Magazine  of  the  School  of.  Agriculture. 
IS 
these  coral  islands  or  atolls.  Darwin’s  theory 
of  the  origin  of  coral  islands  was  that  an  atoll 
began  by  being  a fringing  reef  ; that  it  next  be- 
came a barrier  reef,  and  lastly  a coral  island. 
As  the  sea  bottom  gradually  subsided,  the  coral 
reefs  kept  gradually  rising  as  the  result  of  the 
reef-builders’  work.  The  fringing  reef  would  thus 
tend  to  become  a barrier  reef,  and  after  the 
submergence  of  the  entire  island,  a lagoon  would 
take  its  place,  encircled  by  a coral  ring.  The 
further  researches  of  Mr.  Murray  of  the  “ Chal- 
lenger’’ Expedition  and  certain  facts  relative 
to  coral  structures  which  he  claimed  to  have 
discovered  resulted  in  a counterfeeling  against 
Darwin's  theory  ; but  within  the  last  year  or 
two  the  theory  of  Darwin  has  again  come 
into  favour  owing  to  the  “facts’’  of  Mr.  Murray 
having  been  proved  to  be  very  doubtful. 
Coral-stone  occurs  in  the  north  of  the  Island 
and  is  used  for  building  purposes.  Coral  is 
burnt  for  lime  in  parts  by  the  island — the  lime 
being  used  for  lime-washing  houses,  as  well 
as  for  agricultural  purposes.  Iu  the  latter 
case  quicklime  or  slaked  lime  is  used  for  im- 
proving the  condition  especially  of  peaty  land 
and  heavy  clay  soils.  For  the  many  advantages 
attending  the  ues  of  lime  in  agriculture,  the 
student  is  referred  to  text-books  on  agriculture. 
Coral  lime  may  also  be  used  with  advantage 
in  the  forming  of  composts,  as  the  lime  hastens 
the  decomposition  of  organic  matter  (such  as  sea 
weed),  and  also,  owing  to  its  organic  origin, 
supplies  small  quantities  of  nitrogen  and  phos- 
phoric acid,  derived  from  the  remains  of  the 
coral  polypes. 
Small  quantities  of  red  coral,  apparently 
identical  with  the  Mediterranean  and  Cape 
Verde  species  have  been  found  on  reefs  a short 
distance  from  the  S.  E.  coast  of  Ceylon,  but 
in  such  small  quantities  as  to  be  of  trifling  value. 
FIXATION  OF  FREE  NITROGEN. 
Prof.  Franklaud,  the  well-known  chemist, 
delivered  the  last  of  the  Cantor  lectures  before 
the  Society  of  Arts  on  ‘‘ Recent  Bacteriological 
and  Chemical  Researches.”  Iu  dealing  with  the 
subject  of  nitrification  in  the  soil,  Prof.  Frank- 
land  also  referred  to  the  fixation  of  free  nitro- 
gen by  means  of  leguminous  crops,  and  clearly 
defines  the  latest  results  of  investigation  into 
this  subject.  That  the  fertility  of  the  soil  was 
able  to  be  improved  by  leguminous  crops  was 
known  as  long  ago  as  2,000  years,  but  by  what 
means  this  was  effected  was  not  clear. 
For  nearly  a century  past  agricultural  chemists 
and  vegetable  physiologists  have  been  debating 
as  to  whether  the  free  nitrogen  of  our  atmosphere 
can  be  assimilated  by  plants.  This  question  was 
answered  in  the  negative  by  Boussingault  about 
fifty  years  since.  The  problem  was  again  attack- 
ed by  Lawes,  Gilbert,  and  Pugh  about  thirty 
years  ago,  and  their  answer  was  in  the  negative 
also.  In  the  course,  however,  of  their  continu- 
ous experiments  on  crops  Lawes  and  Gilbert  have 
frequently  pointed  out  that  whilst  that  nitrogen 
in  most  crops  can  be  accounted  for  by  the 
combined  nitrogen  supplied  to  the  land  in  the 
from  of  manures  and  in  rain  water,  yet  in  par- 
ticular leguminous  crops,  such  as  pea,  beans, 
vetches,  and  the  like,  there  is  an  excess  of 
nitrogen  found  in  the  crop  which  cannot  be- 
referred  to  these  obvious  sources. 
The  qnestion  remained  in  this  unsatisfactory- 
state  until  again  revived  by  M.  Bertelot  in  1876, 
whilst,  subsequently  the  most  conclusive  ex- 
periments were  made  by  two  German  investigators 
— Ilellriegel  and  Wilfartli,  who  not  only  showed 
that  this  excess  of  nitrogen  in  the  leguminous 
crops  is  obtained  from  the  atmosphere,  but,  what 
is  more  interesting  to  us,  that  the  assimilation 
is  effected  by  means  of  certain  micro-organisms 
flourishing  in  and  around  the  roots  of  these 
plants,  and  where  these  same  plants  are  grown 
in  sterile  soil  the  fixation  of  free  atmospheric 
nitrogen  does  not  take  place. 
The  manner  in  which  these  micro-organisms 
assist  these  leguminous  plants  in  the  accumula- 
tion of  nitrogen  is  exceedingly  remarkable.  When 
these  organisms  are  present  in  the  soil  they 
occasion  the  formation  of  peculiar  swellings  or 
tuberosities  on  the  roots  of  the  leguminous  plants, 
these  tuberosities  never  being  formed  in  sterile 
soil.  On  miscroscopic  examination  these  tuber- 
cles are  found  to  contain  amongst  their  cells 
a ramifying  growth  which  subsequently  gives 
rise  to  a number  of  small  cells  having  much  the 
appearance  of  bacteria,  although  the  precise  group 
of  organisms  to  which  they  belong  is  one  of  those 
numerous  points  on  which  morphologists  cannot 
agree.  As  long  as  the  point  is  still  sub  judice 
it  is  perhaps  most  convenient  to  call  them  bac- 
teroids.  These  can  be  cultivated  on  artificial 
media  like  ordinary  bacteria. 
The  careful  investigation  of  these  tubercle- 
producing  organisms  has  shown  that  each  species 
of  leguminous  plant  lias  its  particular  bacteriod, 
which  is  more  potent  in  the  formation  of  these 
tubercles  on  the  given  species  than  the  tubercle 
bacteriods  of  other  leguminous  species.  In  this 
connection  some  very  striking  and  highly  instruc- 
tive experiments  have  been  made  by  Professor 
Nobbe,  of  Maraud,  who  has  found  that  if  pure 
cultivation  of  the  bacteroids  from  a pea  tubercle 
are  inoculated  into  the  roots  of  a pea  plant,  a 
more  abundant  growth  and  fixation  of  nitrogen 
by  this  pea-plant  takes  place  than  if  it  is  ino- 
culated with  pure  cultivation  of  the  bacteroid 
from  the  tubercle  of  a lupus  or  a robina,  whilst 
conversely  the  robina  is  more  beneficially  affected 
by  the  application  of  pure  culture  from  robina 
tubercles  than  by  those  from  either  pea-tubercles 
or  lupus-tubercles. 
The  exact  manner  in  which  the  atmospheric 
nitrogen  is  rendered  available  for  these  legumin- 
ous plants  possessing  root  tubercles  is  not  perfectly 
understood,  but  the  general  impression  is  that 
the  micro-organisms  present  in  the  tubercles 
take  up  the  nitrogen  and  elaborate  it  into 
a form  which  can  be  assimilated  by  the  plants. 
Whatever  the  secret  of  the  process  may  be,  it  is 
perfectly  certain  that  the  presence  and  vital  action 
of  the  micro-organisms,  which  give  rise  to  the 
formation  of  the  root  tubercles,  is  the  indispens- 
able factor. 
The  great  importance  of  this  discovery  in  vege- 
table physiology  it  is  unnecessary  to  dwell  upon; 
we  have  to  recognise  in  the  micro-organisms 
the  invaluable  agents  whereby  the  atmospheric 
nitrogen,  which  is  in  itself  worthless  both  to 
animals  and  ordinary  plants,  is  actually  rendered 
available  directly  for  the  nutrition  of  plauts 
