^'^:,wtm'}  Plant  Groups,  their  Constituents  and  Properties.  551 
authority  as  Aspidium ;  the  rhizome,  which  is  the  part  used,  has 
the  same  constituents  which  are  met  with  in  other  ferns,  varying  in 
proportion  in  the  different  species.  Fern  rhizomes  are  generally 
reputed  to  possess  anthelmintic  and  taenifuge  properties,  due  to  the 
presence  of  filicic  acid  or  similar  compound.  Only  a  small  number 
of  ferns  are  indigenous  to  the  northern  hemisphere  ;  the  home  of 
most  of  the  nearly  3,000  species  of  ferns  is  in  damp  localities  of  the 
tropics,  where  quite  a  number  assume  a  tree-like  aspect,  attain  a 
height,  sometimes,  of  fifty  or  sixty  feet,  and  in  habit  resemble  a 
stately  palm.  A  handsome  Cyathea  and  others  have  a  farinaceous 
tissue  which  is  used  as  food  and  establishes  a  resemblance  with  the 
sago  palms,  independently  of  the  general  habit.  The  astringency, 
which  is  observed  in  some  degree  in  the  male  fern,  becomes  so  strong 
in  some  species  that  they  may  be  used  for  tanning.  In  fact,  among 
the  cryptogamous  plants,  only  those  having  vascular  tissue  contain 
tannin,  and  the  ferns  are  the  only  group  in  which  notable  quantities 
of  it  are  met  with.  This  compound — or  rather  class  of  compounds 
— is  more  generally  diffused  among  phanerogamous  plants,  where 
different  kinds  appear  to  be  most  largely  found  in  the  leaves,  in  the 
bark  of  trees,  in  rhizomes,  in  some  perennial  roots,  and  in  such 
morbid  excrescences  like  galls. 
The  gymnospermous  plants,  to  which  class  the  pines  belong, 
though  numerous  as  regards  individuals,  comprise  only  about  450 
species,  two-thirds  of  which  are  coniferous.  The  latter  are  charac- 
terized by  the  production  of  volatile  oils,  many  of  them  being 
hydrocarbons,  and  by  the  secretion  of  resins.  Both  these  classes  of 
compounds,  as  we  have  seen,  occur  only  to  a  very  limited  extent  in 
the  cryptogamae.  But  volatile  oils  and  allied  aromatic  compounds 
are  met  with,  to  some  extent  at  least,  in  most  phaenogamous  orders. 
They  are  produced  in  certain  groups  of  the  grasses,  orchids  and 
lillies  of  the  monocotyledons,  and  likewise  in  certain  genera  or 
species  of  the  violets,  geraniums,  leguminosae,  rosaceae,  compositae 
and  others,  while  these  principles  very  generally  pervade  the  scita- 
mineae,  laurels,  labiatae,  umbelliferae,  the  myrtles,  oranges  and  other 
groups.  A  peculiar  variety  of  volatile  oils,  containing  sulphur, 
imparts  a  unique  pungency  to  the  cruciferous  and  some  allied 
plants;  and  similar  in  composition,  though  different  in  scent,  are 
the  volatile  oils  of  the  alliums  and  of  certain  umbelliferae,  of  which 
the  asafetida  plants  are  representatives. 
