AmAJp°rnr;imrm-}         4  Study  of  Starch  Grains.  183 
takes  place  between  the  iodine  and  starch.  Meyer,  on  the  other 
hand,  does  not  consider  the  so-called  iodide  of  starch  to  be  either  a 
chemical  combination  or  a  mechanical  mixture,  but  says  that  it  is 
"  ein  wohl  definirte  blaue  Losung  von  Iod  in  Starke." 
It  ought  to  be  borne  well  in  mind  that  between  the  pure  blue  and 
red  reaction  of  starches,  intermediate  colors  may  be  produced,  de- 
pending on  the  varying  amounts  of  dextrin  present. 
Another  important  fact  that  may  be  mentioned  in  this  connection 
is  that  if  the  aqueous  solutions  of  iodine  contain  hydriodic  acid,  or 
if  alcoholic  solutions  contain  ethyl  iodide,  there  is  produced  accord- 
ing to  the  amount  of  these  foreign  substances,  as  well  as  the  char- 
acter of  the  starch,  colors  which  vary  from  yellowish  or  reddish- 
yellow  to  reddish  or  reddish-blue  or  even  purple. 
The  starch  in  plant  cells  which  is  still  surrounded  by  the  plastids 
or  imbedded  in  other  materials,  as  resin,  may  be  determined  by 
means  of  a  chloral-iodine  solution,  which  clears  the  other  sub- 
stances and  causes  a  swelling  of  the  grains  which  are  at  the  same 
time  colored  blue.  It  is  useful  in  certain  cases  when  much  resin  is 
present  to  dissolve  out  the  starch  by  heating  the  sections  in  glycerin, 
and  on  the  addition  of  excess  of  water  the  starch  will  separate  from 
the  solution  in  the  form  of  minute  grains. 
A  still  more  characteristic  property  of  starch  grains  is  that  when 
they  are  heated  in  contact  with  water  to  45-770  C,  they  swell  and 
form  a  pasty  mass.  The  temperature  at  which  the  different  starches 
begin  to  form  a  paste  is  likewise  deserving  of  practical  considera- 
tion. 
Not  only  do  starch  grains  swell  when  he  ated  with  water,  but 
they  swell  upon  treatment  with  various  reagents,  as  chloral,  potas- 
sium hydrate,  chlor-zinc-iodide,  chloral-iodine,  etc. 
Various  investigators  since  the  time  of  Nageli,  who  appears  first 
to  have  experimented  with  the  action  of  saliva  on  the  starch  grain, 
have  used  this  and  other  ferments,  as  well  as  other  substances,  as 
reagents  in  the  examination  of  starch.  All  of  this  work  has  been 
done  by  botanists,  and  has  been  for  the  purpose  of  ascertaining  the 
composition  and  structure  of  the  starch  grain.  There  is,  however,  a 
very  practical  application  to  be  made  of  the  character  of  the  interac- 
tion between  reagents  and  starch  grains;  considering  in  this  connec- 
tion the  amount  of  time  and  temperature  required  to  complete  the 
reaction ;  and  the  strength  of  reagent,  etc.    The  writer  has  made 
