INFLUENCE  OF  CULTIVATION  UPON  MEDICINAL  PLANTS.  269 
and  for  tho  evident  preference  of  certain  plants  for  par- 
ticular localities,  we  shall  see  that  they  obey  those  influences  in 
which  heat  and  light  have  the  greatest  share ;  it  is  in  fact  by 
them  that  the  seasons  affect  the  climates,  which  have,  in  their 
turn,  so  powerful  an  action  upon  vegetation.  It  is  heat  and 
light  which  increase  or  diminish  the  respiration,  circulation,  ab- 
sorption, and  exhalation  of  plants;  in  fact,  it  is  to  their  influence 
that  vegetable  secretions  owe  their  peculiar  tastes  and  odors.  A 
plant,  says  M.  Alph.  de  Candolle,  is  a  living  machine  working 
under  the  influence  of  two  external  agents,  heat  and  light,  and 
one  internal,  life.  He  observes,  at  the  same  time,  that  the  old 
botanists  ascribed  to  these  two  agents  a  still  greater  influence, 
for  they  looked  upon  plants  as  thermometers,  the  degrees  of 
which  were  marked  by  the  phenomena  of  vegetation. 
*<  It  is  also  by  heat  and  light  that  we  explain  the  preference 
of  some  plants  for  certain  latitudes,  certain  climateric  zones 
which  they  do  not  go  beyond ;  it  is  also  by  them  that  we  ac- 
count for  the  presence  of  a  great  number  of  species  at  particular 
heights  above  the  level  of  the  sea,  from  whence  they  neither 
ascend  nor  descend  without  destroying  themselves  or  their 
progeny.  It  is  also  to  these  two  agents  that  we  must  attribute 
the  fructification  of  certain  species  in  a  given  spot,  a  fructifica- 
tion which  cannot  be  produced  if  such  plants  be  placed  in  colder 
regions;  such  are,  for  example,  the  olive,  jujube,  etc. 
«  To  these  and  other  causes  of  a  purely  physical  character,  we 
might  add  those  of  a  chemical  nature,  for  although  these  have 
been  altogether  denied  by  some  writers,  they  have,  nevertheless, 
a  great  influence  upon  the  distribution  of  plants  over  the  earth. 
All  botanists,  for  instance,  know  the  preference  of  certain  species 
of  plants  for  silica,  chalk,  etc. ;  of  others,  for  mixed  rocks  and 
gravel,  etc.  Those  species,  again,  which  inhabit  swampy  or 
turfy  ground ;  those  which  select  stagnant  and  warm  waters,  or 
those  which  choose  running  and  cold  streams  ;  and  those  which 
spring  up  in  the  woods,  meadows,  or  upon  walls,  ruins,  etc.,  are 
not  placed  in  such  situations  by  chance  and  without  appreciable 
causes.  Again,  without  referring  to  parasitic  plants,  examples 
of  a  more  extraordinary  predilection  have  been  noticed, — such, 
for  instance,  as  are  presented  to  us  by  alternating  plants — 
namely,  those  which  appear  in  certain  soih  for  one  or  two  years, 
