562  DR.  HOOKER'S  ADDRESS  TO  THE  BRITISH  ASSOCIATION. 
before  honey-feeding  insects  existed,  the  vegetation  of  our  globe 
could  not  have  been  ornamented  with  bright-colored  flowers,  but 
consisted  of  such  plants  as  pines,  oaks,  grasses,  nettles,  &c. 
The  only  other  botanical  paper  of  Mr.  Darwin's  to  which  I 
can  especially  allude,  is  that  "  On  the  Habits  and  Movements  of 
Climbing  Plants,"*  which  is  a  most  elaborate  investigation  into 
the  structure,  modification,  and  functions  of  the  various  organs 
by  which  plants  climb,  twine,  and  attract  themselves  to  foreign 
objects.  In  this  he  reviews  every  family  in  the  vegetable  king- 
dom, and  every  organ  used  by  any  plant  for  the  above  purpose. 
The  result  places  the  whole  subject  in  a  totally  new  light  before 
us.  The  guesses,  crude  observations,  and  abortive  experiments 
that  had  disfigured  the  writings  of  previous  observers  are  swept 
away  ;  organs,  structures,  and  functions  of  which  botanists  had 
no  previous  knowledge  are  revealed  to  them,  and  the  whole  in- 
vestigation is  made  as  clear  as  it  is  interesting  and  instructive. 
The  value  of  these  discoveries,  which  add  whole  chapters  to 
the  principles  of  Botany,  is  not  theoretical  only  ;  already  the 
horticulturist  and  agriculturalist  have  begun  to  ponder  over 
them,  and  to  recognize  in  the  failure  of  certain  crops  the  opera- 
tion of  laws  that  Mr.  Darwin  first  laid  down.  What  Faraday's 
discoveries  are  to  telegraphy  Mr.  Darwin's  will  assuredly  prove 
to  rural  economy,  in  its  widest  sense  and  most  extended  appli- 
cation. 
Another  instance  of  successful  experiment  in  Physiological 
Botany  is  Mr.  Herbert  Spencer's  observations  on  the  circulation 
of  the  sap  and  formation  of  wood  in  plants. f  As  is  well  known, 
the  tissues  of  our  herbs,  shrubs,  and  trees,  from  the  tips  of  their 
roots  to  those  of  their  petals  and  pistils,  are  permeated  by  tubu- 
lar vessels.  The  functions  of  these  have  been  hotly  disputed, 
some  physiologists  affirming  that  they  convey  air,  others  fluids, 
others  gases,  and  still  others  assigning  to  them  far-fetched  uses 
of  a  wholly  different  nature.  By  a  series  of  admirably  contrived 
and  conducted  experiments,  Mr.  Spencer  has  not  only  shown 
that  these  vessels  are  charged  at  certain  seasons  of  the  year  with 
fluid,  but  that  they  are  intimately  connected  with  the  formation 
*  "  Linnean  Transactions,"  vol.  ix,  p.  1. 
f  Ibid.,  vol.  xxv,  p.  405. 
