﻿96 
  

  

  The 
  Vegetative 
  Morphology 
  of 
  Pistia 
  and 
  the 
  Lemnacece. 
  

   By 
  Agnes 
  Arber, 
  D.Sc, 
  F.L.S., 
  Fellow 
  of 
  Newnham 
  College, 
  Cambridge. 
  

  

  (Communicated 
  by 
  Prof. 
  F. 
  W. 
  Oliver, 
  F.RS. 
  Received 
  June 
  17, 
  1919.) 
  

  

  Introduction. 
  

  

  The 
  nature 
  of 
  the 
  duckweed 
  " 
  frond 
  " 
  has 
  presented 
  a 
  baffling 
  problem 
  to 
  

   botanists 
  since 
  the 
  early 
  days 
  of 
  vegetable 
  morphology. 
  A 
  detailed 
  history 
  

   of 
  the 
  views 
  that 
  have 
  been 
  held 
  on 
  the 
  question 
  need 
  scarcely 
  be 
  attempted 
  

   here, 
  since 
  the 
  extreme 
  reduction 
  of 
  the 
  Lemnaceae 
  has 
  offered 
  scope 
  for 
  wild 
  

   surmises 
  of 
  little 
  scientific 
  value.* 
  The 
  principal 
  theories 
  which 
  have 
  been 
  

   put 
  forward 
  may, 
  however, 
  be 
  summarised 
  as 
  follows 
  : 
  — 
  

  

  The 
  " 
  fronds 
  " 
  of 
  the 
  Lemnaceae 
  have 
  been 
  regarded 
  as 
  : 
  — 
  

  

  (1) 
  Entirely 
  axial 
  (Hegelmaier, 
  F. 
  (1868)). 
  

  

  (2) 
  Entirely 
  foliar 
  (Goebel, 
  K. 
  (1891-3)). 
  

  

  (3) 
  Foliar 
  in 
  the 
  distal 
  region 
  and 
  axial 
  in 
  the 
  proximal 
  region 
  (Horen, 
  

   F. 
  van 
  (1869), 
  Engler, 
  A. 
  (1877), 
  and 
  Velenovsky, 
  J. 
  (1907)). 
  

  

  The 
  objections 
  to 
  (1) 
  and 
  (2) 
  are 
  obvious 
  : 
  these 
  views 
  can 
  only 
  be 
  main- 
  

   tained 
  if 
  — 
  in 
  the 
  first 
  case 
  — 
  certain 
  essentially 
  foliar 
  qualities 
  be 
  attributed 
  

   to 
  stem 
  organs, 
  and 
  — 
  in 
  the 
  second 
  case 
  — 
  if 
  leaves 
  be 
  assumed 
  to 
  possess- 
  

   some 
  of 
  the 
  distinctive 
  properties 
  of 
  stems. 
  It 
  is 
  not 
  inconceivable 
  that 
  such 
  

   assumptions 
  might 
  find 
  justification, 
  but 
  they 
  should 
  only 
  be 
  used 
  as 
  a 
  last 
  

   resort, 
  in 
  the 
  case 
  of 
  no 
  adequate 
  explanation 
  on 
  normal 
  lines 
  being 
  forth- 
  

   coming. 
  

  

  The 
  third 
  view, 
  first 
  suggested 
  by 
  van 
  Horen,f 
  has 
  been 
  placed 
  on 
  a 
  

   thoroughly 
  sound 
  basis 
  by 
  Engler'sJ 
  exhaustive 
  comparative 
  study 
  of 
  the 
  

   morphology 
  of 
  the 
  more 
  typical 
  Aracese 
  and 
  of 
  their 
  relation 
  to 
  Pistia 
  and 
  

   the 
  Lemnaceae. 
  There 
  is 
  no 
  doubt 
  that 
  the 
  comparison 
  of 
  the 
  river 
  lettuce 
  

   {Pistia 
  Stratiotes, 
  L.) 
  and 
  the 
  duckweeds 
  supplies 
  the 
  clue 
  to 
  the 
  problem. 
  

   Engler 
  interprets 
  the 
  region 
  of 
  the 
  frond 
  of 
  the 
  Lemnaceae 
  above 
  the 
  basal 
  

   " 
  pockets 
  " 
  (Taschen) 
  as 
  of 
  foliar 
  nature, 
  and 
  he 
  considers 
  the 
  buds 
  developed 
  

   in 
  these 
  pockets 
  as 
  the 
  equivalents 
  of 
  the 
  lateral 
  shoots 
  of 
  Pistia, 
  the 
  main 
  

   difference 
  being 
  that, 
  in 
  the 
  river 
  lettuce, 
  only 
  one 
  bud 
  is 
  developed 
  in 
  

   connection 
  with 
  each 
  leaf, 
  while 
  in 
  the 
  duckweeds 
  there 
  are 
  two, 
  one 
  on 
  each 
  

   side. 
  The 
  position 
  of 
  the 
  buds, 
  which 
  are 
  lateral 
  in 
  relation 
  to 
  the 
  leaf-limb, 
  

   is 
  the 
  same 
  in 
  both. 
  In 
  the 
  Lemnaceae 
  the 
  axis 
  of 
  the 
  vegetative 
  plant 
  is 
  so 
  

  

  * 
  See, 
  for 
  example, 
  Dutailly, 
  G. 
  (1878). 
  

   t 
  Horen, 
  F. 
  van 
  (1869). 
  

   X 
  Engler, 
  A. 
  (1877). 
  

  

  