LETTER TO THE SECEETAEY. XXI 



Minor, presents in the English periodical "Nature" for October 25 an 

 outline of his studies during the season, and this outline when filled out 

 will form a most important report for the Eleventh Annual Eeport of the 

 Survey. It will be seen at a glance that the report will be of the most 

 comprehensive character, and cannot fail to be of the highest interest 

 to our people. The tree vegetation, and especially the coniferse, were 

 made special objects of study, and many obscure points were cleared up. 

 Of a section of the Eocky Mountains comprising Colorado, Wyoming, 

 and Utah, Dr. Hooker says : 



Such a section of the Eocky Mountains must hence contain representatives of three 

 very distinct American floras, each characteristic of immense areas of the continent. 

 There are two temperate and two cold or mountain floras, viz : (1) a prairie flora 

 derived from the eastward ; (2) a so-called desert and saline flora derived from the 

 west ; (3) a subalpine ; and (4) an alpine flora ; the two latter of widely different origin, 

 and in one sense proper to the Eocky Mountain Eanges. 



The principal American regions with which the comparison will have first to be in- 

 stituted are four. Two of these are in a broad sense humid ; one, that of the Atlantic 

 coast, and which extends thence west to the Mississippi Eiver, including the forested 

 shores of that river's western affluents ; the other, that of the Pacific side, from the 

 Sierra Xevada to the western ocean ; and two inland, that of the northern part of the 

 continent extending to the Polar regions, and that of the southern part extending 

 through Xew Mexico to the Cordillera of Mexico proper. 



The first and second (Atlantic plus Mississippi and the Pacific) regions are traversed 

 by meridional chains of mountains approximately parallel to the Eocky Mountains, 

 namely, on the Atlantic side by the various systems often included under the general 

 term Appalachian, which extend from Maine to Georgia, and on the Pacific side by the 

 Sierra Xevada, which bound California on the east. The third and fourth of the 

 regions present a continuation of the Eocky Mountains of Colorado and Utah, flanked 

 for a certain distance by an eastern prairie flora extending from the British Possessions 

 to Texas, and a western desert or saline flora, extending from the Snake Eiver to Ari- 

 zona and Mexico. Thus the Colorado and Utah floras might be expected to contain 

 representatives of all the various vegetations of North America, except the small trop- 

 ical region of Florida, which is confined to the extreme southeast of the continent. 



The most singular botanical feature of Xorth America is unquestionably the marked 

 contrast between its two humid floras, namely, those of the Atlantic plus Mississippi, 

 and the Pacific one ; this has been ably illustrated and discussed by Dr. Gray in various 

 communications to the American Academy of Sciences, and elsewhere, and he has further 

 largely traced the peculiarities of each to their source, thus laying the foundations for 

 all future researches into the botanical geography of Xorth America ; but the relations 

 of the dry intermediate region either to these or to the floras of other countries had not 

 been similarly treated, and this we hope that we have now materials for discussing. 



Dr. Hooker sums up the results of the joint investigations of Dr. Gray 

 and himself, aided by Dr. Gray's previous intimate knowledge of the 

 American flora, from the Mississippi to the Pacific coast : 



That the vegetation of the middle latitudes of the continent resolves itself into three 

 principal meridional floras, incomparably more diverse than those presented by any 

 similar meridians in the Old World, being, in fact, as far as the trees, shrubs, and many 

 genera of herbaceous plants are concerned, absolutely distinct. These are the two 

 humid and the dry intermediate regions above indicated. 



Each of these, again, is subdivisible into three, as follows : 



1. The Atlantic slope plus Mississippi region, subdi visible into (a) an Atlantic, (/3) 

 a Mississippi Valley, and (y) an interposed mountain region with a temperate and sub- 

 alpine flora. 



