in  its  Effect  on  Cultivation. 
655 
rises  to  40°,  and  will  perfect  their  seed  as  late  in  the  season  as 
the  ground  retains  a temperature  of  45°;  if  not  ripe  belore  the 
temperature  falls  below  this,  they  remain  unripe,  and  will  wither 
if  kept,  though  they  may  of  course  be  eaten  when  they  are  fresh 
dug  even  in  that  case.  Heat  and  moisture  increase  their  power 
of  vegetation,  hence  in  the  moist  and  warm  year  of  1828  there 
was  the  most  splendid  crop  of  potatoes  in  the  British  Islands  that 
has  been  known  for  a century.  A humid  climate  and  a soil 
abounding  in  organic  matter  are  circumstances  particularly 
favouring  their  growth,  and  they  are  thus  similar  to  oats  ; they 
exhaust  strong  clayey  soils,  but  flourish  on,  without  appearing  to 
injure,  light  and  peaty  soils.  They  also  appear  to  delight  in 
saline  matter,  and  it  is  found  by  experience  that  they  succeed  best 
in  Ireland,  Scotland,  and  the  western  parts  of  England,  Lanca- 
shire and  Cheshire  being  the  English  counties  where  they  are 
most  successfully  cultivated.  Potatoes  commonly  fail,  if  at  all, 
in  consequence  of  the  dryness  of  June  and  July,  and  therefore 
early  planting  is  best  for  them. 
The  culture  of  turnips  is  in  our  islands  highly  artificial.  In  a 
state  of  nature  turnips  would  (if  sown)  in  our  climate  spring  up 
in  February,  flower  in  May  or  beginning  of  June,  and  ripen 
their  seed  in  July  or  August.  But  it  was  found  that  turnips 
possessed  the  valuable  property  in  our  climate  of  being  arrested 
in  their  progress,  and  that  after  the  formation  of  their  root  and  its 
perfection,  the  cold  of  winter  would,  by  checking  their  growth, 
prevent  their  tendency  to  perfect  their  seed,  and  maintain  them 
in  this  state  till  the  rise  of  the  temperature  in  the  spring  was 
sufficient  to  allow  their  progress.  The  great  purpose  therefore 
for  which  the  turnip  is  cultivated  is  to  supply  food  for  cattle 
during  winter.  But  as  the  value  of  turnips  as  food  is  only  great 
before  they  begin  to  raise  up  a stalk  for  the  purpose  of  perfecting 
their  seed,  and  they  being  for  the  purpose  of  food  of  the  most 
value  just  at  the  time  when  they  begin  to  rise  into  stalk,  the  great 
purpose  to  be  accomplished  is  to  arrest  their  progress  just  at  the 
critical  time — if  arrested  sooner  they  not  coming  to  their  full 
size  ; and  if  permitted  to  grow  longer,  part  of  the  root  being  con- 
verted into  woody  fibres.  Turnips  will  vegetate  at  all  tempera- 
tures above  40° ; when  the  temperature  sinks  below  that  they 
cease  to  grow,  therefore  by  sowing  them  in  time  for  them  to 
attain  their  full  size  before  the  temperature  sinks  below  40°,  and 
yet  not  so  early  as  to  allow  them  time  to  run  to  seed,  their  growth 
will  be  checked,  and  they  may  be  kept  in  perfection  till  the  tem- 
perature again  rises  above  that  height.  The  season  when  the 
effect  of  our  climate  checks  the  growth  of  the  turnip  plant  is 
generally  November,  and  the  time  when  the  temperature  rises 
sufficiently  high  to  again  allow  of  their  growth  is  the  early  part 
of  March,  but  in  Scotland  this  rise  of  temperature  does  not  com- 
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