RYDBERG: PHYTOGEOGRAPHICAL NOTES 299 
these may be lacking in one or more of the districts or subdivisions, 
especially in Districts 8, 11, and 12, but some also in Districts 
4, 6, and 7. 
I. PLANTS WITH BOREAL-SYLVAN DISTRIBUTION 
In this discussion the word ‘‘sylvan’’ applied to a plant does 
not mean that it grows only in the deep woods, but that its distribu- 
tion has taken place in connection with the Northern Woods, and 
that it does not grow in the prairie or plains regions. The plant 
may be a forest species in the true sense or it may grow in open 
woods, thickets, or among rocks in more open places. The essen- 
tial point is that its migration east or west has taken place around 
and north of the plains, and not across them along the water- 
courses. To this category belong the transcontinental trees and 
most of the shrubs. The most important of the former is the 
quaking aspen, Populus tremuloides, also found in the Subalpine 
Zone. : 
In this and subsequent lists, species which are marked 
are rare in the Southern Rockies; those marked ‘‘}”’ attain their 
best development at higher altitudes and reach the Alpine zone; 
those marked ‘‘t”’ develop best lower down and reach the plains; 
those followed by ‘‘(Eur.)”’ are found also in Europe and usually 
also in northern Asia. The nomenclature is that of the author’s 
Flora.* 
the KD 
a. Forest species 
Trees 
Populus tremuloides . Salix Bebbiana 
Populus balsamifera** Betula papyrifera** 
Shrubs 
Juniperus sibirica (Eur.) Lepargyraea canadensis 
Dasiphora fruticosa (Eur.) Arctostaphylos Uva-ursi (Eur.) 
Chamaepericlymenum Linnaea americana 
candense Distegia involucrata 

* Flora of The Rocky Mountains and Adjacent Plains. New York. 1917. 
