Potato
     
     White potatoes  are  edible  tubers  that  grow  at  the  end  of
underground stems  of the  plant Solanum  tuberosum, a  member  of the
NIGHTSHADE family.   Above  ground, the  plant has a  stem and coarse,
dark green  leaves resembling those of the  tomato.  Its flowers range
from white  to purple,  and  occasionally fruits develop that resemble
small green   tomatoes.   Like  the berries  of the  nightshade, these
fruits   are poisonous.   The  tuber's skin ranges from tan to purple,
and its  flesh is  usually white  or light  yellow, although  in  some
Andean varieties  it is  purple.   The tuber  has external   buds,  or
"eyes," that can sprout into new plants.  These  eyes, rather than the
seeds of the fruit, are planted to  grow a new crop.
     
      Food  Value   According to  food experts, a diet of potatoes and
milk will   supply  all the  nutrients the human body needs.  Potatoes
contain plentiful  carbohydrates  and  some  protein,  calcium,    and
niacin.   They also  contain a considerable amount of  vitamin C. They
are cooked fresh and can also be frozen or  dried.  They are processed
into flour, starch, and alcohol  and are used as fodder, especially in
Europe.  In the  United States, annual per-capita consumption is 19 kg
(42   lb) of  fresh potatoes and 13 kg (30 lb) of processed  potatoes,
such as frozen french fries and potato chips.
     
      Cultivation    Potatoes  are  the  fourth  largest  world  crop,
surpassed only   by wheat, rice, and corn.  The leading potato-growing
countries in  1990 were the USSR, Poland, and the United  States.  The
leading U.S.  states, in order of production,  were Idaho, Washington,
Maine, Oregon, and Colorado.
     
     Potatoes need a cool growing season, with temperatures  averaging
16 deg  to 18 deg C (61 deg to 64 deg F).  Small,  eyed tubers (or cut
portions of  larger tubers)  are planted  10 to 15 cm (4 to 6 in) deep
in well-drained,  loamy soil.   When the plant tops emerge, additional
soil is  used to   protect  the tubers  from  insects  and  accidental
exposure to   the  sun. (Sunlight  causes a  potato to  turn green and
produce a  poison, solanin.) Potatoes then mature in 100 to  130 days.
The tubers  can be stored for up to 8 months at  temperatures slightly
above freezing, but at temperatures  below 5 deg C (41 deg F), some of
the potato  starch turns  to sugar. Chemicals applied to stored tubers
or growing   plants  can delay  sprouting, because  sprouting  greatly
reduces the food value of potatoes.
     
      History   The  potato plant is native to the Peruvian Andes.  It
was   cultivated in  Peru and  Bolivia and,  along with  maize, was  a
staple of  the Inca  diet.   It was  introduced into  Europe by  16th-
century Spanish  explorers.   Because the  potato is a  nightshade, it
was at first shunned as a food by Europeans.
     
      It  was supposedly  endowed with  powers such as the ability  to
cure impotence, and so long as the plant remained rare  in Europe, its
price often  reached astronomical  heights.   By 1573, however, it was
being grown in Spain, and shortly  thereafter in the Low Countries and
in Switzerland.
     
     The English were familiar with the potato by 1586 but  considered
it merely a botanical curiosity.  Only in the  mid-18th century was it
first planted as a food crop there.
     
      Because  the plant  grows well  in cool,  moist areas  and is  a
reliable, uncomplicated  crop, potatoes  soon became  a  major    food
staple, particularly  in Ireland.   (Today  the vegetable    is  often
called the "Irish" potato, to distinguish it from  the unrelated sweet
potato.) When  a potato  blight caused   the  crop to  fail (1845-47),
about 1  million people  perished   in Ireland  and  an  equal  number
emigrated.   Immigrants had   already  brought  the  potato  to  North
America by  then, and  it   was first  grown in volume in Londonderry,
N.H., in 1719.