Am'Nov.1!'imrm'}^^^^  Groups,  their  Constituents  and  Properties.  549 
their  supposed  nutritive  properties  depend.  Mucilaginous  com- 
pounds, analogous  in  composition,  are  widely  distributed  among 
plants ;  few,  if  any,  being  entirely  free  from  such.  But  occasionally 
mucilage  is  met  with  in  abundance  as  one  of  the  characteristics  of 
phaneragamous  plant-groups;  for  instance,  in  Malvaceae,  particu- 
larly in  the  mallow  tribe  proper  ;  in  the  tuber-bearing  orchids,  and 
in  the  bulb-bearing  liliaceae.  We  meet  with  it  in  the  epithelial 
layer  of  the  seeds  of  flax,  quince,  certain  salvias,  plantagos  and  of 
many  other  plants;  we  find  it  in  leaves,  like  benne,  senna  and 
buchu ;  and  we  observe  it  as  an  exudation,  most  likely  caused  by 
internal  metamorphosis,  upon  shrubs  and  trees  of  the  genus  Astra- 
galus, the  Mimoseae,  Pruneae  and  other  groups  of  the  dicotyledons. 
Closely  allied  to  these  mucilaginous  bodies,  which  are  known  to  be 
carbohydrates,  are  others  in  the  composition  of  which,  apparently, 
less  hydrogen  is  contained,  though  this  still  awaits  final  settlement 
by  renewed  researches  with  the  means  at  present  afforded  by 
science.  I  refer  to  what  are  known  as  pectin-com pounds  and  which 
are  met  with  in  sweet  and  acidulous  fruits  generally,  such  as  the 
lemon  and  orange,  the  currant  and  gooseberry,  the  pear  and  other 
pomeae,  the  peach  and  allied  pruneae,  the  raspberry  and  blackberry, 
the  pumpkin  and  melon,  the  cranberry  and  huckleberry,  and  many 
others.  But  these  pectin  bodies  have  also  been  found  in  other 
parts  of  plants,  and  notably  in  certain  biennial  and  perennial  fleshy 
roots,  usually  associated  with  sugar,  and,  like  the  latter,  constituting 
most  likely  a  reserve  compound  in  the  carrot  and  parsnip,  the 
turnip,  peony,  and  in  the  officinal  yellow  dock,  senega  and  gen- 
tian. These  amorphous  compounds  present  many  difficulties  in 
their  purification  and  liberation  from  coloring  and  other  princi- 
ples ;  yet  it  is  now  generally  conceded  that  the  metapectic  acid 
of  the  beet  and  other  plants  is  in  reality  identical  with  arabic 
acid  of  gum  acacia. 
While  the  algae  grow  submersed,  the  lichens  are  air  plants,  usu- 
ally fastened  upon  rocks,  walls  or  the  bark  of  woody  plants.  Some 
are  of  industrial  value  as  the  source  of  litmus  or  cudbear ;  others 
are  used  as  food,  principally  in  the  arctic  regions,  and  a  species  of 
Lecanora  has,  by  some  scientists,  been  regarded  as  the  manna  of 
the  Hebrews.  The  species  which  have  been  employed  medicinally 
were  regarded  as  being  valuable  in  pectoral  complaints,  and  of  these 
the  so-called  Iceland  moss  still  holds  its  place  in  the  Pharmacopoeia. 
