﻿Setchell: 
  The 
  marine 
  sperm 
  atophytes 
  5C9 
  

  

  that 
  the 
  isocryme, 
  or 
  line 
  of 
  mean 
  minimum 
  temperature 
  for 
  

   2^^ 
  C, 
  touches 
  the 
  coast 
  of 
  eastern 
  Australia 
  in 
  the 
  vicinity 
  of 
  

   Cape 
  York. 
  In 
  the 
  cases 
  of 
  the 
  five 
  species 
  which 
  are 
  credited 
  to 
  

   both 
  a 
  subtropical 
  and 
  an 
  adjacent 
  temperate 
  zone, 
  two 
  of 
  these 
  

   species, 
  viz., 
  those 
  belonging 
  to 
  the 
  genus 
  Phyllospadix, 
  are 
  not 
  

  

  4 
  

  

  certainly 
  members 
  of 
  the 
  flora 
  of 
  the 
  subtropical 
  zone 
  and 
  also 
  

   are 
  inhabitants 
  of 
  the 
  littoral 
  belt 
  where 
  temperature 
  relations 
  

  

  ■ 
  # 
  

  

  are 
  complicated 
  by 
  the 
  influence 
  of 
  air 
  temperatures 
  and 
  consider- 
  

   able 
  insolation 
  at 
  periods 
  of 
  low 
  water. 
  It 
  may 
  be 
  for 
  these 
  

   reasons 
  that 
  these 
  two 
  species 
  also 
  extend 
  into 
  the 
  Lower 
  Boreal 
  

   Zone 
  as 
  is 
  indicated 
  by 
  some 
  statements 
  as 
  to 
  their 
  distribution. 
  

   Two 
  other 
  of 
  the 
  two-zoned 
  species 
  are 
  mud 
  and 
  shallow-water 
  

   species 
  and 
  probably 
  are 
  normally 
  subtropical, 
  extending 
  over 
  into 
  

   spots 
  in 
  the 
  temperate 
  zone 
  where 
  the 
  combination 
  of 
  shallow 
  

   quiet 
  waters 
  and 
  higher 
  air 
  temperature 
  allows 
  the 
  air 
  and 
  the 
  sun 
  

   to 
  raise 
  the 
  temperature 
  of 
  the 
  water 
  some 
  5° 
  C* 
  or 
  even 
  more, 
  

   for 
  a 
  sufficient 
  length 
  of 
  time 
  each 
  season 
  or, 
  at 
  least 
  in 
  an 
  occa- 
  

   sional 
  season 
  of 
  fairly 
  frequent 
  occurrence. 
  The 
  two 
  species 
  just 
  

   referred 
  to 
  are 
  Zostera 
  nana 
  Roth 
  and 
  Z. 
  pacifica 
  S. 
  Watson. 
  

   It 
  may 
  also 
  be 
  mentioned 
  here 
  that 
  it 
  is 
  by 
  no 
  means 
  sufficiently 
  

   certain 
  as 
  to 
  the 
  exact 
  limits 
  of 
  the 
  distribution 
  of 
  either 
  of 
  these 
  

   two 
  species- 
  The 
  fifth 
  species 
  of 
  this 
  two-zoned 
  group 
  is 
  Cytno- 
  

   docea 
  antarctica 
  (Labill.) 
  Endl. 
  This 
  species 
  occurs 
  on 
  the 
  western 
  

   as 
  w^ell 
  as 
  on 
  the 
  southern 
  coasts 
  of 
  Australia 
  and 
  on 
  the 
  northern 
  

   shores 
  of 
  Tasmania. 
  This 
  seems 
  likely, 
  for 
  similar 
  reasons 
  to 
  

   those 
  advanced 
  in 
  the 
  case 
  of 
  Zostera 
  capricorni, 
  to 
  be 
  a 
  case 
  of 
  

   seasonal 
  invasion 
  of 
  the 
  South 
  Subtropical 
  Zone 
  from 
  the 
  South 
  

   Temperate 
  Zone. 
  It 
  may 
  be, 
  however, 
  an 
  invasion 
  in 
  just 
  the 
  

   opposite 
  direction, 
  viz., 
  from 
  the 
  South 
  Subtropical 
  to 
  the 
  South 
  

   Temperate 
  Zone, 
  since 
  according 
  to 
  Tepper 
  (1881) 
  the 
  South 
  

   Temperate 
  plants 
  increase 
  only 
  by 
  a 
  peculiar 
  and 
  specialized 
  

   vegetative 
  splitting 
  oflF 
  of 
  buds 
  or 
  short 
  branchlets. 
  

  

  The 
  cases 
  of 
  species 
  credited 
  to 
  three 
  or 
  more 
  zones 
  are 
  more 
  

   complex 
  if 
  we 
  undertake 
  to 
  show 
  that, 
  in 
  spite 
  of 
  this 
  extended 
  

   distribution, 
  they 
  are 
  to 
  be 
  regarded 
  as 
  steno- 
  rather 
  than 
  eury- 
  

   thermal. 
  There 
  are 
  two 
  of 
  these, 
  at 
  least 
  at 
  present, 
  considered 
  

   to 
  be 
  of 
  very 
  extensive 
  distribution, 
  viz., 
  Ilalophila 
  ovalis 
  (R. 
  Br.) 
  

   Hook. 
  f. 
  and 
  Zostera 
  marina 
  L. 
  The 
  first 
  seems 
  to 
  center 
  in 
  the 
  

  

  