IV.  Annual  Growth  and  Winter  Protec-
tion of  Shoots  and  Buds."^

728.  Winter  coiLditioiL8.f  —  While  herbs  are
subjected  only  to  the  damp  warm  atmosphere
of  summer,  woody  plants  are  also  exposed  dur-
ing the  cold  dry  winter,  and  must  protect  them-
selves against  such  conditions.  The  air  is  dryer
in  winter  than  in  summer;  while  at  the  same
time  root  absorption  is  much  retarded  by  the
cold  soil.  Then,  too,  the  osmotic  activity  of
the  dormant  twig-cells  being  much  reduced,  the
water-raising  forces  are  at  a  minimum.  It  is
easy  to  see,  therefore,  that  a  tree  in  winter  is  prac-
tically under  desert  conditions.  Moreover,  it  has
been  found  by  various  investigators,  contrary  to
the  general  belief,  that  cold  in  freezing  is  only  indi-
rectly the  cause  of  death.  The  real  cause  is  the
abstraction  of  water  from  the  cell  by  the  ice  crys-
tals forming  in  the  intercellular  spaces.  Death
ensues  because  the  water  content  is  reduced  below
the  danger-point  for  that  particular  cell.  It  was
formerly  thought  that  on  freezing,  the  cells  in  the
tissue  were  ruptured.  This  is  not  so.  Ice  almost
never  forms  within  the  cell,  but  in  the  spaces
between.  Freezing  then  is  really  a  dmng  proc-
ess, and  dryness,  not  cold,  causes  death  in  winter.
To  protect  themselves  in  winter,  trees  provide
various  waterproof  coverings  for  the  exposed  sur-
faces and  reduce  the  acti\'ity  of  the  protoplasm
so  that  it  will  be  less  easily  harmed  by  the  loss  of
water  abstracted  by  the  freezing  process.
