174.  Action  of  root  hain  on  insoloble  Biibstaiioet.  Aoidityot
root  bain. — If  we  take  a  seeiJIing  which  has  been  grown  in  a
germinator,  or  in  the  folds  of  cloths  or  paper,  so  that  the  roots  are
free  from  the  soil,  and  touch  the  moist  root  hairs  to  blue  litmus
paper,  the  paper  becomes  red  in  color  where  the  root  hairs  have
come  in  contact.  This  is  the  reaction  for  the  presence  of  an  add
salt,  and  indicates  that  the  root  hairs  excrete  certain  acid  sub-
stances. This  add  property  of  the  root  hairs  serves  a  very  im-
portant function  in  the  preparation  of  certain  of  the  elements  of
plant  food  in  the  soil.  Certain  of  the  chemical  compounds  of
potash,  phosphoric  acid,  etc.,  become  deposited  on  the  soil  par-
ticles, and  are  not  soluble  in  water.  The  acid  of  the  root  hairs
dissolves  some  of  these  compounds  where  the  particles  of  soil  are
in  close  contact  with  them,  and  the  solutions  can  then  be  taken  up
by  the  roots.  Carbonic  acid  and  other  acids  are  also  formed  in
the  soil,  and  aid  in  bringing  these  substances  into  solution.
