^ovemberS"}    A  Botanical  Excursion  to  Mexico.  589 
Mexican  can  be  classified  into  several  ecological1  communities,  as 
follows  : 
(1)  Hydrophytic  Community,  composed  of  Hydrophytes,  or 
water.plants. 
(2)  Xerophytic  Community,  composed  of  Xerophytes,  or  desert- 
plants. 
(3)  Halophytic  Community,  composed  of  salt-loving  plants. 
(4)  Mesophytic  Community,  including  those  plants  found  in  in- 
termediate situations,  such  as  plants  of  the  tropical  forests,  palm  for- 
ests, bamboo  thickets,  temperate  deciduous  forests,  sub-tropical 
evergreen  forests,  and  plants  of  the  arctic,  alpine  and  prairie 
regions. 
The  Valley  of  Mexico  is  especially  suited  to  ecological  inquiries. 
It  is  an  elevated  circular  valley,  closed  in  by  two  distinct  ranges  of 
hills,  the  oldest  porphyritic  rocks  to  the  north  and  east,  which,  be- 
fore volcanic  disturbances  began,  sloped  gradually  southward  toward 
the  Isthmus,  and  the  newer  volcanic  mountains  to  the  south  and 
west,  built  up  in  a  later  period  and  closing  off  to  the  south  and 
east  the  gradually  sloping  plain,'  thus  forming  the  basin-shaped 
plateau  known  as  the  Valley  of  Mexico.  Ajusco,  the  oldest  vol- 
canic peak,  stands  like  a  sentinel  on  the  southern  rim  of  the  basin, 
and  from  its  summit  to  the  base  of  the  valley  extends  a  lava  bed, 
known  locally  as  the  Pedregal.  On  the  southeast  rim  of  the  Valley 
rises  the  ice- capped  peak  of  Popocatepetl  (17,780  feet),  and  to  the 
eastward,  connected  with  the  conical  volcano,  the  ridged  back-bone 
of  Ixtaccihuatl,  also  snow-capped,  considered  by  geologists  to  be  built 
of  the  older  porphyritic  rocks.  Along  the  base  of  the  eastern 
range  three  fresh-water  lakes,  Texcoco,  Chalco  and  Xochimilcho, 
are  found,  while  the  partially  drained  basin  of  Lake  Texcoco 
forms  an  alkaline  plain,  stretching  along  the  Mexican  Railroad  to  the 
hills,  which  jut  southward  into  the  plain  at  Guadalupe. 
The  lakes  and  communicating  ditches  furnish  the  hydrophytes  ; 
the  alpine  summits  of  Popocatepetl,  Ixtaccihuatl  and  Ajusco 
(13,612  feet),  the  alpine  plants;  the  lava  beds  afford  a  large  number 
of  xerophytes,  while  the  alkali  plain  near  Lake  Texcoco  grows  a 
number- of  halophytes,  which  reach  their  greatest  numerical  devel- 
1  Ecology  is  the  study  of  plants  with  reference  to  their  environmental  con- 
ditions, and  covers  the  field  of  the  so-called  biology  of  plants. 
