(311) 
Pinus ponderosa Jeffreyt Cornus Nuttallit 
Pinus Coulteri Azalea occidentale 
Abies concolor Uva-ursi Parryana 
Heyderia decurrens * Uva-urst patula 
* Juniperus occidentalis Uva-ursi Pringler 
*Salix Scouleriana * Leptodactylon Hookeri 
Quercus Kelloggi Eriodictyon Parryt 
* Castanopsis sempervirens Ramona pachystachya 
Ribes nevadensis Pentstemon jacintensts 
Rubus leucodermts Sambucus mexicana 
Rubacer velutinus *Symphoricarpus Parishu 
Cercocarpus ledifolius Chrysothamnus stenophyllus 
Cercis occidentalis Artemesia tridentata 
* Ceanothus cordulatus Tetradymia canescens 
The species designated by an asterisk (*) are confined to the upper 
parts of the Transition and extend into the Canadian Zone. 
A study of the geographical distribution of the species found 
in the Transition Zone shows that about sixty per cent. are limited 
to California (including San Pedro Martir Mountain) or extend 
only into the southern part of the Cascade Mountains; fifteen 
per cent. extend northward along the Coast or the Cascade Moun- 
tains of Oregon and Washington; ten per cent. are common to 
California and the Great Basin, and twelve per cent. occur through- 
out the Western Arid Area. 
To express the phytogeographical conditions of this zone ac- 
curately it is evident that the divisions must reckon with the 
large percentage of purely Californian or endemic species. ‘The 
simple statement that the flora belongs to the Transition Zone, 
as is usually the case, or that it is in the Western Arid Area, is 
misleading, as many of the species are not found in other parts 
of these divisions. It is essential that the areas recognized by 
Merriam, and based upon moisture, be divided into sub-areas, 
which are chiefly the result of a third factor—geographical segregation. 
Without undertaking a discussion of the entire Western Arid 
Area, the California mountains, on account of the large number 
of endemic genera and species, are recognized as a distinct division 
of the area. This may be termed the Sierran Sub-area. 
The Sierran Sub-area is best characterized by Pinus ponderosa 
Jeffreyi, Pinus Lambertiana and Heyderia decurrens. Its geo- 
graphical limits on the north are most satisfactorily defined by 
