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 a result, 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 Siirila.v, 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 desert forms. 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 Bacc/iaris, Ccrcns, Condalia, Covillca, Franseria, 

 Kramcria, Fouquicria, Kccberlinia, and Olneya, which occur in the arid 

 regions of North America only. Ep/u'dra, Prosopis, and Zisyfi/uts have 

 nearly worldwide distribution, since they, occur both in the Old and the New 

 Worlds and in both hemispheres, but not in colder regions. Celt is 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. 



