STRUCTURAL STUDIES ON SOUTHWESTERN 
CACTACEAE. 
CARLETON E. PRESTON. 
(WITH NINE FIGURES) 
Tuat the Cactaceae, a family in every respect appearing to 
be of recent origin, are unique in the extent of their adaptive 
modifications to arid conditions is a commonplace. Not only 
does this adaptation affect the external parts as gross structures, 
but in still greater measure does it work changes upon the 
minute anatomy by modifying the histological elements. These 
internal changes, keeping pace with and supplementing the 
external, raise this family to the first rank among the desert 
plants. A critical study of the minute anatomy from the stand- 
point of ecology, therefore, should be most fruitful. Moreover, 
since the family is one of late origin, in which definite groups 
have so far not crystallized out, but between which nearly all 
connecting links are present, a general comparative examination 
of internal structure should afford at least some further clue as 
to the lines upon which the various groups have developed or 
are still developing, adding to or confirming the evidence already 
obtained from the study of external characters. 
Viewed from the standpoint of the systematist, this paper, 
being a study of the general histology of eight southwestern 
forms, can by itself be of little value, serving only as an addition 
to the number of species already critically examined, Although 
in collecting the material for study it was my intention to choose 
such forms as could be considered typical, I found, as work went 
on, that too much variation occurs to admit of their use in this 
way except upon the broadest lines. In order to do complete 
and Satisfactory systematic work on the basis of structural char- 
acters, evidently one would have to compare, not only all the 
genera within the family, but at least the larger subdivisions and 
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