Great Basin: Arizona- New Mexico District. 575 
short speels of rainy weather. Such are the winter perennials (December 
—-March) with rootstocks and bulbs which develop new shoots and form leaves 
and flowers under the influence of the rising temperature and moisture and the 
winter annuals the seeds of which germinate in the wet period and soon pass 
away. Ihe spinose änd succulent forms of the dry fore-summer (April— June), 
such as cacti, yuccas, woody herbs, shrubs and trees, flower under the 
influence of the advancing temperature on parts which have stored water 
earlier in the season. The short humid mid-summer follows with a second 
burst of bloom of summer annuals and perennials followed by the dry after- 
summer which sees the ripening of fruits and seeds’). A numeric statement 
of the proportion of the various kinds of desert plants is reserved for treat- 
ment in a later section under “Colorado Desert Vegetation”. 
ı. The Desert plains, Mesas, Dunes and Gravel Hills. 
Desert Plain Formation. The Tularosa Desert may be considered as 
a typic one for New Mexico. Airiplex canescens and Koeberlimia spinosa form 
low trees associated with a suffrutescent Suaeda, a tufted Sporobolus and occa- 
sional specimens of a Zycium. In low spots and along the margins of cla 
bottomed washes covered with a crust of alkali appears Spirostachys occidentalis. 
Toward the middle of the Tularosa Valley the principal facies consists of 
Atriplex canescens with areas in which Yucca radiosa or Opuntia arborescens 
are abundant. This formation also extends into the Painted Desert or 
desert of the Little Colorado River in Arizona. The vegetation is scattered 
and scanty and consists of such characteristic arid land forms as Arriplex 
canescens, A. confertifolia, Sarcobatus vermiculatus, Dicoria Brandegei, Oxy- 
taenia acerosa, Tetradymia canescens, Ephedra sp., Yucca angustifolia and cac- 
tuses of several genera. With these grow Bouteloua oligostachya, Lupinus 
capitatus, Mirabilis multiflora, Riddellia tagetina and Zinmia grandiflora. This 
desert flora reached the Painted Desert er way of the Grand Canyon of the 
Colorado. 
Two plant formations in New Mexico may be distinguished where the sand dunes occur, 
viz: the sand dune formation and the dune hollow formation. 
Dune Formation: The most characteristic plant of the dunes is Rhus trilobata:) 
occurring in hemispheric bushes 4 to 8 feet high and binding the sand. Other characteristie 
woody plants of the dunes are Atriplex (Obione) canescens, two nm of rn 2 Chryso- 
thamnus) and Yucca radiosa. — A marked peculiarity of the white t a cotton wood 
(Populus) is occasionally found in the lower dunes, reaching a er in gr but ae more 
than 10 to 15 feet high while the mesquite is absent. 
Dune Bottom Formation: The bottoms among the dunes have a characteristic vegetation 
of a grama grass (Bouteloua), clumps of Ephedra and about the water holes, Distichlis spicata, 
1) Mac Dovcar, D. T.: Bot. Features of N. A. Deserts, loc. eit.; Course of Vegetation in 
southern Arizona. The Plant World. Sept. Oct. Nov. Dec. 1908; 'THORNBER, J. J.: Relation of 
plant Growth and Vegetation Forms to climatie Conditions, The Plant Wohl Xu: 1—7- 
2) Covirze, F. V. and MacDouvcar, .D. T.: Desert Botanical Laboratory of the Carnegie 
Institution, Publication 6, 1903. 
