Observations  on  Parasitic  Funrji. 
417 
mune  (fig.  5),  which  stunts  its  growth  and  turns  its  leaves  of  a 
brown  colour ; but,  on  account  of  their  tough  nature,  is  unable 
to  destroy  them. 
The  same  species  attacks  several  sorts  of  poppy  on  the  leaves 
and  stalks,  producing  moist  blotches  on  the  latter  in  damp 
weather,  and  frequently  killing  the  plants. 
Wild  angelica  and  other  herbaceous  plants  suffer  in  a similar 
way  from  the  same  species  as  the  last,  as  well  as  from  the  attacks 
of  Botrytis  ganglioniformis. 
The  leaves  of  the  common  stinging-nettle  are  infested  by  a 
smaller  species  of  O'idium,  which  grows  in  dense  tufts,  and 
closely  resembles  a species  which  I have  found  on  the  young 
shoots  and  roots  of  the  potato.  It  causes  the  leaves  to  curl  back, 
when  they  appear  covered  with  the  white  mould,  but  soon  after 
turn  black  and  die.  I have  also  once  or  twice  found  a species  of 
Botrytis  on  the  leaves,  which  I presume  is  the  Botrytis  urticae  of 
Mile.  Libert,  but  it  seems  to  be  rare  in  this  neighbourhood  on 
the  above  plant,  and  closely  resembles  B.  parasitica.  A bunch 
of  nettles  with  its  leaves  reflexed  and  spotted  with  mildew  pre- 
sents nearly  the  same  appearance  as  potato  haulm  under  the 
attack  of  Botrytis  infestans. 
Groundsel  is  often  much  infested  by  Uredo  senecionis  (Tab. 
D.  1),  as  well  as  by  Botrytis  ganglioniformis  and  Otdium  com- 
mune, and  the  effect  produced  by  them  is  precisely  similar  to 
that  of  Botrytis  infestans  on  the  potato  plant ; the  leaves  and 
stems,  however  luxuriant,  soon  become  blotched  beneath  the 
mildewed  spots,  and  the  latter  double  down  at  the  places  where 
the  thickest  tufts  had  grown ; the  leaves  (D.  2)  turn  brown,  and 
hang  dangling  for  a time  ; at  length  they  fall  away,  leaving  only 
the  blackened  stems.  The  roots  remain  generally  sound  to  the 
last,  but  I have  found  red  blotches  upon  them  which  caused  them 
to  rot  off. 
The  shepherd's  purse  has  afforded  another  very  remarkable 
case  of  the  same  kind  ; indeed,  on  no  plant  whatever  has  the 
effect  of  parasitic  mildew  appeared  more  clear  and  self-evident 
than  on  this.  It  is  very  commonly  attacked  both  by  Botrytis 
parasitica  and  Uredo  Candida,  but  the  Botrytis  is  by  far  the  most 
powerful  agent  of  the  two.  At  first  a few  scattered  plants  of  it 
appear  on  the  stem,  but  in  a short  time  a dense  tuft  is  formed, 
which  soon  encircles  them  to  an  extent  of  two  or  three  inches 
(Tab.  B.)  : this  part  next  assumes  a sw'ollen,  gouty  appearance, 
covered  with  the  mildew  as  with  fine  hoar  frost.  Sometimes  these 
parts  are  swollen  to  four  times  their  natural  size,  and  for  a time 
no  discolouration  takes  place,  but  as  soon  as  the  mildew  has 
fructified  and  run  its  course,  a moist,  brown,  gangrenous  blotch 
invariably  results  to  the  extent  of  the  mildew,  at  least,  and  the 
2 G 2 
