550   Plant  Groups,  their  Constituents  and  Properties.  {Ami?00v.!imarm' 
Its  bitter  principle,  cetraric  acid,  has  been  recently  employed  medi- 
cinally ;  to  what  extent  it  is  related  to  the  bitter  principles  of  other 
lichens  is  not  known. 
Coloring  matters — others  than  chlorophyll — are  formed  by  all 
plants  with  but  few  exceptions,  and  are  not  confined  to  plants  of 
low  organization.  Aside  from  those  principles  imparting  color  to 
flowers  and  fruit,  the  chemistry  of  which  is  still  involved  in 
obscurity,  many  of  the  industrially  important  chromogenes  and 
coloring  matters  have  been  investigated  to  a  larger  or  more  limited 
extent,  like  indigo  and  alizarin,  and  the  color-compounds  present  in 
or  obtainable  from  logwood,  quercitron  bark,  red  saunders,  saffron 
and  others  ;  but  little  or  no  chemical  relation  exists  between  these 
bodies,  and  with  the  exception  of  indigo  and  quercitrin  they  are 
mostly  met  with  only  in  one  plant,  or  in  a  few  which  are  botanically 
related.  However,  the  yellow  crystals  of  chrysophanic  acid  yielded 
by  a  Parmelia  are  identical  with  the  same  compound  prepared  from 
rhubarb  or  yellow  dock  of  the  Polygonaceae,  or  from  senna  and 
from  araroba  of  the  Leguminosse.  To  what  extent  this  principle, 
or  such  yielding  it,  may  be  distributed  throughout  the  vegetable 
kingdom,  must  be  left  to  future  researches. 
The  few  species  of  the  liver  mosses,  mosses  and  equisetums, 
which  were  formerly  medicinally  employed,  have  long  since  been 
discarded  from  regular  practice,  and  but  very  little  is  known  of  their 
chemical  constituents.  Of  more  than  passing  interest  is  the  pres- 
ence, in  the  species  of  equisetum,  of  large  amounts  of  silica,  doubt- 
less serving  for  the  purpose  of  protection.  Though  the  same 
mineral  compound  is  present  in  small  percentage  in  the  ash  of  most 
plants,  the  equisetums  share  only  with  the  grasses,  sedges  and  a  group 
of  the  palms  the  very  general  distribution  of  silica  and  its  relatively 
large  proportion. 
In  regard  to  the  medicinal  properties  of  the  ferns,  we  meet  with 
a  notable  distinction  between  the  effects  of  the  foliaceous  fronds  and 
those  of  the  rhizomes.  In  the  former,  we  find  mucilaginous  prin- 
'  ciples  predominating,  and  associated  with  mildly  astringent  and 
slightly  aromatic  qualities  ;  these  fern-leaves  have,  therefore,  been 
used  as  remedies  in  pectoral  complaints,  for  which  purpose  the 
European  and  North  American  species  of  Adiantum,  known  as 
Maidenhair,  are  still  employed.  However,  the  only  ferns  officinal, 
according  to  our  and  most  other  Pharmacopoeias,  are  known  by  our 
