178  A  Study  of  Starch  Grains.  {Am'£?iiJ^m- 
some  extent  by  the  agency  of  the  wind ;  whereas  it  is  replaced  by 
cellulose  in  seeds  where  germination  is  a  long  time  in  being  effected. 
Starch,  however,  is  the  substance  most  generally  stored  by  the  plant, 
and  this  is  found  in  connection  with  other  principles,  as  oil  (in  corn) 
and  proteids  (in  wheat),  etc.  The  starch  which  we  find  in  rhizomes, 
tubers,  bulbs  and  seeds  owes  its  origin,  like  the  transitory  starch, 
to  the  leucoplastids  which  change  the  sugar  solutions  which  they 
receive  through  the  moving  protoplasm  back  to  a  more  stable 
form,  viz.,  the  starch  grain.  It  is  then  called  "  depot,"  *  storehouse" 
or,  more  frequently,  "  reserve  "  starch.  The  starch  grains  of  this 
class  differ  from  either  "  transitory  "  or  "  assimilation  "  starch  in 
that  they  are,  as  a  rule,  quite  large  and  the  grains  are  more  charac- 
teristic for  the  different  plants  in  which  they  are  produced ;  this  is 
especially  marked  in  the  starch  we  find  in  rhizomes,  tubers  and 
other  metamorphosed  stems. 
ORIGIN  OF  GROWTH   OF  GRAIN. 
This  brings  us,  then,  to  the  consideration  of  the  starch  grain 
itself.  In  some  cases  we  observe  a  distinct  centric  or  eccentric  mark- 
ing, around  which  lamellae  or  layers  may  extend.  The  centric  or  ec- 
centric marking  is  spoken  of  as  the  "  hilum"  or  "  nucleus  "  of  the 
grain.  The  terms  "  hilum  "  and  "  nucleus  "  are,  however,  open  to 
criticism,  inasmuch  as  they  are  employed  in  botanical  language  for 
another  specific  purpose.  The  expression  "  point  of  origin  of 
growth"  would  be  better  on  account  of  its  being  less  confusing,  and 
is,  moreover,  descriptive  and  accurate.  The  "  point  of  origin  of 
growth,"  is  the  part  of  the  starch  grain  that  was  first  formed  by  the 
plastid  and  according  as  new  products  were  added  we  have  a  nearly 
spherical  or  elliptical  or  irregular  shaped  grain. 
The  point  of  origin  of  growth  in  some  starch  grains  is  only  dis- 
cerned by  the  use  of  reagents  or  polarized  light.  When  it  is  distinct 
it  may  be  described  as  being  either  a  nearly  spherical  mark,  or  a 
distinct  fissure.  In  some  cases  there  are  several  fissures  which 
cross  one  another.  All  of  these  marks  are  to  be  employed  practi- 
cally in  distinguishing  those  starch  grains  in  which  it  is  possible  to 
discern  the  botanical  source. 
CONSTITUTION  OF  STARCH  GRAINS. 
We  have  observed  in  some  grains,  as  the  result  of  additions  or 
growth,  that  there  are  layers  or  lamellae.    What  does  this  mean  ? 
