1903] FLORA OF SOUTHERN CALIFORNIA 269 
plants bears to the whole number of species in each area; those 
in the footing show the proportion of each class in reference to 
the whole flora of southern California. The right-hand column 
shows the proportion which the total flora of each area bears to 
the whole flora. 
It appears by these tables that there is a notable difference in 
the development of the various classes of plants in the several 
areas. Thus the Desert has the largest proportion of shrubs and 
the smallest of perennials herbs—a condition which is exactly 
reversed in the Nevadan area. The Desert and the Cismontane 
areas have nearly an equal percentage of annual species, and 
each has more than twice as many as the Nevadan. It also 
appears that the Cismontane has nearly as many species as both 
the other areas combined. The percentage of arboreal species 
is unexpectedly found to be nearly the same in each region, but 
could the comparison be made between the number of individual 
trees in each area, the Nevadan would far exceed the others. 
The principal cause of these differences is doubtless to be 
found in the climatic character of the several areas. The short 
season of winter rainfall in the two Sonoran areas permits the 
development of annual plants, but is unfavorable to perennial 
herbs. The cooler climate, and the numerous living streams 
and springs in the Nevadan area are more favorable to perennials 
than are the conditions in the other areas. Why the proportion 
of shrubby species should be so much ‘smaller in the Nevadan 
than in either of the other areas is less evident, but it is prob- 
ably due to the occupation of the land by trees, whose shade 
discourages the multiplication of shrubs. But it is also a fact 
that the chaparral of this area, while extensive, is composed of 
fewer species than is the same formation in the other areas. 
ADAPTATION OF PLANTS TO CLIMATIC CONDITIONS. 
The chief condition of their environment, as I have already 
stated, to which the plants of southern California have to adapt 
themselves, those which are paludose or aquatic excepted, is the 
aridity of the climate, resulting from prevalent high temperature, 
and a scanty and irregular precipitation. This necessity to some 
