414  The  Structure  of  the  Starch  Grain.    { timber,  wo^1 
1.  The  starch  grain  consists  of  a  membrane  which  is  insoluble  in 
water,  and  a  more  or  less  soluble  content,  as  pointed  out  by  Raspail. 
2.  It  develops  from  a  centric  or  excentric  point,  to  which  layer 
after  layer  is  added,  a  view  first  advanced  by  Fritsche  (4),  and  sub- 
sequently enlarged  upon  by  Schimper  (5),  who  demonstrated  that  its 
growth  is  dependent  upon  the  function  of  the  leucoplastids. 
3.  The  content  of  the  grain  consists  of  at  least  two  different  sub- 
stances, as  first  pointed  out  by  Nageli  (2),  and  later  confirmed  by 
Meyer  (6),  Schimper  (5),  and  others,  who  showed  in  addition  that 
the  structure  might  be  compared  to  that  of  sphero-crystalloids. 
THE  ACTION  OF  IOUIN  UPON  STARCH. 
The  effect  of  iodin  upon  starch  has  been  the  subject  of  consider- 
able investigation  and  speculation  by  both  botanists  and  chemists. 
Mylius  (7)  held  to  the  view  that  the  blue  coloration  formed  on  the 
addition  of  iodin  to  starch  is  due  to  the  formation  of  a  compound 
of  hydriodic  acid  and  starch.  Meineke  (8)  also  contended  that 
hydriodic  acid  is  essential  to  the  formation  of  the  blue  color  of  an 
iodin-starch  solution.  Stocks  (9),  Seyfert  (10),  and  Rouvier  (11)  are 
of  the  opinion,  however,  that  this  colored  compound  does  not  contain 
hydriodic  acid,  the  last-named  investigator  having  shown  that  it 
may  be  formed  in  the  presence  of  alcohol  as  well  as  water,  and  all 
of  them  agreeing  that  a  true  chemical  compound  (iodid  of  starch) 
is  formed. 
On  the  other  hand,  Kuster  (12)  and  Meyer  (6)  believe  that  the 
so-called  iodid  of  starch  is  not  a  true  chemical  compound,  but  that 
the  iodin  and  starch  are  combined  in  much  the  same  way  as  the 
ingredients  in  an  emulsion.  This  latter  view  has  been  more  or  less 
sanctioned  on  account  of  the  statements  in  the  text-books  that  the 
iodin  is  removed  from  the  combination  on  treatment  with  carbon 
disulfid.  My  observations,  however,  show  that  if  the  starch  is  in 
excess  of  the  iodin  (1  gramme  of  potato  starch  and  10  c.c.  of  a 
solution  containing  01  per  cent,  of  iodin  and  05  per  cent,  of  potas- 
sium iodid),  or  if  the  iodin  and  starch  be  in  such  proportion  that  all 
of  the  free  iodin  is  taken  up,  and  the  mixture  is  dried  before  adding 
the  carbon  disulfid,  even  on  allowing  it  to  act  for  six  months,  the 
carbon  disulfid  is  not  colored ;  that  is,  does  not  take  up  the  iodin, 
the  same  being  true  of  alcohol  and  chloroform.  This  result,  taken 
together  with  the  previous  experiments  in  the  treatment  of  starch 
