186  Pipitzahoic  Acid  and  Species  of  Perezia.  {^'A^u'iss*™' 
slender  tube,  distinctly  two-lipped,  with  the  three-toothed  exterior  lip 
longer  than  the  interior,  with  two  teeth ;  the  anthers  are  long  caudate, 
with  a  more  or  less  prominent  lanceolate  tip  or  crest-like  appendage. 
The  akenes  are  elongated  cylindrical  or  slightly  angled,  often  some- 
what spindle-shaped,  with  a  discoid  apex,  bearing  a  pappus  of  copious 
capillary,  somewhat  scabrous  bristles.  All  the  species  are  perennials^ 
with  more  or  less  rigid  leaves,  with  the  simple  stem  bearing  the  white 
or  purplish  flowers  in  solitary  heads  or  in  corymbs.  They  are  exclu- 
sively confined  to  the  warmer  parts  of  the  American  continent,  and 
the  40  or  50  species  known  are  equally  divided  between  its  southern 
and  northern  divisions.  Those  occurring  in  the  latter  are  found  in 
the  highlands  of  Mexico  and  the  adjacent  parts  of  Central  America, 
extending  beyond  the  Mexican  border  into  the  territory  of  the  United 
States  as  far  north  as  the  34°  of  north  latitude. 
The  North  American  species  belong  all  to  a  group  distinguished  by 
the  similarity  of  all  the  florets  within  one  head,  the  three-toothed 
exterior  lip  of  the  corolla  being  even  in  the  marginal  flowers,  scarcely 
if  at  all  longer  than  the  interior,  forming  the  well-marked  natural 
section  Acourtia,  established  first  as  a  proper  genus  by  De  Candolle. 
In  the  group  embracing  the  South  American  species,  the  Perezias  pro- 
per, found  mostly  south  of  the  Equator,  the  interior  lip  of  the  corolla 
is  considerably  shorter  than  the  ligulate  exterior.  For  the  establish- 
ment of  the  characters  of  the  species  belonging  to  the  first  of  these 
groups,  and  for  the  determination  of  the  limits  of  their  distribution, 
we  are  indebted  to  Prof.  Gray,  who  has  particularly  given  many  years 
of  his  arduous  labors  to  the  elucidation  of  the  most  prominent  feature 
of  the  North  American  flora,  the  difficult  order  of  Composite,  with 
such  eminent  and  distinguished  success.  The  characters  of  these 
plants  were  before  but  vaguely  defined,  and  variously  understood ; 
hence  we  find  them  referred  to  various  genera ;  some  were  described 
under  the  genus  Dumerilia,  Less.,  others  as  species  of  Trixis  and  Prous- 
tia,  section  Thelecarpus  and  Acourtia,  D.  C.  Of  the  24  North  Ameri- 
can species  recognized  by  Gray  seven  are  found  within  the  south- 
western territory  of  the  United  States ;  they  were  mostly  brought  to 
light  during  later  years  by  the  explorations  of  the  arid  regions  between 
southwestern  Texas  and  the  Pacific  Ocean.  The  first  five  of  the  spe- 
cies enumerated  below,  the  flora  of  the  United  States  has  in  common 
with  northern  Mexico,  and  the  two  following  seem  to  be  confined  to 
its  limits. 
