ORIGINATION OF SELI--GENERATING MATTER 615 
stomata show special constructions, and all parts of the shoot heavily 
coated and hardened; root-systems have been extended horizontally 
and the individuals thus isolated, being more or less accommodated 
to soils containing a large proportion of salts. The spinose, stubby, 
and switchlike perennials which result from such action are char- 
acteristic of low, inclosed desert basins, like the Salton, and those 
of southern Africa, and central Asia, where the scanty rainfall does 
not occur within such regular limits as to make distinct moist seasons. 
The second form of desert vegetation is one in which the absorp- 
tive function has become highly developed and the capacity gained 
for conserving the surplus water taken up during the moister seasons. 
The Cactaceae are the most prominent representatives of this type 
in North America, and some of this group, as well as other species 
representing a wide range of families, have the capacity for sufficient 
water to meet the needs of the individual for a decade, while forms 
are known which might carry out their cycles of reproduction for a 
quarter of a century by the use of the surplus accumulated within 
their bodies. Such succulents display not only the reduction of the 
shoot and of the foliar surfaces together with induration of the 
epidermis, but have also this capacity for accumulating water and 
are hence desert plants par excellence, representing the apex of 
specialization to desiccation. 
As a total result of the slow desiccation of any region, therefore, 
a very important proportion of the flora would consist of moisture- 
requiring species, or mesophytes, and the remainder would be 
included in two classes, the spinose forms with reduced shoots and 
roots, and the succulents with atrophied shoots, but with the addi- 
tional development of storage structures in some organ of the shoot 
or root. The total number of species within an arid region is not 
less than that of the most densely closed tropical area, but the num- 
ber of individuals is less, the interrelations of the individuals and 
species are not identical, and the competitive struggle for existence is 
of a nature widely different from that of a tropical forest. Increase 
in aridity tends to localization in distribution, and increase in humid- 
ity to diffuseness. 
Evidence of the existence of xerophytes in previous periods of 
desiccation is extremely scanty. Calamites and lycopods with a 
