116  APPLIED  BIOLOGY

nitrogenous  excretions  (those  containing  nitrogen)  formed  in
animals.  While  the  plant  cannot  eliminate  these  substaaces
directly,  it  is  interesting  to  note  that  many  of  them,  along
with  mineral  substances  taken  up  with  water  from  the  soil,
become  stored  in  leaves,  fruit,  seeds,  and  bark  —  all  of
which  parts  of  plants  are  frequently  detached,  resulting
in  the  elimination  from  the  plants  of  useless  and  possibly
sometimes  harmful  substances.  In  many  other  cases  sub-
stances of  no  further  use  to  the  plant  may  be  stored  per-
manently in  harmless  forms  in  stem  or  roots.
This  habit  of  storing  certain  useless  substances  is  peculiar
to  plants.  As  plants  grow  older  the  amount  of  stored  sub-
stance, especially  mineral  matter  from  the  soil,  increases.
Thus  a  leaf  collected  and  burned  in  early  summer  leaves
little  ash  as  compared  with  a  similar  one  taken  in  late  autumin.
Throughout  the  growing  season  water  has  been  evaporating
and  leaving  behind  in  the  leaves  the  mineral  materials  carried
up  from  the  soil.
Animals  do  not  store  excess  or  useless  mineral  substances
and  excretions,  but  eliminate  them  daily,  dissolved  in
water  which  is  discharged  by  means  of  the  kidney-system.
Since  most  plants  cannot  discharge  water  in  liquid  form,
storage  of  useless  mineral  matter  and  some  excretions  is
the  plant's  only  possible  way  of  doing  the  same  kind  of
necessary  work  which  the  kidneys  of  animals  do.
