Am.  Jour.  Pharm. 
April,  1921. 
Hybridization  in  Plants. 
253 
contrasted  unit  characters  are  crossed,  nevertheless,  when  they  take 
into  consideration  a  multiplicity  of  factors,  there  are  discrepancies 
in  the  working  out  of  the  theory  which  remains  unexplained  to  the 
satisfaction  of  geneticists. 
There  is,  accordingly,  always  an  element  of  doubt  as  to  what 
the  results  of  a  new  cross  may  be,  until  the  combination  lias  been 
produced  and  tested. 
METHODS  OF  CROSSING  PLANTS. 
In  the  majority  of  instances  among  Spermatophytes,  the  plants 
bear  flowers  having  both  male  reproductive  organs  (stamens)  and 
female  reproductive  organs  (carpels)  in  the  same  bloom.  In  some 
cases,  as  in  the  genera  Zea,  Croton,  Ricinus,  Pinus,  Castanea  and 
Quercus,  both  of  these  sets  of  essential  organs  are  borne  on 
the  same  plant,  but  in  different  flowers.  In  still  others,  as  the 
genera  Cannabis,  Humulns,  Salix,  Popiilus,  etc.,  the  stamens  and 
carpels  are  borne  in  flowers  of  different  plants  of  the  same  species. 
In  the  practice  of  crossing,  care  must  be  exercised  to  the  end 
that  plants  are  not  fertilized  with  their  own  pollen  or  with  pollen 
from  any  other  than  the  desired  source.  Accordingly,  if  the  plant 
to  be  hybridized  has  the  stamens  and  carpels  in  the  same  flower, 
as  for  examples,  Aconite  or  Larkspur  plants,  the  stamens  must 
be  removed  from  the  flower  in  the  bud  condition,  before  the  anthers 
have  burst  and  discharged  their  pollen.  This  act  is  known  as 
'•emasculation"  and  is  absolutely  essential  in  order  to  prevent  self- 
fertilization.  In  protandrous  plants,  e.  g.,  those  with  flowers  in 
which  the  stamens  elongate  and  mature  their  pollen  before  the 
ripening  of  the  carpels,  it  is  necessary  to  emasculate  the  buds  quite 
early,  while  in  protogynous  plants,  e.  g.,  those  with  flowers  in  which 
the  carpds  mature  before  the  stamens,  emasculation  can  be  deferred 
until  just  before  the  opening  of  the  flower  bud. 
In  all  cases  where  petals  are  found  in  the  floral  make-up,  these 
are  either  pried  apart  or  cut  away.  The  stamens  are  pulled  out 
by  forceps,  care  being  taken  not  to  injure  the  pistil.  The  bud  is 
then  enclosed  in  a  paper  bag  in  order  to  prevent  pollen  from  any 
other  souroe  being  brought  to  the  pistil.  After  waiting  a  sufficient 
length  of  time  to  allow  the  pistil  to  reach  maturity,  the  paper  bag 
is  removed  and  pollen  from  the  desired  variety  dusted  on  the  pistil. 
The  bud  is  again  covered  with  the  paper  bag  and  allowed  to  remain 
for  several  days  or  longer  until  fertilization  has  taken  place.  A 
labelled  tag,  indicating  the  nature  of  the  cross,  is  then  tied  b.y  fine 
