JVo.l.] GRAY AND HOOKER ON THE ROCKY MOUNTAIN FLORA. 9 



arctic flora. Any list will therefore be to some extent arbitrary. For 

 example, in the Atlantic alpine list, while Gardamine helUdifolia, Silene 

 acaulis, Slbbaldia procumhens^ GnaphaUnm supinwnj Rhododendron Lappo- 

 nicum, Diapensia Lapponica, and the like, are strictly and exclusively 

 alpine, Ar^enaria Grcenlandica and Genm radiatum {Peelcii) are included 

 for reasons which any botanist who has ascended these mountains will 

 appreciate, although a form of the Arenaria sparingly occurs at low 

 levels in Southern New England and New York, and both on the tops 

 of the higher Alleghanies, where no characteristically alpine species 

 accompany them, and where such summits as are bare of trees are not 

 woodless on account of cold or any other incident of mere elevation. 



Notwithstanding the geographical extent of the country over which it 

 is spread, the North American alpine flora is meagre in species com- 

 pared with that of Europe. This will abundantly appear in the com- 

 parison to be made in another part of this report. Reasons connected 

 with geographical configuration and climate will account for this, but it 

 must also be remembered that the botany of the European Alps is 

 thoroughly known ; that of the Rocky and other western mountains 

 quite imperfectly so. 



II. — The Forest Region. 



1. Its trees. 



The most cou.spicuous portion of the vegetation of a country, and the 

 most important under more than one point of view, is its trees. Their 

 importance is most manifest in the district under consideration, where 

 less than a quarter of the area is capable of producing them, and of 

 which, owing to fires and other causes, only about half of what Major 

 Powell designates as " timber regions " are actually covered with forest. 

 Toward the north the case is more or less altered, especially in British 

 America, where, in a wide tract with moderatelj' abundant and well 

 distributed rainfall, and summers not excessively warm, the Atlantic 

 and Pacific forests join and intermingle. Southward, and indeed nearly 

 up to the northern boundary of the United States, trees a,re borne only 

 on the mountains and high plateaux, and along the immediate banks of 

 streams descending from these. 



The species of the whole Rocky Mountain region (taken in the widest 

 extent) which may claim the name of trees — even of treelike shrubs — 

 are not long to enumerate.* They are these : 



Sapindus marginatiis, Willd. Morus microphijUa, Buckley. 



Acer grandidoitatum, Nutt. Pojndus angustiJvUa, James. 



Negundo aceroides, Moench. Populus halsamifera, L. 



* We arc much aided in this accouut by Prof. C. S. Sargeut's article on The Forests 

 of Central Nevada, in Amer. Journ. Sci., ser. 3, xvii, June, 1879. and hy his Catalogue 

 of the Forest Trees of North America, 1880, printed by the United States Census 

 Bureau. 



