218 
Supplement  to  the  ^'Tropical  Agriculturist.”  [Sept.  1,  1896, 
THE  ^'UTRITIVE  PROCESS  IN  PLANTS. 
(Prof.  J.  Reynolds  Green,  D.Sc.,  f.r.s.) 
Turning  to  more  permanent  stores  than  the  cells 
of  the  leaf  afford,  how,  for  instance,  is  the  struc- 
ture which  we  know  by  the  name  of  potato  formed  ? 
When  we  cut  it  we  find  that,  though  bulky  and 
solid,  it  is  not  woody  but  is  of  a succulent 
consistency,  its  firmness  being  largely  due  to 
distention  of  its  substance  by  water.  If  we  cut  a 
thin  section  of  it  and  look  at  it  through  the 
miscroscope  we  find  it  tc  be  made  up  of  nothing 
but  cells,  and  that  the  greater  number  of  them  are 
gorged  with  grains  of  starch.  These  are  much 
larger  than  the  grains  in  the  leaf  and  have  a 
complicated  structure,  they  are  in  shape  irregularly 
oval,  and  their  surfaces  are  marked  by  nearly 
concentric  lines  of  striation,  dividing  them 
apparently  into  layers.  The  centre  of  these  cells 
is  not  the  geometrical  centre  of  the  grain,  but 
lies  near  the  smaller  end,  and  the  rings  or  laj'ers 
are  much  narrower  at  that  end  than  at  the  other. 
How  did  the  starch  get  there  ? There  is  no 
chlorophyll  in  the  part  known  technically  as  the 
tuber,  nor  has  light  access  to  it  during  its  forma- 
tion and  growth. 
The  appearance  of  the  starch  there  coincided  in 
point  of  time  with  its  removal  from  the  leaves, 
which  we  have  seen  first  formed  it.  To  remove 
it  a process  of  transformation  was  necessary,  of 
just  the  opposite  kind  to  that  which  led  to  its 
transitory  appearance.  The  chloroplastid  con- 
verted starcli  into  sugar,  something  else  must 
have  again  converted  the  sugar  into  starch.  This 
was  the  first  and  essential  step,  for  the  solid 
grain  of  starch  could  not  pass  through  the  wall  of 
the  leaf  cell  and  so  travel  from  place  to  place  in 
the  plant.  It  has  been  ascertained  in  the  last 
few  years  that  this  change  is  brought  about  in 
the  leaf  by  the  action  of  a so-called  enzyme,  or 
unorganized  ferment,  known  as  diastase,  the  same 
body  as  has  been  known  for  so  many  years  to 
be  the  active  agent  in  the  saccharification  of 
malt.  The  identification  of  euzj’me  in  the  leaf 
was  difficult,  but  there  is  now  no  doubt  of  its 
presence.  By  its  agency,  particularly  during  the 
hours  of  darkness,  when  the  constructive  activity 
of  the  leaf  was  suspended,  tlie  transitory  starch 
in  the  leaf-cells  was  converted  into  sugar.  This 
sugar  which  is  known  as  maltose,  or  malt  sugar, 
made  its  way  from  the  leaf  down  the  stem  into 
the  tuber,  which  is  really  only  an  underground 
branch  which  becomes  very  much  swollen  and  does 
not  increase  in  length.  It  found  a convenient 
path  in  those  vascular  bundles  which  supplied  the 
leaf  with  water  from  tlie  root,  and  whicli  serve 
therefore  as  the  channels  of  conduction  not  only 
for  water  but  also  for  the  elaborated  products 
which  the  leaf  has  found.  On  reaching  the  cells 
of  the  tuber  a conversion  of  the  sugar  into  starch 
again  took  place.  In  these  cells  there  are  a 
number  of  small  protoplasmic  corpuscles,  much 
like  chloroi^hyll  grains,  only  without  the 
colouring  matter.  They  are  called  leuco]}  last  ids 
because  they  are  white  or  rather  colourless.  In 
their  nature  they  resemble  chloroplastids  very 
closely,  turning  green  when  exj^osed  for  a consider- 
able time  to  light.  They  not  only  resemble 
chloroplastids  thus,  but  they  behave  very  much 
like  in  relation  to  the  stream  of  sugar  solution 
which  reaches  them.  Absorbing  them,  they  form 
in  the  substance  of  their  bodies  starch,  which  they 
pour  out  from  some  portion  of  their  surface.  Thus 
a little  profusion  of  starch  appears  at  one  side  of 
a leucoplastid ; soon  another  outpouring  takes 
place  which  flows  round  the  first  one ; a third 
follows,  and  a fourth,  and  so  on,  the  successive 
layers  causing  the  striated  appearance  we  have 
seen  to  be  characteristic  of  the  adult  grain.  Their 
activity  lasts  much  longer  than  that  ot  the 
chloroplastid,  and  the  starch  grain  is  therefore 
much  larger.  Their  activity  continues,  indeed, 
till  they  are  completely  used  up  and  disappear. 
It  is  not  very  easy  to  see  these  leucoplastids  in 
the  potato ; they  can  be  aeen,  however,  more 
easily  in  other  plants  where  they  are  larger  and 
not  round  [e.y.,  the  cells  of  some  orchids).  In  a 
few  places,  such  as  some  pollen  grains,  starch 
grains  are  found  in  large  numbers  without  the  aid 
of  leucoplastids  so  far  as  we  know  at  present. 
They  seem  in  this  case  to  be  constructed  by  the 
protoplasm  of  the  cell. 
GENERAL  ITEMS. 
The  following,  wdtli  reference  to  ticks  perhaps, 
the  most  troublesome  pest  of  cattle  in  the  tropics, 
is  from  Nature.  In  Ceylon,  tlie  favourite  remedy 
against  ticks  is  coconut  oil  with  wliicli  infested 
animals  are  smeared,  generally  a mixture  of 
common  salt  and  oil  being  used  for  the  purpose  : — 
“Dr.  M.  Francis,  Veterinarian  of  the  Experi- 
ment Station,  has  drawn  our  attention  to  an 
account  by  him  of  the  method  of  destroying 
ticks  on  the  cattle  of  Texas,  and,  as  the  study 
of  the  tick  pest  is  one  of  his  principal  duties, 
this  description  is  of  great  value.  After  several 
unsuccessful  attempts  to  destroy  the  pest  by 
various  means,  the  dipping  process  has  been 
adopted  in  Texas  with  very  gratifying  results. 
A large  vat  of  five  thousand  gallons  capacity 
is  used,  and  the  cattle  are  forced  to  swim 
through  it.  Various  carbolic  and  arsenical  sheep- 
dips  w'ere  employed  as  solutions  in  the  vat,  but 
the  results  were  not  satisfactory  ; either  the 
cattle  had  to  be  kejit  in  the  dips  for  too  long 
a time  in  order  to  kill  all  the  ticks,  or  they 
were  irritated  by  the  solutions.  This  led  Dr. 
Francis  to  try  the  ell'ect  of  oil  in  destroying  the 
ticks.  It  is  well-known  that  grease  or  oil,  of 
almost  any  kind,  is  fatal  to  insects,  lice,  &c., 
and  knowm  facts  as  to  the  life-history  and  struc- 
ture of  ticks  gave  presumptive  evidence  that  oil 
might  be  successfully  substituted  for  the  various 
commercial  dips  which  had  been  employed.  A 
layer  from  three-quarters  to  one  inch  in  thick- 
ness, of  crude  cotton-seed  oil  on  the  w’ater  in 
the  vat  was  first  used,  the  cattle  being  forced  to 
swim  through  the  vat,  so  that  wdien  thej'  emerged 
they  were  covered  perfectly  wdth  oil.  This  had 
no  apparent  ett'ect  on  the  cattle,  but  was  found 
to  be  exceedingly  fatal  to  the  tick,  and  was 
very  much  superior  to  any  other  treatment  tried. 
Dips  of  different  nature  w'ere  experimented  with, 
but  none  as  yet  used  has  given  such  satisfac- 
tory results  as  the  cotton-seed  oil.  Kerosene 
emulsion  was  found  to  have  no  practical  value  ; 
crude  petroleum  irritates  the  skin,  and  emulsifies 
■with  water;  resin  oil  is  useless  for  the  jnirpose ; 
corrosive  suldimate  is  too  dangerous  and  is  not 
very  fatal  to  ticks  even  in  solution  l'2o0  in  waiter, 
an(t  tobacco  sheep-dips  have  no  jiractical  value. 
Dr.  Francis  is  at  present  studying  the  cfl’ects 
of  other  oils,  the  most  ])romislng  being  We.st 
ATrginia  Black,  a mineral  oil.” 
