588  A  Botanical  Excursion  to  Mexico.  {K£S' 
and  place  this  on  a  piece  of  clean  white  paper.  Drop  upon  this  one 
drop  of  a  5  per  cent,  solution  of  ferric  chloride.  The  drop  remains 
for  some  time  in  the  form  of  a  globule,  unabsorbed.  If  senna  alone 
be  present,  the  powder  as  seen  through  the  drop  remains  compara- 
tively unchanged.  If  any  chestnut  leaves  be  present  they  will  be 
shown  by  the  particles  under  the  drop  in  less  than  thirty  seconds, 
turning  a  dark  blue  or  black  color  (tannin).  A  very  small  per  cent, 
of  such  an  adulterant  as  chestnut  leaves  can  thus  be  detected.  The 
solution  of  ferric  chloride  should  not  remain  on  the  powder  more 
than  one-half  minute  to  give  the  most  delicate  reaction. 
The  epidermal  cells  of  the  chestnut  leaves  were  found  to  average 
25  micromillimetres  in  diameter. 
Powdered  senna,  adulterated  with  chestnut  leaves,  may  be  exam- 
amined  under  a  microscope  and  the  adulteration  detected  by  the 
presence  of  the  tracheids  and  pitted  cells  which  compose  the  mid- 
rib of  the  chestnut  leaf.  The  finding  of  bundles  of  these  fibres  in 
senna  powder  is  good  evidence  of  adulteration.  Occasionally,  frag- 
ments of  these  wood  cells  may  be  found  in  senna  powder,  but  they 
are  rare. 
A  BOTANICAL  EXCURSION  TO  MEXICO. 
By  John  W.  Harshberger. 
Mexico  is  a  profitable  place  for  the  botanist  to  visit,  on  account  of 
the  richness  of  the  flora,  numerically  speaking,  and  because  of  the 
assemblage  in  one  geographical  region  of  many  species  peculiar  to 
certain  physical  or  meteorological  zones.  The  aquatic  flora,  the 
alpine  flora,  the  desert  and  tropical  floras,  are  all  found  associated 
within  a  radius  of  a  few  miles.  Altitude  and  the  distribution  of  the 
rainfall  explain  this  somewhat  interesting  feature  of  the  flora.  The 
problems  presented  to  the  botanist  for  solution  are  very  intricate, 
and  it  is  necessary  for  him,  in  order  to  solve  the  more  difficult  ques- 
tions, to  spend  considerable  time  in  the  field,  where  the  plants  are  to 
be  found  surrounded  by  natural  conditions. 
Each  of  the  plant  communities  into  which  the  flora  of  a  country, 
as  vast  as  Mexico,  can  be  divided,  can  be  distinguished  by  the  com- 
ponent plants,  which,  together  with  their  collective  features,  give 
character  to  the  vegetation  of  a  particular  meteorological,  geologi- 
cal or  physical  region  of  the  earth's  surface.    Such  a  flora  as  the 
