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Observations  on  Parasitic  Fungi. 
slices  of  the  shoot  under  the  microscope,  I find,  however,  that  its 
mycelium  runs  amongst  the  healthy  tissue  (Tab.  C.  2).  I was 
also  greatly  surprised  on  discovering  that  it  was  not  confined  to 
this  part  of  the  plant,  but  that  it  actually  grows  in  comparatively 
large  tufts  on  the  small  hairs  with  which  these  shoots  are  clothed 
(Tab.  C.  3). 
Many  wonderful  phenomena  have  been  brought  to  light  by 
means  of  the  microscope ; but  I know  of  no  botanical  discovery 
more  curious  than  this.  I have  named  it  Oidium  setarum.  It 
preys  upon  the  healthy  hairs  until  they  are  exhausted,  when  they 
turn  brown,  and  their  fluid,  vitiated  by  decomposition,  is  trans- 
mitted to  the  parts  beneath.  Having  dug  up  potatoes  for  ex- 
amination, in  every  stage  of  growth,  I find  the  roots  generally 
more  or  less  diseased,  and  this  mildew  frequently  upon  them,  but 
not  on  the  tubers. 
On  a review  of  the  above  observations,  and  after  a careful  ex- 
amination of  the  various  specimens  which  I have  collected, 
clearly  displaying  the  insidious  and  predatory  habits  of  several 
species  of  parasites,  very  nearly  allied  to  that  found  on  almost 
every  part  of  the  potato  plant,  it  is  obvious  that  the  latter  ex- 
hibits no  sign  whatever  of  any  exemption  from  their  power,  which 
occasionally  lays  low  even  the  denizens  of  the  soil.  On  the  con- 
trary, it  has  suffered,  as  might  be  expected  from  its  foreign  origin 
and  succulent  nature,  more  severely  than  many  of  them. 
Under  a firm  conviction  that  this  is  the  case,  it  may  not  be  un- 
interesting to  inquire  by  what  process  it  is  probable  that  the 
gangrenous  blotches  on  the  potato  and  other  plants  are  pro- 
duced. 
Most  chemical  authors  attribute  them  to  putrefaction  or  oxy- 
dation  of  the  tissue  of  the  plants ; but.  in  what  way  that  first  com- 
mences is  confessedly  involved  in  mystery.  It  is,  however,  my 
belief  that  this  destructive  force  is  set  up  within  the  plants  by 
means  of  the  decay  of  the  internal  filaments  or  roots  of  the  mil- 
dews which  vegetate  on  their  surface.  Decaying  matter  being 
thus  secretly  introduced  among  the  tissues  produces  at  first  a 
small  spot,  which  soon  spreads  iuto  a blotch  by  contagion.  That 
ammonia  is  given  out  abundantly  during  the  decomposition  of 
these  fungi  is  certain,  both  from  the  pungent  smell  which  they 
emit  when  enclosed  in  a box,  and  from  other  tests  to  which  I 
have  submitted  them.  It  is  the  property  of  ammonia  to  turn 
vegetable  substances  brown,  and  such  is  the  effect  produced  by 
the  decomposition  of  these  fungi  in  the  juices  of  plants  ; indeed, 
from  their  gelatinous  nature  and  the  homogeneous  dark  mass 
which  the  parts  beneath  a mildewed  patch  ultimately  present,  I 
am  of  opinion  that  the  entire  substance  of  their  internal  filaments 
is  dissolved  in  the  sap  of  plants. 
