i9i 2] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 201 



of Yucca. This plant has slender wandlike branches which are 

 regularly winter-killed for several inches. Other plants very 

 abundant here are Croton texensis, the spiny ragweed (Franseria 

 acanthicarpa), Orobanche multi flora, and Cenchrus tribuloides. 



The steeper hills of this formation are too xerophytic even for 

 Bigelovia, and here the low shrubby composite Hymenatherum 

 acerosum is the most abundant plant. Associated with it usually 

 are Crassina (Zinnia) grandiflora, Ephedra trifurca, whose leafless 

 stems both look and feel like a branched Equisetum, and several 

 species of Eriogonum. The Crassina has a method of seed dis- 

 persal that is not mentioned in any text with which I am familiar. 

 The very large ligules of its ray flowers, instead of dropping off, 

 become dry and papery, and when the seeds are ripe, the whole 

 head separates from the stem and goes rolling off over the plain 

 and hills, a diminutive tumbleweed. 



The arroyos of this dissected edge of the mesa show an inter- 

 esting succession of societies, characterized by successively smaller 

 plants as one ascends. If sufficiently large to deposit considerable 

 sand, their lower courses are occupied by the desert willow (Chilop- 

 sis saligna), a plant with pretty Catalpa-like blossoms. Its leaves, 

 however, resemble very closely those of such a willow as Soli* 

 longifolia. It is the tallest shrub outside of the mountains and the 

 cottonwood forest, reaching a height of 15-20 ft. 



Ascending the arroyo this society is replaced by one in which 

 Fallugia paradoxa is dominant. This rosaceous plant is very slow 

 to drop its leaves, retaining them until late in the winter. It has 

 pure white blossoms and plumose fruit. It grows to a height of 

 3-4 ft. in dense thickets, which are even more dense underground, 

 where about half of the stems are found, in which respect it 

 resembles the famous mesquite of more southern regions, the plant 

 which gave rise to the expression that in New Mexico one <k climbs for 

 water and digs for wood." Here grow also two low perennial ever- 

 green composites, Berlandiera lyrata and Melatn podium cinereum. 

 After the summer rains there appears here, as on the mesa, a rela- 

 tively abundant growth of annuals, among which the composites 

 Hymenopappus flavescens, Thelespcrma gracile, and Bailey a mul- 

 tiradiata, together with Pentstemon ambiguus, are characteristic. 



