﻿RELATION 
  OF 
  AQUATIC 
  PLANTS 
  TO 
  SUBSTRATUM. 
  487 
  

  

  relative!}^ 
  large 
  volume 
  of 
  oxygen 
  present 
  in 
  the 
  deeper 
  water 
  of 
  lakes 
  

   (about 
  7.6 
  c. 
  c, 
  per 
  liter 
  of 
  water). 
  He 
  thinks 
  it 
  probable 
  that 
  the 
  

   migrations 
  of 
  animals 
  from 
  the 
  superficial 
  water 
  toward 
  the 
  bottom 
  

   and 
  back 
  again 
  aid 
  diffusion 
  by 
  mechanicall}^ 
  mixing 
  the 
  water, 
  thus 
  

   maintaining 
  the 
  oxygen-content 
  of 
  its 
  deeper 
  laj-er. 
  He 
  has 
  found 
  

   that 
  the 
  percentage 
  of 
  oxygen 
  at 
  a 
  depth 
  of 
  2^5 
  meters 
  in 
  Lake 
  Con- 
  

   stance 
  is 
  6.68 
  c. 
  c. 
  per 
  liter 
  and 
  has 
  shown 
  by 
  experiment 
  (1896, 
  p. 
  17) 
  

   that 
  a 
  content 
  of 
  3.3 
  c. 
  c. 
  per 
  liter 
  is, 
  if 
  continuousl}^ 
  maintained, 
  

   more 
  than 
  sufficient 
  for 
  the 
  support 
  of 
  sensitive 
  fishes, 
  such 
  as 
  trout. 
  

   To 
  what 
  extent 
  this 
  oxygen 
  of 
  the 
  deeper 
  layers 
  of 
  water 
  owes 
  its 
  

   origin 
  to 
  plants 
  of 
  any 
  sort 
  is 
  not 
  known, 
  but 
  there 
  is 
  no 
  reason 
  to 
  

   believe 
  that 
  an}^ 
  appreciable 
  part 
  of 
  it 
  is 
  due 
  to 
  the 
  larger 
  rooted 
  plants 
  

   of 
  the 
  shore 
  region. 
  Hoppe-Seyler 
  does 
  not 
  attempt 
  to 
  account 
  for 
  

   its 
  presence. 
  It 
  is 
  quite 
  possible 
  that 
  the 
  seasonal 
  inversion 
  in 
  which 
  

   the 
  surface 
  layer 
  is 
  carried 
  to 
  the 
  bottom 
  assists 
  in 
  maintaining 
  the 
  

   oxygen 
  supply 
  at 
  very 
  great 
  depths. 
  The 
  carbon 
  dioxid 
  present 
  in 
  

   Lake 
  Constance 
  Hoppe-Seyler 
  found 
  to 
  exist 
  chiefly 
  in 
  the 
  form 
  of 
  

   carbonates; 
  but 
  little 
  of 
  it 
  (8.14 
  mg. 
  per 
  liter 
  of 
  water 
  at 
  147 
  m. 
  depth) 
  

   exists 
  free. 
  From 
  these 
  results 
  the 
  conclusion 
  may 
  be 
  drawn 
  with 
  

   entire 
  definiteness 
  that 
  even 
  at 
  great 
  depths 
  in 
  the 
  lake 
  and 
  very 
  near 
  

   the 
  bottom 
  only 
  little 
  carbon 
  dioxid 
  is 
  present 
  uncombined, 
  and 
  there- 
  

   fore 
  no 
  hindrance 
  to 
  the 
  respiration 
  of 
  the 
  animals 
  of 
  the 
  lake 
  can 
  

   occur 
  from 
  the 
  carbon 
  dioxid 
  tension 
  even 
  at 
  such 
  depths. 
  

  

  The 
  observations 
  of 
  Hoppe-Seyler, 
  then, 
  show 
  that 
  the 
  upper 
  layers 
  

   of 
  the 
  water 
  of 
  the 
  lake 
  to 
  a 
  depth 
  of 
  2 
  meters 
  are 
  practically 
  satu- 
  

   rated 
  with 
  oxygen, 
  not 
  only 
  where 
  larger 
  aquatic 
  plants 
  are 
  growing, 
  

   but 
  where 
  there 
  are 
  no 
  such 
  plants. 
  These 
  plants 
  can 
  therefore 
  have 
  

   no 
  practical 
  effect 
  in 
  increasing 
  the 
  oxygen 
  content 
  of 
  the 
  superficial 
  

   layer 
  of 
  water. 
  Since 
  his 
  observations 
  show 
  further 
  that 
  in 
  no 
  part 
  of 
  

   the 
  lake, 
  even 
  at 
  great 
  depths, 
  and 
  in 
  other 
  situations 
  destitute 
  of 
  larger 
  

   aquatic 
  plants, 
  is 
  there 
  more 
  than 
  a 
  small 
  quantity 
  of 
  uncombined 
  

   carbon 
  dioxid 
  present, 
  it 
  is 
  clear 
  that 
  the 
  larger 
  plants 
  are 
  not 
  essential 
  

   for 
  the 
  removal 
  of 
  this 
  gas 
  from 
  the 
  water. 
  It 
  is 
  removed 
  rather 
  as 
  a 
  

   free 
  gas, 
  by 
  the 
  formation 
  of 
  carbonates. 
  The 
  statement, 
  however, 
  

   that 
  the 
  larger 
  aquatic 
  plants 
  can 
  not 
  be 
  regarded 
  as 
  essential 
  for 
  the 
  

   furnishing 
  of 
  oxygen 
  to 
  the 
  animals 
  of 
  a 
  lake 
  or 
  for 
  the 
  removal 
  of 
  

   carbon 
  dioxid 
  injurious 
  to 
  those 
  animals 
  must 
  be 
  understood 
  as 
  applying 
  

   onl}" 
  to 
  lakes 
  of 
  considerable 
  size 
  — 
  not 
  to 
  small 
  ponds 
  nor 
  to 
  standing 
  

   aquaria. 
  

  

  Since 
  the 
  larger 
  plants 
  are 
  scarcely 
  used 
  directly 
  as 
  food 
  by 
  fishes 
  

   and 
  are 
  of 
  no 
  demonstrated 
  aeration 
  importance 
  in 
  lakes, 
  it 
  remains 
  to 
  

   determine 
  whether 
  the}^ 
  form 
  one 
  of 
  the 
  links 
  in 
  the 
  chain 
  of 
  nutritive 
  

   relations 
  that 
  stretches 
  from 
  the 
  water 
  and 
  the 
  soil 
  to 
  the 
  higher 
  fishes; 
  

   whether, 
  in 
  other 
  words, 
  the 
  plants 
  have, 
  in 
  addition 
  to 
  their 
  mechan 
  

   ical 
  role, 
  a 
  nutritive 
  role 
  also. 
  If 
  we 
  follow 
  it 
  backward 
  from 
  the 
  fish, 
  

  

  