Observations  on  Parasitic  Fungi. 
445 
The  pea  is  frequently  clothed  with  the  white  mildew  O'idium 
monilioides  (fig.  5),  occasionally  succeeded  by,  and  often  con- 
founded with,  Erysiphe  communis,  from  both  being  found  on  the 
same  leaf ; but  as  both  species  have  distinct  spores  enclosed  in 
very  different  receptacles,  their  connexion  must  be  accidental  ; 
and  as  this  Oidium  infests  a great  many  other  plants,  I propose 
calling  it  Oidium  commune.  I should  not  venture  on  this  inno- 
vation, had  I not  carefully  examined  every  plant  which  I shall 
name  as  being  attacked  by  this  species,  under  a high  power  of  a 
compound  microscope,  and  satisfied  myself  that  no  difference 
exists,  except  that  which  arises  from  the  nature  of  the  plants  on 
which  it  subsists,  and  between  a young  and  mature  specimen  of 
the  fungus  itself.  I have  also  grown  it  on  the  pea  and  other 
plants  by  scattering  on  their  leaves  spores  from  mildewed  grapes. 
Among  the  plants  attacked  by  this  mildew  are  the  cucumber, 
melon,  and  gourd ; all  the  varieties  of  cabbage  and  turnip ; dog- 
wood, acacia,  maple,  sycamore,  and  apple-trees  ; rose,  gooseberry, 
and  peach-trees;  the  hop  and  grape  vine;  the  verbena,  aquilegia, 
and  several  other  flowers.  Its  effects  are  more  or  less  injurious, 
in  proportion  as  the  plants  on  which  it  grows  are  hard  and  tough, 
or  tender  and  succulent. 
Cucumber  and  melon  plants  are  soon  overrun,  rendered  un- 
productive and  eventually  killed  by  it.  The  cabbage  becomes 
stunted;  the  leaves  turning  yellow  beneath  the  mildew,  and  soon 
rotting.  Yellow  spots  also  ensue  on  the  leaves  of  turnips,  caus- 
ing them  to  fade  prematurely  ; the  bulbs  become  pithy,  frequently 
spotted  inside,  and  keep  badly  when  stored.  The  leaves  are,  at 
the  same  time,  subject  to  the  attack  of  Botrvtis  parasitica  (8), 
which  produces  similar  results.  The  leaves  of  the  sycamore  and 
other  trees,  although  not  often  destroyed  by  this  mildew,  on 
account  of  their  tough  nature,  are  still  so  choked  by  it,  that  they 
cannot  perform  their  proper  functions,  and  the  trees  do  not  thrive. 
It  kills  the  young  shoots  of  peach-trees,  and  spoils  the  fruit — and 
prevents  the  rose  from  blooming  in  its  full  beauty.  On  the  hop 
plant,  its  effects  are  well-known  to  be  so  destructive,  that  it  is 
quite  unnecessary  to  describe  them.  The  grape-vine  appears  to 
have  become  a much  more  recent  prey  to  this  pest — my  first  ex- 
perience of  it  being  in  1846,  and  I have  not  heard  of  any  one 
who  had  noticed  it  previously.  I was  walking  at  the  time  in  the 
garden  of  the  Horticultural  Society,  discussing  the  subject  of  the 
potato  mildew  with  Mr.  Thompson,  when  a vine  at  the  end  of  one 
of  the  hothouses  (outside)  attracted  my  observation  ; the  leaves 
being  thickly  spotted  with  patches  of  this  mildew.  Next  year, 
many  crops  were  attacked  by  it,  both  indoors  and  out ; and  last 
year  it  became  general,  in  many  cases  destroying  the  entire  crops. 
It  is  remarkable  that  the  mildew  attacks  the  fruit,  leaves,  petioles, 
VOL.  XI.  2 G 
