﻿The 
  Vegetative 
  Morphology 
  o/ 
  Pistia 
  and 
  the 
  Lemnacece. 
  99 
  

  

  When 
  we 
  turn 
  to 
  Engler's 
  figures 
  and 
  description 
  of 
  the 
  Pistia 
  shoot, 
  we 
  

   find 
  no 
  indication 
  of 
  anything 
  corresponding 
  to 
  the 
  " 
  pockets 
  " 
  of 
  the 
  

   Lemnacese, 
  while 
  the 
  buds 
  are 
  represented 
  as 
  situated 
  between 
  the 
  ligule 
  and 
  

   the 
  limb 
  of 
  the 
  leaf. 
  No 
  drawings 
  are 
  given 
  showing 
  the 
  actual 
  origin 
  of 
  the 
  

   buds 
  from 
  the 
  axis, 
  the 
  diagrammatic 
  transverse 
  sections 
  of 
  shoots 
  being 
  all 
  

   taken 
  above 
  the 
  level 
  of 
  the 
  apex 
  of 
  the 
  axis. 
  Engler's 
  paper 
  was 
  published 
  

   in 
  the 
  seventies 
  of 
  the 
  last 
  century, 
  that 
  is 
  to 
  say 
  before 
  the 
  microtome 
  had 
  

   become 
  one 
  of 
  the 
  recognised 
  instruments 
  of 
  botanical 
  research. 
  Now 
  it 
  

   happens 
  that 
  the 
  extreme 
  abbreviation 
  of 
  the 
  axis 
  in 
  Pistia 
  makes 
  it 
  almost 
  

   impossible 
  to 
  follow 
  the 
  detailed 
  relations 
  of 
  the 
  parts 
  of 
  the 
  shoot 
  without 
  

   the 
  help 
  of 
  serial 
  sections 
  ; 
  this 
  difficulty 
  is 
  enhanced 
  by 
  the 
  fact 
  that 
  the 
  

   levels 
  at 
  which 
  the 
  leaves 
  become 
  free 
  from 
  the 
  axis 
  are 
  not 
  consistent 
  with 
  

   their 
  actual 
  order 
  of 
  development 
  (cf. 
  figs. 
  3-7). 
  It 
  is 
  probably 
  for 
  these 
  

   reasons 
  that 
  my 
  re-examination 
  of 
  Pistia 
  has 
  led 
  to 
  somewhat 
  different 
  results 
  

   from 
  those 
  of 
  Engler 
  — 
  results 
  which 
  serve, 
  however, 
  to 
  corroborate 
  and 
  extend 
  

   his 
  general 
  view 
  regarding 
  the 
  relation 
  of 
  Pistia 
  to 
  the 
  Lemnacese. 
  

  

  It 
  is 
  not 
  easy 
  in 
  this 
  country 
  .to 
  obtain 
  supplies 
  of 
  Pistia 
  plants 
  with 
  

   actively 
  growing 
  axillary 
  shoots 
  suitable 
  for 
  microtoming, 
  and 
  the 
  present 
  

   account 
  is 
  based 
  upon 
  material 
  in 
  which 
  only 
  two 
  buds 
  proved 
  perfectly 
  

   adapted 
  for 
  the 
  purpose. 
  Fortunately 
  one 
  of 
  these, 
  which 
  was 
  entirely 
  

   vegetative, 
  displayed 
  the 
  general 
  structure 
  with 
  great 
  clearness. 
  I 
  am 
  

   indebted 
  to 
  the 
  Director 
  of 
  the 
  Royal 
  Botanic 
  Gardens, 
  Kew, 
  for 
  living 
  

   plants, 
  and 
  to 
  Prof. 
  A. 
  C. 
  Seward, 
  F.R.S., 
  for 
  herbarium 
  material. 
  

  

  The 
  axis 
  of 
  the 
  developing 
  bud 
  of 
  Pistia, 
  which 
  is 
  illustrated 
  by 
  the 
  

   transverse 
  sections 
  shown 
  in 
  figs. 
  3-7, 
  bore, 
  firstly, 
  a 
  zone 
  of 
  adventitious 
  

   roots, 
  r, 
  in 
  fig. 
  3, 
  immediately 
  followed 
  by 
  a 
  sheathing 
  scale-leaf, 
  l 
  x 
  . 
  This 
  

   was 
  not 
  attached 
  to 
  the 
  axis 
  in 
  the 
  median 
  line, 
  but 
  considerably 
  to 
  one 
  

   side, 
  thus 
  showing 
  an 
  asymmetry 
  which 
  was 
  also 
  displayed 
  by 
  the 
  sheathing 
  

   region 
  of 
  the 
  normal 
  leaves. 
  The 
  broader 
  free 
  segment 
  was 
  again 
  fused 
  

   with 
  the 
  axis 
  at 
  a 
  second 
  point 
  of 
  the 
  circumference, 
  thus 
  forming 
  a 
  small 
  

   pocket 
  (hp.). 
  Within 
  this 
  pocket 
  a 
  bud, 
  bi, 
  was 
  developed 
  from 
  the 
  

   axis. 
  The 
  second 
  leaf, 
  h, 
  which 
  was 
  a 
  normal 
  foliage 
  leaf, 
  behaved 
  in 
  an 
  

   essentially 
  similar 
  manner, 
  with 
  certain 
  modifications 
  due 
  to 
  the 
  possession 
  

   of 
  a 
  differentiated 
  limb. 
  Below 
  the 
  level 
  at 
  which 
  the 
  limb 
  became 
  free 
  from 
  

   the 
  axis, 
  there 
  was 
  a 
  distinct 
  sheath 
  forming 
  a 
  free 
  flap, 
  S2, 
  on 
  one 
  side, 
  while 
  

   on 
  the 
  other 
  side 
  it 
  was 
  free 
  for 
  a 
  short 
  distance 
  laterally 
  and 
  then 
  again 
  

   attached 
  to 
  the 
  axis, 
  thus 
  forming 
  a 
  pocket 
  (s%p.) 
  containing 
  a 
  bud, 
  b 
  2 
  . 
  

   Higher 
  up, 
  the 
  margin 
  of 
  the 
  pocket 
  became 
  free 
  like 
  the 
  opposite 
  sheath- 
  

   wing 
  (cf. 
  figs. 
  5 
  and 
  6), 
  and, 
  higher 
  still, 
  this 
  sheath 
  detached 
  itself 
  entirely 
  

   from 
  the 
  limb, 
  and 
  the 
  two 
  wings 
  fused 
  into 
  a 
  free 
  axillary 
  ligule 
  (fig. 
  7). 
  

  

  VOL. 
  XCI. 
  — 
  B. 
  H 
  

  

  