iqi2] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 199 



age for ten years: days entirely clear, 219.4; days partly clear, 

 104.9; days cloudy, 38.4. 



Plant formations and associations 



Floristically the country is very interesting, as it is the meeting 

 place of the northern and eastern flora with that of the arid south- 



west. 



mountains the 



the flora of the east would be able to recognize at least the genus 



b luZiV/ "^ ^**^ *•-"*- £> 



of nearly every plant encountered, while upon the mesa, with the 

 exception of Gaura and Salsola, scarcely a genus would be familiar. 



RIVER VALLEY FORMATIONS 



i. Cottonwood forest 



Along the Rio Grande, where the water-table is never very far 

 from the surface, there occurs an open and more or less pure forest 

 of Populus Wislizenii. The trees are small, due probably to the 

 operations of the native ranchers in their search for fuel and fence 

 posts, for individual trees of this species planted in dooryards are 

 veritable giants in girth. Scattered throughout this forest and 

 especially along the banks of the streams are a few willows, clumps 

 of the shrubs Baccharis Wrightii and Cassia baithinioides , while on 

 the ground grow J uncus balticus, Trifolium Rydbergii, Aster spino- 

 sus, and a little grass. This forest is monotonously uniform and 

 poor in species. 



2. Juncus-Houttuynia association 



Alternating with the last in its possession of the river banks 

 is a meadow-like association of which Juncus balticus and Hout- 

 tuynia californica are the dominant plants. Just what factors 

 determine which of these two associations will take possession of a 

 given area is not clear to the writer. However, it would seem that, 

 given time enough, the cottonwoods will occupy most of the 

 situations. The Juncus-Houttuynia association, however, is not a 

 necessary stage in the formation of a cottonwood forest, as the 

 latter may develop directly from a mud bank. Whenever a mud 

 bank is exposed for a few weeks in summer, a vigorous growth at 

 once appears, of which young cottonwoods, willows, cat-tails, and 



