622 Bray: THE TIsSuES OF SOME OF THE 
ena in such a type as the star-cactus (Aviocarpus fissuratus), for ex-— 
ample, were under investigation, it is very certain that these could 
not be made fully clear without a very accurate and detailed knowl- 
edge of the machinery through which they operate, z. ¢., the various 
tissue systems in the organism. It is in just the unique—#ina 
sense pathological — types like this and various other Cactaceae, 
Yucceae, Agaveae and others that the study of vital processes will 
be keenest, and these also are just the types in which the study of 
the tissues from the physiological point of view gives the greatest 
zest. 
Since the study is distinctly a physiologic-anatomical one, the 
classification of tissues on the basis of function will be the prefer- 
able one as indicated in Haberlandt (/. c.) —for example, assim- 
ilation tissue will embrace cells whose main function is that of 
carbon assimilation, regardless of whether they are morpholog- 
ically the cortex of a stem or the mesophyl of a foliage leaf or 
the superficial zone in a leaf of a succulent type such as Agave. 
But it is always to be noted what morphological parts have be- 
come devoted to certain functions, as that of assimilation, since it 
is desired to know why the cells of certain topographical regions 
of a plant have been chosen for this or that function. 
In the present instance a study has been begun upon the tissues 
of a few of the most specialized of the ecological forms occurring 
in the very arid or desert region of Trans-Pecos Texas — a region 
which in this and in a subsequent geographic-ecological paper is 
designated the “ Sotol Country,” because of the predominance of 
the Sotol or Dasylirion Texanum upon the most characteristic 
parts of the region. From thé breaks of the Devil’s River west- 
ward across the Pecos and beyond to the front ranges of the 
Rocky Mountain system and far southward in Mexico, is @ vast 
limestone region well advanced in the progress of erosion, consist- 
ing of high conical hills and long ridges or divides with even slopes 
and with wide draws leading finally into deep-cut canyons such as 
those of the Pecos and Rio Grande and numerous side branches. 
This great area is thickly strewn with coarser or finer limestone 
débris resulting from the weathering of the soft upper patt of the 
formation. There is no rich soil-covering to hide the rocks, and 
the vegetation is the scant and open formation characteristic of 
