 
  venation 
  to 
  make 
  

  

  this 
  view 
  untenable. 
  But 
  for 
  positive 
  evidence 
  we 
  must 
  look 
  to 
  the 
  internal 
  

  

  structure. 
  Attention 
  was 
  drawn 
  some 
  years 
  ago 
  by 
  a 
  Japanese 
  botanist* 
  to 
  

  

  certain 
  features 
  of 
  the 
  leaf 
  anatomy, 
  but 
  the 
  peculiar 
  arrangement 
  of 
  the 
  

  

  vascular 
  bundles 
  appears 
  to 
  have 
  been 
  hitherto 
  overlooked. 
  I 
  have 
  found, 
  on 
  

  

  cutting 
  transverse 
  sections, 
  that, 
  in 
  addition 
  to 
  more 
  than 
  one 
  row 
  of 
  normally 
  

  

  orientated 
  strands 
  (n.b., 
  in 
  figs. 
  1 
  and 
  2), 
  there 
  is, 
  towards 
  the 
  upper 
  surface 
  of 
  

  

  the 
  leaf, 
  a 
  series 
  of 
  bundles 
  with 
  inverted 
  orientation, 
  i. 
  b. 
  In 
  the 
  paper 
  

  

  already 
  cited 
  I 
  have 
  given 
  reasons 
  for 
  regarding 
  the 
  presence 
  in 
  a 
  leaf 
  of 
  such 
  

  

  inverted 
  bundles 
  as 
  an 
  indication 
  of 
  petiolar 
  origin. 
  We 
  may 
  thus 
  conclude 
  

  

  that 
  the 
  relation 
  of 
  the 
  leaf-limb 
  of 
  Pistia 
  Stratiotes 
  to 
  its 
  ligule, 
  together 
  

  

  with 
  the 
  evidence 
  of 
  its 
  form, 
  venation, 
  and 
  vascular 
  anatomy, 
  appear 
  to 
  

  

  * 
  Domin, 
  K. 
  (1911). 
  

   + 
  Arber, 
  A. 
  (1918). 
  

   } 
  Ito, 
  T. 
  (1899). 
  

  

  