J.iimaty  il,  lt)00. 
JOURNAL  OF  HORTICULTURE  AND  COTTAGE  GARDENER. 
31 
THE  ATMOSPHERE,  AND  THE  ATMOSPHERIC 
FOOD  OF  PLANTS. 
Though  this  all-important  subject  has  been  treated  from  time  to 
time  in  the  Journal  of  Horticulture,  we  do  not  remember  its  having 
been  presented  more  comprehensively  and  clearly  than  in  a  lecture  by 
Professor  Black,  published  among  others  some  time  since  by  the  New 
Zealand  Department  of  Agriculture,  but  only  received  by  us  a  lew 
weeks  ago.  It  is  reproduced  with  slight  abridgement  as  follows  : — 
Of  the  seventy  elements  that,  in  various  states  of  combination, 
make  up  the  crust  of  the  earth,  thirteen  enter  into  the  composition 
of  plants,  and  may  therefore  be  regarded  as  plant  food.  Five  of 
these  are  metals — viz.,  potassium,  sodium,  calcium  (the  metal  in 
lime),  magnesium,  and  iron.  The  other  eight  are  non-metallic — viz., 
carbon,  oxygen,  hydrogen,  nitrogen,  chlorine,  sulphur,  phosphoius, 
and  silicon.  Of  these,  carbon  is  supplied  almost  exclusively  by  the 
air;  nitrogen  partly  by  the  air,  but  mainly  from  the  soil.  All  tne 
others  are  supplied  by  the  soil  and  water. 
The  atmospheric  air  is  a  mixture  of  various  gases,  of  which 
nitrogen  and  oxygen  are  the  most  abundant,  making  up  between 
them  more  than  98  per  cent,  of  the  whole.  The  carbonic  acid, 
though  amounting  to  only  about  6  parts  in  10,000  by  weight,  is  of 
the  very  highest  importance  to  plants ;  and  there  are  small  quantities 
of  ammonia,  nitric  acid,  and  nitrous  acid,  all  containing  nitrogen  ; 
and,  in  the  air  of  large  manufacturing  towns,  sulphurous  acid  and 
sulphureted  hydrogen.  There  is,  of  course,  also  the  vapour  of  water, 
which  is  very  variable  in  quantity,  being  evidently  less  abundant  just 
after  a  three  days’  heavy  rain  than  just  before  such  an  occurrence. 
Besides  all  these,  there  is  another  gas  named  argon,  recently  dis¬ 
covered  in  the  air,  and  comprising  about  of  its  total  volume. 
Disregarding  the  small  quantities  of  aqueous  vapour,  of  argon,  and  of 
carbonic  acid,  and  the  other  still  rarer  constituents  named,  we  may 
take  100  gallons  of  air  as  containing  79  gallons  of  nitrogen  and 
21  gallons  of  oxygen ;  or,  if  we  take  it  by  weight  instead  of  volume. 
lOU  tons  of  air  contains  77  tons  of  nitrogen  and  23  tons  of  oxygen. 
The  proportion  of  carbonic  acid  gas  in  the  air  may  be  stated  as 
4  gallons  in  IQ, 000  gallons  of  air,  or  about  6  tons  in  10,000  tons  of 
air.  Small  though  this  proportion  of  carbonic  acid  gas  is,  yet  when 
the  enormous  quantity  of  atmospheric  air  is  considered,  it  will  be 
seen  that  the  weight  of  this  gas  that  rests  on  every  acre  of  surface 
of  land  and  sea  all  round  the  earth  amounts  to  about  21  tons.  'I'he 
sources  of  this  carbonic  acid  gas  are  varied.  First,  there  is  the 
breathing  of  animals.  The  exhaled  air  is  mainly  nitrogen  and  oxygen, 
but  it  also  contains  the  vapour  of  water  and  about  4  per  cent,  of 
carbonic  acid.  It  is  therefore  about  one  hundred  times  richer  in 
carbonic  acid  gas  than  the  inhaled  air.  This,  therefore,  is  a  constant 
and  important  source  of  the  gas,  as  all  animals  of  every  sort  and 
every  size  aie  busy  all  life  long,  night  and  day,  asleep  or  awake,  on 
land  or  in  the  sea  or  air,  perpetually  making  carbonic  acid  gas. 
Secondly,  the  burning  of  fire  is  another  productive  source.  A  ton  of 
West  Coast  coal  in  burning  evolves  about  2^  tons,  and  the  lignites 
from  IJ  to  2  tons,  of  carbonic  acid.  Indeed  3  lbs.  of  carbon  in  every 
kind  of  fuel  produces  about  11  lbs.  of  this  gas. 
There  are,  thirdly,  small  contributions  from  what  is  called  the 
burning  of  limestone.  In  this  process  (in  which,  however,  noihing 
is  burnt  but  the  fuel  that  heats  the  limestone)  every  100  tons  of  pure 
limestone  evolves  44  tons  of  carbonic  acid  gas,  and  leaves  a  residue 
of  56  tons  of  burnt  lime  or  quicklime  in  the  kiln.  Fourthly,  in  the 
process  of  fermentation  of  the  Sugar  in  brewing,  wine  making, 
whisky,  &c.,  180  lbs.  of  sugar  splits  up  into  92  lbs.  of  alcohol  and 
88  lbs.  of  carbonic  acid.  There  are  also,  fifthly,  notable  quantities  of 
this  gas  produced  by  the  decay  or  oxidation  of  dead  organic  remains 
— the  bodies  of  plants  and  animals ;  and  there  is,  lastly,  the  most 
prolific  source  of  all  in  the  exhalations  issuing  from  the  craters  and 
crevices  of  extant  and  extinct  volcanoes,  and  from  numerous  other 
fissures  communicating  with  the  interior  of  the  earth.  Those  that 
best  know  tell  us  that  from  this  source  alone  there  is  probably  seven 
times  as  much  carbonic  acid  gas  heaved  into  the  atmosphere  as  there 
is  produced  by  all  the  other  sources  put  together.  Of  the  atmospheric 
gases,  taking  an  equal  bulk  of  each,  carbonic  acid  is  the  heaviest, 
argon  next,  oxygen  next,  and  then  nitrogen.  The  numbers  represent¬ 
ing  the  relative  weights  of  these,  in  the  order  in  which  they  are 
named  above,  are  22,  20,  16j  and  14  respectively ;  aqueous  vapour  is  9, 
and  ammonia  8L 
If  in  the  atmosphere  these  gases  behaved  like  a  mixture  of  liquids 
such  as,  say,  mercury,  water,  and  ether,  of  which  the  heaviest  would 
go  to  the  bottom,  the  others  resting  on  it  in  the  order  of  their  specific 
gravity,  we  should  have  all  the  heavy  carbonic  acid  settling  down  into 
the  valleys  and  low-lying  lands  and  plains,  and  covering  these  with  a 
deadly  pall  to  the  depth  of  many  feet.  The  waters  of  the  oceans 
would  in  that  case  also  dissolve  a  large  quantity  of  the  gas,  and  life 
on  land  or  sea  would  be  impossible.  Such,  however,  is  not  the  case, 
for,  in  obedience  to  the  law  of  the  diffusion  of  gases,  all  gases  that  do 
not  act  chemically  on  each  other  distribute  themselves  uniformly 
throughout  the  whole  space  which  they  jointly  occupy.  By  the 
operation  of  this  la#  (which  is  not  really,  though  apparently,  incon- 
sistent  with  the  law  of  gravitation)  the  carbonic  acid  produced  at  the 
surface  of  the  ground  rapidly  diffuses  itself  away  up  all  through  the 
atmosphere,  and  maintains  everywhere  its  proportion  of  about  4  gallons 
in  every  10,000  gallons  of  air. 
(To  be  continued.) 
PRUNING. 
Much  has  been  written  on  this  importanfoperation  in  gardening, 
much  has  been  taught,  and  much  either  apparently  forgotten  by  some 
performers  with  the  knife  or  misunderstood.  Pruning  nray  be  divided 
into  two  distinct  methods,  the  one  carried  out  under  the  cut  and  dried 
formula  of  “  prune  to  a  given  number  of  buds  or  eyes,”  as  if  regard¬ 
less  of  the  future  ;  the  other  in  which  thought  and  care  for  the  after¬ 
growth  guide  the  action  of  the  operator.  When  there  is  a  great 
amount  of  pruning  to  be  done,  it  may  be  impossible  to  avoid  a  certain 
proportion  from  being  carried  out  in  a  mechanical  manner ;  yet  who 
shall  say  that  this  machine-like  method  has  not  gone  too  far  in  many 
instances  ?  It  is  all  too  frequently  seen  in  the  persistent  stumping 
back  of  trees  growing  in  rich  soil,  until  there  is  nothing  but  a  thicket 
of  useless  growth  each  season,  instead  of  a  crop  of  health-giving  fruit. 
No  trouble  appears  to  be  taken  by  some  persons  to  thin-out  and  expose 
the  leaves  on  what  ought  to  be  the  main  branches  to  the  influence  of 
light  and  air ;  yet  there  are  thousands  of  trees  that  have  no  distinct 
main  branches,  but  have  been  distorted  into  dense  bushes,  and  the 
trimming  of  these  into  form  yearly  is  called  “  pruning.” 
A  gardener  found  in  a  new  situation  some  old  espalier  Apples, 
which  had  been  innocent  of  fruit  for  years.  They  bore  heavy  crops  of 
rampant  stems  that  had  been  closely  cut  off  and  burned  time  after 
time.  No  amount  of  persuasion  would  induce  the  owner  to  part  with 
these  treasured  trees,  so  there  was  nothing  for  it  but  to  make  the  best 
of  them.  By  leaving  a  few  of  the  strong  shoots  along  the  top  tier  of 
branches  about  15  inches  asunder,  just  taking  off  the  unripened  tips. 
The  second  spring  afterwards,  the  shoots  so  treated  were  wreathed  with 
blossom  buds,  and,  as  the  weather  was  favourable  for  setting  excellent 
fruit  followed.  Nor  was  this  all.  The  lower  branches  gradually 
improved,  as  if  steadily  invigorated  by  giving  the  “  old  stagers  ”  their 
heads.  True  the  clusters  of  exhausted  spurs  were  thinned,  and  the 
branches  cleansed,  and  those  that  remained  gave  what  gardeners  would 
call  a  “good  sprinkling”  of  Apples.  It  might  have  been  better  to 
have  uprooted  these  old  trees  and  planted  young  ones,  but  in  this  case 
as  in  many  others  the  gardener  was  not  a  free  agent.  The  owner 
would  not  hear  of  parting  with  his  gnarled  friends,  and  there  they 
remain,  and  nothing  that  the  “  new  ”  gardener  did  gave  more  pleasure 
than  the  attention  he  so  usefully  applied  to  the  old  family  favourites. 
This  desire  of  owners  to  retain  almost  useless  fruit-bearers  in  their 
gardens  opens  up  another  part  of  the  subject.  In  the  matter  of  Vines 
it  would  be  hard  to  say  how  many  vineries  in  this  country  would  be 
made  vastly  more  profitable  if  the  worn-out  occupants  were  consigned 
to  the  rubbish  fire  and  young  thrifty  canes  planted  in  their  stead. 
But  if  the  old  Vines  must  perforce  remain,  is  the  best  always  made  of 
them  ?  We  read  from  time  to  time  of  the  rejuvenation  of  ancient 
Vines  by  the  renewal  of  borders  and  such-like  commendable  means, 
but  cases  are  not  at  all  rare  where  the  stereotyped  habit  of  pruning  to  a 
certain  number  of  buds  at  the  base  of  the  current  season’s  growth  is 
continued.  The  same  decrepit  rods  remain  year  after  year,  their  curled 
and  twisted  spurs  weakening  accordingly,  some  dying,  others  dwindling 
until  they  have  scarcely  strength  with  which  to  start  into  growth. 
Would  it  not  be  better  to  vary  the  procedure  for  a  time,  by  pruning 
to  some  of  the  best  buds  on  the  lateral  shoots,  and  also  by  starting 
young  canes  from  the  base,  so  that  these  might  afterwards  take  the 
place  of  the  veterans,  which  could  be  gradually  cut  out  to  make  room 
for  them  ?  I  am  convinced  that  better  crops  of  Grapes  of  finer  quality 
might  be  obtained  by  carrying  out  these  simple  methods. 
Retui’ning  to  outdoor  fruits,  experience  has  proved  conclusively 
that  decided  advantages  are  gained  by  keeping  these  thinly  disposed,  the 
removal  of  worn  out  branches,  and  inducing  new  ones  to  replace  them. 
Take  the  case  of  Peaches  on  walls.  I  have  seen  old  trees  condemned, 
yet  by  careful  treatment  in  this  manner  they  have  been  made  to  render 
a  good  account  of  themselves  for  a  number  of  years.  I  have  also  in 
mind  a  number  of  large  Apple  trees  which  were  benefited  in  the  same 
way,  these,  after  a  time,  producing  fruit  which  could  not  be  regarded 
as  discreditable  if  produced  by  much  younger  trees.  I  should  like  to 
uproot  many  old  trees  and  plant  young,  also  to  have  new  ranges  of 
vineries  to  plant,  but  have  no  such  privilege.  In  the  meantime,  I  try 
to  make  the  best  of  what  is  obsolete,  and  remain — Submerged. 
Acacia  pubescens. — There  appears  to  be  a  slightly  increased 
interest  in  the  beautiful  section  of  Acacias  that  are  suitable  for  green¬ 
house  culture,  and  this  is  one  of  the  best  of  them.  Foliage  and  flowers 
are  alike  extremely  graceful,  and  it  rapidly  makes  fine  specimens. 
The  flowers  are  produced  in  the  earliest  months  of  the  year,  and  growth 
starts  soon  after.  Quite  a  cool  greenhouse  with  ample  root  moisture 
suits  it  well,  and  for  the  sake  of  making  room,  as  well  as  the  health  of 
the  plants,  they  may  be  stood  outside  from  the  end  of  May  until 
October.  Under  these  very  simple  conditions  this  useful  plant  thrives 
admirably. — B.  S.  E, 
