ENVIRONMENTAL CONDITIONS IN RELATION TO STRUCTURE. 5 
latter it is spongy. The immediate reason for this variation is not clear. 
The environmental conditions of the two classes of plants may be, as far 
as one can determine, quite the same. The cause of this must evidently 
be looked for elsewhere and, as will be shown below, may perhaps be asso- 
ciated with the character of the structures exterior to the chlorenchyma. 
A noticeable feature of many of the desert plants as opposed to those of 
the humid regions—a feature very conceivably related to the distribution of 
chlorophyll in the stems—is the open character or, in a measure, the loose- 
ness of growth. This is characteristic of both trees and shrubs. Among 
the shrubs this appearance is due in part to the relatively small number of 
branches and in part to the small size of the leaves. Quite likely the latter 
is the leading reason in either trees or shrubs. As aresult, all portions of 
the plant are exposed either to direct sunlight or to very strong illumination 
at all times during the day. The light conditions are such in consequence 
that wherever chlorophyll is to be found, even in the oldest parts, as it is 
in Parkinsonia, photosynthesis can take place. 
On the other hand, the various positions attained by the branches as 
related to the incident rays of light insure a certain degree of protection 
from the most intense light, as is found in such plants as Smilax, of the 
Florida scrubs, for example, by the erect posture of the leaves. 
In considering the affinities of the plants which have been under obser- 
vation and their distribution, it is of interest to note that their nearest 
relatives are desertforms. As one result of this fact, the possibility of com- 
paring congeners growing in desert and in humid regions is in many cases 
precluded and one important source of evidence as to the direct origin of 
these plants is thrown out. Those plants which are confined to North or 
South America include Baccharis, Cereus, Condalia, Covillea, Franseria, 
Krameria, Fouquieria, Keberlinia, and Olneya, which occur in the arid 
regions of North America only. Zphedra, Prosopis, and Zizyphus have 
nearly worldwide distribution, since they occur both in the Old and the New 
Worlds and in both hemispheres, but not in colder regions. Ceétis is the 
only marked exception and has representatives in cold temperate and 
humid regions, as well as in the warm and dry regions, and is practically 
cosmopolitan in distribution. 
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