41  o  Plant  Morphology  and  Taxonomy.        { ^ptembw.wos1' 
We  thus  see  that  morphology  has  become  a  science,  which  is  not 
merely  concerned  with  the  form  and  structure  of  organs,  but  is  more 
especially  concerned  in  the  study  of  the  factors  which  influence  form 
and  structure,  and  it  is  therefore  not  only  one  of  the  most  interest- 
ing departments  of  biological  science,  but  may  be  regarded  as  its 
very  soul,  as  pointed  out  by  Darwin.  According  to  Goebel  there 
are  two  branches  of  experimental  organography  to  be  considered  i 
(1)  The  reciprocal  influence  of  organs  upon  one  another,  which  is 
termed  correlation. 
(2)  The  influence  of  external  factors,  which  are  designated  by 
Herbst  as  formative  stimuli. 
The  reciprocal  influence  of  one  organ  upon  another  may  be  either 
quantitative  or  qualitative,  although  there  is  no  sharp  distinction 
between  the  two.  In  quantitative  correlation  certain  primordia  are 
suppressed  while  others  apparently  develop  and  enlarge  at  their 
expense,  as  for  example,  it  is  a  common  occurrence  for  most  of  the 
ovules  to  be  entirely  suppressed  while  the  available  nutritive  mate- 
rial is  used  in  the  development  of  the  few  remaining  ones,  as  in 
the  horsechestnut.  In  qualitative  correlation  an  organ  may  assume 
a  different  direction,  and  even  the  function  of  another  part,  if  this 
be  injured,  removed  or  arrested  in  its  development.  As  an  example 
of  this  Goebel  mentions  that  the  transformation  of  the  leaflets  of 
pea  may  be  hindered  by  removing  other  leaflets.  This  plasticity  is 
not  confined  to  any  one  organ,  but  is  more  or  less  characteristic  of 
all  parts  of  the  plant  by  reason  of  certain  latent  properties  of  the 
protoplasm.  The  external  factors  influencing  the  development  of 
organs  may  be  enumerated  as  follows :  Light,  moisture,  temperature 
and  gravity,  and  those  of  a  purely  mechanical  nature  as  well. 
As  a  result  of  the  studies  in  experimental  morphology,  or  experi- 
mental organography,  as  it  is  termed  by  Goebel,  certain  underlying 
principles  have  been  deduced,  to  which  attention  should  be  directed. 
These  were  brought  out  in  an  excellent  paper  on  "  The  Cardinal 
Principles  of  Morphology,"  read  before  the  Society  for  Plant  Mor- 
phology and  Physiology,  in  1 900,  by  Professor  Ganong,  who  was  a 
student  of  Goebel's. 
The  principles  formulated  by  Professor  Ganong  are  as  follows : — 
(1)  The  Continuity  of  Origin. — No  functional  structure  ever  arises  de  novo, 
but  only  from  the  modification  of  a  pre-existing  structure. 
(2)  Opportunism. — The  direction  taken  in  metamorphosis  is  not  determined 
