The Environment. 23 



than attribute to the high vapor-tension a general dampering effect upon 

 evaporation, both from the plant and from the soil, but it is not improb- 

 able that research will discover plant-structures which are specifically 

 related to atmospheric humidity, especially as it has been shown (Lloyd, 

 19050) that the ocotillo and probably other plants have the ability to 

 take advantage of rain which has not yet reached the earth. 



TOPOGRAPHY AND SOIL. 



The surface of the high plateau of Mexico on which the guayule 

 finds its home is broken up into mountain ranges of various extent, sep- 

 arated by wide, flat valleys or "bolsones." The middle reaches (playas) 

 of these valleys are nearly level and have a deep, fine, alluvial soil, 

 containing a vast amount of capillary water. In this soil the mesquite 

 is generally found in abundance, and often of large size. Within these 

 flats are frequently found more or less extensive areas (alkali spots, salt 

 spots) where salts have accumulated and where the salt-bushes (Atriplex 

 'sp.) only may be found. 



From the periphery of these alluvial plains, extending to the foot- 

 hills of the mountain ridges, is a gentle slope of low gradient, the foot- 

 slope, characterized by a gravelly soil (plate 5), which becomes more 

 and more stony as the foot-hills are approached. Here the soil is fre- 

 quently very shallow and may be confined to the crevices of the under- 

 lying rock. This condition becomes still more marked in the hills proper, 

 where the edges of the strata are often exposed and where the vegetation is 

 confined to the intervening fissures. The most widely distributed plants 

 of the foot-slope and adjacent ridges, and therefore the most characteristic, 

 are the alvarda or ocotillo (Fouquieria splendens) , the palma samandoca 

 (Samuelld earner osa Trelease), and the Cedros sotol (Dasylirion cedro- 

 sanum Trelease) - 1 The gobernadora or Mexican grease wood (Covillea sp.) 

 is also a very common plant of the foot-slopes and ridges, but is to be 

 found also in the alluvial plains and is therefore less characteristic. 



Of the species of Parthenium found in the region, the guayule is 

 confined to the foot-slopes and foot-hills, 2 being also abundant in hills 

 not above about 7,000 feet in altitude. It is therefore, like some of its 



1 Dasylirion cedrosanum Trelease (n. sp.). 



Subacaulescent. Leaves slightly roughened on the dorsal angles, pale, the 

 upper face glaucous, somewhat fibrous-brushy at tip, broad (20 mm.), 1.5 m. or 

 more long: prickles mostly 10 to 15 mm. apart, yellow or at length reddened at 

 tip, 3 to 5 mm. long, moderately heavy, upcurved or hooked, the whitish-yellow 

 intervening margin roughened by minute hyaline tipped denticles. Branches of 

 the narrow inflorescence rather elongated, about 7 by 60 mm. Fruit narrowly 

 elliptical, 4 to 5 by 7 to 9 mm., deeply and acutely notched, the style much shorter 

 than the wings. 



Cedros, Zac., Mexico, Lloyd, No. 118 the type, No. 82, 1908; Kirkwood, 

 No. 96, 1908. 



Allied to D. wheekri and D. graminifolium, from both of which it differs in 

 its smaller fruit not widened upwardly and with shorter style more conspicuously 

 surpassed by the wings A (fig. 4, and on the extreme right of fig. A., pi. i). 



The type is in the Herbarium of the Missouri Botanical Garden; cotypes in 

 the Gray and National herbaria. 



2 It is generally believed by those familiar with the plant that it affects more 

 particularly the south slopes, and this accords in general with my observations, 

 though it must not be inferred that it does not grow at all on north slopes. 



