in  its  Effect  on  Cultivation. 
649 
that  of  England.  Yet  though  in  these  situations  they  vegetate 
with  a luxuriance  unknown  in  Spain  or  Italy,  yet  they  never  pro- 
duce fruit,  even  in  places  whose  mean  temperature  is  higher 
than  that  of  Italy,  and  you  would  look  in  vain  to  these  ap- 
parently luxuriant  Irees  for  that  fruit  they  cannot  produce  with- 
out a summer  temperature  higher  than  they  can  there  attain. 
Effect  of  Climate  on  the  Growth  of  Grass. — I shall,  in  writing 
on  this  subject,  include  under  the  popular  term  Grass  all  the 
different  species  commonly  cultivated  for  their  leaves  and  stems, 
not  including  those  particular  cases  where  they  are  cultivated 
for  their  seeds. 
Grass  vegetates  with  the  air  at  any  temperature  above  the 
freezing  point  and  the  ground  at  a temperature  varying  from  40° 
to  36°,  and  any  temperature  less  than  this  prevents  its  growth 
entirely.  On  the  other  hand,  unless  there  be  a great  quantity 
of  moisture  in  the  air  it  will  scarcely  vegetate  with  a degree  of 
heat  greater  than  66°,  unless  the  ground  be  moistened  naturally 
or  artificially.  It  will  vegetate  best  with  the  air  on  the  point  of 
saturation ; hence  on  a porous  subsoil  and  in  a dry  atmosphere 
the  pastures  are  during  summer  burnt  up  by  drought.  From 
many  careful  observations  made  in  our  own  country  as  to  quan- 
tities of  stock  kept,  and  length  of  grass  in  different  seasons,  as 
well  as  by  the  accounts  given  by  Humboldt,  &c.,  I have  come  to 
the  following  conclusions  respecting  the  influence  of  climate  on 
the  growth  of  grasses  generally  : — 
1st.  That  the  growth  of  grass  is  always  proportionate  to  the 
heat  of  the  air,  if  a sufficiency  of  moisture  be  present  in  the 
atmosphere. 
2nd.  That  in  our  climate  the  moisture  present  is  rarely  suffi- 
cient to  allow  the  temperature  to  have  full  effect  when  the  tem- 
perature exceeds  56°,  but  if  moisture  be  artificially  supplied,  as 
by  irrigation,  to  catch-water  meadows,  then  vegetation  will  still 
proceed  in  proportion  to  the  heat. 
3rd.  That  when  the  temperature  of  the  air  is  between  36°  and 
41°,  grass  will  only  vegetate  with  a fifth  part  of  the  force  that  it 
will  when  the  temperature  is  56°.  Thus  land  that  will  keep  ten 
sheep  per  acre  in  the  latter  case,  will  only  keep  two  in  the 
former.  That  from  41°  to  46°  its  growth  is  two-fifths  or  double 
that  of  its  growth  when  the  temperature  is  under  41°,  and  it  will 
then  keep  four  sheep  instead  of  two.  Again,  from  46°  of  tem- 
perature to  50°  its  growth  will  rise  to  seven-tenths,  or  it  will  keep 
on  the  same  ground  from  five  to  seven  sheep ; and  from  50"  to 
56"  it  generally,  unless  assisted  by  an  artificial  addition  of 
moisture,  arrives  at  its  maximum,  but  if  the  month  of  June  be 
very  moist,  it  will  continue  to  grow  with  an  increase  of  force  up 
to  60".  Hence  generally  grass  with  the  same  temperature  grows 
