﻿APR. 
  19, 
  1922 
  BLAKE: 
  acanthospermum 
  i^rom 
  gai^apagos 
  islands 
  201 
  

  

  forms, 
  although 
  closely 
  related, 
  are 
  not 
  connected 
  by 
  intermediates, 
  

   they 
  are 
  here 
  treated 
  as 
  species. 
  

  

  Since 
  the 
  days 
  of 
  Darwin's 
  voyage 
  on 
  the 
  Beagle, 
  the 
  Galapagos 
  

   Archipelago 
  has 
  been 
  a 
  classical 
  region 
  for 
  the 
  study 
  of 
  the 
  evolution 
  

   of 
  closely 
  allied 
  forms 
  of 
  both 
  plants 
  and 
  animals. 
  The 
  three 
  species 
  of 
  

   Acanthospermum 
  here 
  discussed 
  make 
  an 
  interesting 
  addition 
  to 
  the 
  

   list 
  of 
  plant 
  groups 
  represented 
  on 
  different 
  islands 
  by 
  distinguishable 
  

   forms 
  so 
  closely 
  related 
  that 
  their 
  origin 
  from 
  a 
  common 
  ancestor, 
  

   and 
  presumably 
  at 
  no 
  great 
  distance 
  in 
  the 
  past, 
  is 
  incontestable. 
  

   The 
  abundant 
  material 
  representing 
  two 
  of 
  these 
  forms, 
  moreover, 
  

   affords 
  a 
  basis 
  for 
  a 
  greater 
  degree 
  of 
  assurance 
  as 
  regards 
  their 
  prob- 
  

   able 
  distinctness 
  than 
  has 
  often 
  been 
  the 
  case 
  previously. 
  

  

  As 
  already 
  mentioned, 
  Acanthospermum 
  is 
  closely 
  allied 
  to 
  Melampo- 
  

   dium. 
  Melampodium 
  is 
  an 
  American 
  genus 
  of 
  about 
  43 
  species 
  ranging 
  

   from 
  Kansas 
  to 
  Brazil, 
  and 
  represented 
  by 
  one 
  introduced 
  species 
  in 
  

   the 
  Philippine 
  Islands, 
  but 
  not 
  known 
  from 
  the 
  Galapagos 
  Islands. 
  

   Acanthospermum 
  includes, 
  with 
  the 
  two 
  species 
  here 
  described, 
  ten 
  

   species, 
  native 
  in 
  the 
  West 
  Indies, 
  South 
  America, 
  and 
  the 
  Galapagos 
  

   Islands, 
  and 
  introduced 
  in 
  North 
  and 
  Central 
  America 
  and 
  in 
  the 
  Old 
  

   World. 
  In 
  both 
  genera 
  only 
  the 
  ray 
  flowers 
  are 
  fertile, 
  and 
  each 
  achene 
  

   is 
  closely 
  enveloped 
  and 
  hidden 
  by 
  the 
  corresponding 
  subtending 
  

   phyllary. 
  The 
  compound 
  structures, 
  called 
  "fruits" 
  for 
  the 
  sake 
  of 
  

   brevity, 
  are 
  armed 
  in 
  Acanthospermum 
  with 
  several 
  or 
  many 
  spines 
  or 
  

   hooked 
  prickles. 
  In 
  Melampodium 
  the 
  achene-enclosing, 
  phyllaries 
  

   are 
  smooth 
  or 
  merely 
  tuberculate, 
  and 
  are 
  in 
  one 
  section 
  developed 
  

   at 
  apex 
  into 
  a 
  cup 
  or 
  hood 
  which 
  may 
  be 
  prolonged 
  into 
  a 
  single 
  short 
  

   or 
  long 
  often 
  recurved 
  horn 
  on 
  the 
  apical 
  outer 
  margin. 
  

  

  The 
  most 
  remarkable 
  character 
  of 
  the 
  three 
  species 
  of 
  Acantho- 
  

   spermum 
  here 
  considered 
  is 
  the 
  variability 
  in 
  the 
  armament 
  of 
  the 
  

   fruits, 
  a 
  feature 
  quite 
  without 
  parallel 
  in 
  the 
  other 
  species 
  of 
  the 
  genus. 
  

   In 
  this 
  respect 
  A. 
  leptolobum 
  is 
  by 
  far 
  the 
  most 
  variable. 
  Although 
  

   this 
  species 
  happens 
  to 
  be 
  represented 
  by 
  far 
  more 
  material 
  than 
  the 
  

   others 
  (34 
  sheets, 
  as 
  opposed 
  to 
  8 
  of 
  A. 
  brachyceratum 
  and 
  2 
  of 
  A. 
  

   lecocarpoides) 
  , 
  this 
  cannot 
  be 
  considered 
  the 
  explanation 
  of 
  the 
  varia- 
  

   bility, 
  since 
  the 
  extremes 
  represented 
  in 
  figure 
  1, 
  d-], 
  are 
  sometimes 
  

   found 
  in 
  a 
  single 
  head. 
  Especially 
  noteworthy 
  is 
  the 
  type 
  of 
  fruit 
  

   represented 
  in 
  figure 
  1, 
  /. 
  Technically 
  this 
  fruit 
  by 
  itself 
  would 
  be 
  

   referred 
  to 
  Melampodium. 
  The 
  absence 
  of 
  spines 
  in 
  this 
  type 
  of 
  fruit 
  

   seems 
  to 
  be 
  due 
  to 
  a 
  loss 
  of 
  vigor 
  or 
  nourishment, 
  as 
  indicated 
  by 
  the 
  

   comparatively 
  small 
  size, 
  and 
  not 
  to 
  infertility, 
  for 
  the 
  seed 
  is 
  quite 
  

  

  