3J' 



Bulletin American Museum of Natural History. [Vol. XXII, 



bined effects of altitude and slope-exposure, the effects of the latter 

 being here manifested in an unusual degree. Where the walls of the 

 canon face north or north east the uppermost tree-zone consists of 

 Douglas and balsam firs {Pseudotsuga douglasii and Abies concolor) — 

 northern species which do not occur elsewhere in the canon. Below 

 this is a belt of pines (Pinus ponderosa) , succeeded in turn by a belt 

 of junipers [Juniperus occidentalis monospermd\ and pihon [Pinus 

 cdulis], usually more or less mingled with pines. Immediately 'below 

 the pinon belt is a zone which corresponds in the main to the Desert 

 of the Little Colorado; but since it has humid as well as arid areas, 

 forms of vegetation unknown on the desert interrupt its stretches of 

 cactuses, yuccas, and greasewoods. Still lower down another zone is 

 encountered which may be recognized by the presence of huge 

 cactuses, arborescent opuntias, agaves whose tall stems are conspicuous 

 land-marks, and many of the plants characteristic of the Lower 

 Colorado and Gila regions, together with subtropical humid forms 

 and a certain percentage of species not found elsewhere. The com- 

 plex and interacting effects of radiation and refraction, of aridity 

 and humidity, of marked differences in temperature at places of equal 

 altitude on opposite sides of the canon, of every possible angle of 

 slope-exposure, and of exposure to and protection from winds and 

 storms, produce a diversity of climatic conditions the effect of which 

 on the animal and vegetable life of the canon has been to bring into 

 close proximity species characteristic of widely separated regions, 

 and to crowd the several life zones into narrow parallel bands along 

 the sides of the canon — -bands which expand and contract in con- 

 forming with the ever-changing surface. The same conditions modify 

 and alter the species there present in the manner in which the evolu- 

 tion of new species is brought about. In short, the Grand Canon 

 of the Colorado is a world in itself, and a great fund of knowledge 

 is in store for the ^philosophic biologist whose privilege it is to study 

 exhaustively the problems there presented." 



Coville and McDougal give the following brief sketch of the flora 

 of the Bright Angel Trail :^ "A visit was made to the Grand Canon 

 of the Colorado with the expectation that its lower elevations would 

 afford lodgment for many desert plants, and that a descent from the 

 timbered rim at 6,866 feet, to the river at 2,436 feet, would permit 

 the traveller to see in a brief trip a wide range of desert vegetation. 

 Although the descent is full of botanical interest, and does carry one 



1 Desert Botanical Laboratory of the Carnegie Institution. Washington, Carnegie Institution. 

 November, 1903, 58 pp., 29 pll. and 4 text-figs. 



