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Transactions Texas Academy of Science. 
this in the case of South America 1 as would be expected where the 
cacti reach a very noteworthy development and where Prosopis, 
several zygophyllaceous genera and Mentzelias are more numer- 
ously represented in the deserts of Chila and' Argentine than in 
the region here discussed. Perhaps it would not be inaccurate to 
say that this formation contains a larger percentage of species 
peculiarly North American than any other that could be selected. 
Here are many Nyctages, Mallows and the very interesting com- 
posite group of Tubuliflorae-Helenieae which is strikingly a Lower 
Sonoran group and many of the genera quite limited to rocky 
plateaus of the arid Southwest. But more important floristically, 
of course, are the groups which embrace the more striking ecologi- 
cal types of the formation, — dwarf shrubs like Krameria, Micro- 
rhamus, Parkinsonia ; the wandlike Fouquiera, and the “all thorn” 
Koeberlinia ; the dracenoid lilies embracing Yuccas, Nolina, Dasy- 
lerion and Hesperaloe, the Agaves, and finally, the most unique 
group of the cacti. On the basis of its floral and ecological dis- 
tinctness, one gains the impression that this southwestern plateau 
desert has had a more effective barrier about it than has the conti- 
nent as a whole and the statement is justified that it is to be re- 
garded as a distinct floral province and an originative or creative 
center for ecological forms like those in other deserts but from 
quite distinct floristic groups. 
ecological forms and their occurrence in the vegetation. 
The plant forms in the sotol vegetation are constructed upon a 
relatively few general types not unlike those which have been 
formed by similar environment in other arid regions of the earth, 
but for the most part floristically distinct. The dominant factor 
in shaping these adaptation forms has been the water supply. In 
some plants it has led to abnormal storage of water and other pro- 
ducts and the whole plant structure has been subordinated to this. 
In others, quite the contrary course has been followed and plants 
of this type are strikingly devoid of succulent tissue — i. e., Chap- 
arral shrubs. Other plants meet the emergency of insufficient 
water supply by virtual encystment of their living substance and 
consequent drying and shrinking or folding of their mechanical 
parts, and yet others exist the greater part of the time as resistent 
seed capable of germinating and completing a vegetative cycle 
1 See Relation of Flora of Lower Sonoran Zone to Arid Regions of South 
America, Bot. Gaz. XXVI, 1898, pp. 121-147. 
