﻿Yol. 
  53.] 
  ME. 
  A. 
  C. 
  SEWARD 
  ON 
  CYCADEOIDEA 
  GIGANTEA. 
  33 
  

  

  to 
  the 
  hypodermal 
  sclerenchyma 
  ; 
  in 
  Macrozcvmia 
  Denisoni, 
  imme- 
  

   diately 
  underneath 
  the 
  epidermis. 
  Some 
  of 
  the 
  petioles 
  seen 
  on 
  the 
  

   polished 
  tangential 
  surface 
  of 
  a 
  large 
  block 
  of 
  Bennettites 
  Gibson- 
  

   ianus 
  in 
  the 
  British 
  Museum 
  possess 
  a 
  peripheral 
  band 
  of 
  darker 
  

   tissue, 
  which 
  is 
  probably 
  periderm. 
  In 
  the 
  leaf-bases 
  of 
  B. 
  Gibson- 
  

   ianus, 
  in 
  the 
  immediate 
  neighbourhood 
  of 
  the 
  lateral 
  inflorescences, 
  

   there 
  is 
  no 
  cork 
  or 
  sclerenchyma 
  ; 
  in 
  such 
  cases 
  the 
  petioles 
  would 
  

   retain 
  the 
  power 
  of 
  further 
  growth 
  by 
  cell-division. 
  

  

  The 
  identity 
  of 
  the 
  ramenta 
  with 
  those 
  of 
  Bennettites 
  has 
  already 
  

   l)een 
  emphasized, 
  also 
  the 
  apparently 
  close 
  resemblance 
  between 
  the 
  

   vascular 
  bundles. 
  Without 
  discussing 
  at 
  length 
  the 
  comparative 
  

   structure 
  of 
  the 
  vascular 
  bundles 
  of 
  recent 
  and 
  fossil 
  Cycadean 
  

   petioles, 
  it 
  is 
  advisable 
  to 
  draw 
  attention 
  to 
  one 
  or 
  two 
  important 
  

   features. 
  ■ 
  In 
  a 
  transverse 
  section 
  of 
  a 
  Cycadean 
  leaf-stalk, 
  we 
  find 
  

   the 
  vascular 
  bundles 
  are 
  distinctly 
  collateral 
  in 
  form, 
  with 
  the 
  proto- 
  

   xylem 
  elements 
  on 
  that 
  side 
  of 
  the 
  xylem 
  next 
  the 
  phloem. 
  The 
  

   main 
  portion 
  of 
  the 
  xylem 
  consists 
  of 
  tracheids 
  internal 
  to 
  the 
  proto- 
  

   xylem, 
  which 
  have 
  been 
  developed 
  centripetally 
  ; 
  external 
  to 
  the 
  

   protoxylem, 
  however, 
  there 
  occur 
  a 
  few 
  isolated 
  tracheids, 
  or 
  in 
  

   some 
  cases 
  a 
  fairly 
  thick, 
  crescent-shaped 
  band, 
  developed 
  in 
  a 
  

   centrifugal 
  direction. 
  1 
  Bundles 
  in 
  which 
  the 
  primary 
  xylem 
  is 
  both 
  

   centripetal 
  and 
  centrifugal 
  have 
  been 
  termed 
  mesarch. 
  The 
  nature 
  

   of 
  such 
  bundles 
  has 
  been 
  discussed 
  by 
  Solms-Laubach 
  in 
  his 
  

   ' 
  Fossil 
  Botany' 
  (p. 
  257), 
  and 
  more 
  recently 
  by 
  Williamson 
  and 
  

   Scott 
  (Phil. 
  Trans, 
  vol. 
  clxxxvi. 
  1895, 
  B, 
  p. 
  713). 
  If 
  we 
  examine 
  a 
  

   petiolar 
  vascular 
  bundle 
  in 
  the 
  basal 
  portion 
  of 
  a 
  frond-axis, 
  the 
  

   centripetal 
  portion 
  of 
  the 
  xylem 
  is 
  found 
  to 
  be 
  much 
  more 
  con- 
  

   spicuous 
  than 
  in 
  the 
  upper 
  part 
  of 
  the 
  petiole, 
  and 
  composed 
  of 
  

   radially-arranged 
  tracheids. 
  This 
  is 
  seen 
  to 
  be 
  the 
  case 
  in 
  Macro- 
  

   zamia 
  Denisoni, 
  and 
  Mettenius 
  figures 
  a 
  good 
  example 
  of 
  such 
  a 
  

   bundle 
  in 
  the 
  petiole 
  of 
  Cycas 
  revoluta. 
  2 
  A 
  comparison, 
  therefore, 
  

   of 
  the 
  fossil 
  vascular 
  bundles 
  with 
  those 
  in 
  the 
  basal 
  portions 
  of 
  

   recent 
  petioles 
  shows 
  a 
  fairly 
  close 
  agreement. 
  There 
  is 
  still 
  need, 
  

   however, 
  of 
  a 
  more 
  accurate 
  knowledge 
  as 
  to 
  the 
  anatomy 
  of 
  the 
  

   recent 
  Cycadean 
  vascular 
  bundle 
  and 
  its 
  exact 
  relation 
  to 
  that 
  of 
  

   Bennettites 
  and 
  other 
  fossils. 
  The 
  chief 
  difference 
  between 
  the 
  

   fossil 
  and 
  recent 
  bundles 
  seems 
  to 
  consist 
  in 
  the 
  more 
  considerable 
  

   development 
  in 
  the 
  former 
  of 
  the 
  centrifugal 
  portion 
  of 
  the 
  xylem. 
  

  

  The 
  genus 
  Bennettites 
  has 
  frequently 
  been 
  referred 
  to 
  in 
  the 
  

   description 
  of 
  the 
  Portland 
  stem 
  ; 
  this 
  generic 
  name 
  was 
  proposed 
  by 
  

   Carruthersin 
  1870 
  for 
  stems 
  possessing 
  the 
  following 
  characters 
  : 
  — 
  

   * 
  Trunk 
  ovoid, 
  in 
  transverse 
  section 
  elliptical, 
  covered 
  with 
  the 
  some- 
  

   what 
  long 
  permanent 
  bases 
  of 
  the 
  petioles. 
  Medulla 
  entirely 
  cellular, 
  

   with 
  numerous 
  gum-canals. 
  Wood 
  consisting 
  of 
  a 
  thin 
  interrupted 
  

  

  1 
  These 
  centrifugal 
  tracheids 
  are 
  shown 
  in 
  a 
  figure 
  of 
  Cycas 
  revoluta, 
  in 
  De 
  

   Bary's 
  ' 
  Comparative 
  Anatomy' 
  (1884), 
  p. 
  336. 
  See 
  also 
  Mettenius, 
  'Beitrage 
  

   zur 
  Anatomie 
  der 
  Cycadeen,' 
  Abh. 
  k. 
  sachs. 
  Gesellsch. 
  Wissensch. 
  1860 
  ; 
  Kraus, 
  

   Priugsh. 
  Jahrb. 
  vol. 
  iv. 
  1865-66, 
  etc. 
  The 
  petioles 
  of 
  Cycas 
  media, 
  Br., 
  and 
  

   Dioon 
  edule, 
  show 
  a 
  larger 
  development 
  of 
  these 
  centrifugal 
  tracheids 
  than 
  is 
  

   found 
  in 
  some 
  other 
  species. 
  2 
  Mettenius, 
  op. 
  supra 
  cit. 
  pi. 
  i. 
  fig. 
  7 
  (d). 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  209. 
  d 
  

  

  