﻿216 
  

  

  A 
  Preliminary 
  Account 
  of 
  the 
  Meiotic 
  Phenomena 
  in 
  the 
  Pollen 
  

   Mother-Cells 
  and 
  Tapetum 
  of 
  Lettuce 
  (Lactuca 
  sativa). 
  

  

  By 
  E. 
  Euggles 
  Gates, 
  Ph.D., 
  F.L.S., 
  Eeader 
  in 
  Botany, 
  University 
  of 
  London 
  

  

  (King's 
  College). 
  

  

  (Communicated 
  by 
  Prof. 
  J. 
  B. 
  Farmer, 
  F.E.S. 
  Eeceived 
  March 
  1, 
  1920.) 
  

  

  On 
  examining 
  some 
  lettuces 
  which 
  Prof. 
  William 
  Bateson, 
  F.E.S., 
  was 
  

   growing 
  in 
  his 
  experimental 
  grounds 
  at 
  Merton, 
  I 
  was 
  struck 
  by 
  the 
  differences 
  

   between 
  a 
  certain 
  Cos 
  lettuce 
  (Sutton's 
  "Dwarf 
  Perfection") 
  and 
  a 
  rogue 
  

   from 
  it 
  which 
  showed 
  some 
  resemblances 
  in 
  leaf 
  characters 
  to 
  a 
  cabbage 
  

   lettuce. 
  Prof. 
  Bateson 
  kindly 
  permitted 
  me 
  to 
  collect 
  some 
  cytological 
  

   material 
  of 
  the 
  variety 
  and 
  its 
  rogue, 
  and 
  a 
  subsequent 
  study 
  of 
  the 
  pollen 
  

   development, 
  particularly 
  in 
  the 
  variety, 
  has 
  developed 
  several 
  points 
  of 
  so 
  

   much 
  general 
  interest 
  that 
  a 
  preliminary 
  account 
  of 
  certain 
  features 
  was 
  

   deemed 
  desirable. 
  So 
  far 
  as 
  the 
  work 
  has 
  progressed 
  no 
  constant 
  difference 
  

   between 
  the 
  variety 
  and 
  its 
  rogue 
  has 
  yet 
  appeared, 
  the 
  chromosome 
  number 
  

   being 
  the 
  same 
  in 
  both. 
  The 
  present 
  account, 
  so 
  far 
  as 
  known, 
  applies 
  equally 
  

   to 
  the 
  variety 
  and 
  the 
  rogue. 
  Only 
  the 
  main 
  points 
  of 
  special 
  interest 
  in 
  the 
  

   meiotic 
  history 
  will 
  be 
  touched 
  upon 
  in 
  this 
  communication. 
  

  

  The 
  material 
  was 
  fixed 
  in 
  various 
  chrom-acetic 
  acid 
  solutions, 
  some 
  of 
  

   which 
  gave 
  very 
  good 
  fixation, 
  and 
  the 
  sections 
  were 
  chiefly 
  stained 
  with 
  

   Heidenhain's 
  iron-alum 
  hematoxylin. 
  In 
  the 
  resulting 
  preparations 
  the 
  

   structural 
  features 
  both 
  of 
  the 
  chromatin 
  and 
  the 
  cytoplasm 
  were 
  beautifully 
  

   clear, 
  the 
  presence 
  of 
  latex 
  tubes 
  causing 
  no 
  difficulties 
  and 
  requiring 
  no 
  

   special 
  treatment. 
  

  

  One 
  of 
  the 
  striking 
  features 
  first 
  observed 
  was 
  the 
  variety 
  of 
  conditions 
  in 
  

   the 
  tapetum, 
  all 
  transitions 
  between 
  pollen 
  mother-cells 
  and 
  tapetal 
  cells 
  being- 
  

   found. 
  This 
  was 
  all 
  the 
  more 
  unexpected 
  as 
  occurring 
  in 
  a 
  composite, 
  and 
  

   since 
  the 
  Composite 
  stand 
  high 
  among 
  flowering 
  plants 
  the 
  condition 
  can 
  

   scarcely 
  be 
  regarded 
  as 
  a 
  primitive 
  one. 
  The 
  tapetal 
  cells 
  frequently 
  become 
  

   greatly 
  elongated 
  and 
  are 
  in 
  some 
  cases 
  very 
  narrow, 
  but 
  they 
  lie 
  almost 
  

   invariably 
  with 
  their 
  long 
  axes 
  lengthwise 
  of 
  the 
  loculus. 
  They 
  become 
  

   binucleate 
  by 
  a 
  mitotic 
  division 
  of 
  the 
  nucleus 
  about 
  the 
  time 
  of 
  synapsis 
  in 
  

   the 
  pollen 
  mother-cells, 
  and 
  many 
  of 
  them 
  later 
  become 
  tetranucleate 
  by 
  

   another 
  mitosis. 
  

  

  These 
  tapetal 
  cells 
  vary 
  enormously 
  in 
  size, 
  both 
  in 
  the 
  binucleate 
  and 
  the 
  

   tetranucleate 
  conditions. 
  The 
  chromatin 
  of 
  the 
  nuclei, 
  particularly 
  in 
  the 
  

   binucleate 
  cells, 
  in 
  certain 
  cases 
  shows 
  appearances 
  which 
  are 
  indistin- 
  

  

  