122 THE PLANT 
increase of air-spaces is correlated with reduction of the palisade, and a 
decided increase in the sponge. An increase in water supply is indicated by 
the absence of storage tissues, and the reduction of the vascular system, 
which, however, is more closely connected with a diminished need for 
mechanical support. 
166. Plant types. The necessity for decreasing or increasing water loss 
in compensation of the water supply has made it possible to distinguish two 
fundamental groups of plants upon the twofold basis of habitat and struc- 
ture. These familiar groups, xerophytes and hydrophytes, represent two 
extremes of habitat and structure, between which lies a more or less vague, 
intermediate condition represented by mesophytes. These show no char- 
acteristic modifications, and it is consequently impossible to arrange them 
in subgroups. Xerophytes and hydrophytes, on the other hand, exhibit 
marked diversity among themselves, a fact that makes it desirable to 
recognize subgroups, which 
correspond to fundamental 
differences of habitat or 
adaptation. It is hardly neces- 
sary to point out that these 
types are not sharply defined, 
or that a single plastic species 
may be so modified as to ex- 
hibit several of them. The 
Fig. eh ee ana @ extremes are always clearly 
defined, however, and they 
indicate the specific tendency of the adaptation shown by other members 
of the same group. 
167. Xerophytic types. With the exception of dissophytes, all xero- 
phytes agree in the possession of a deep-seated root system, adapted to 
withdraw water from the lower moist layers, and to conserve from loss 
from the upper dry layers. Reservoirs are developed in the root, however, 
in relatively few cases. The stem follows the leaf more or less closely in 
its modification, except when the leaf is greatly reduced or disappears, in 
which event the stem exhibits peculiar adaptations. While the leaf is by 
far the most strikingly modified, it is a difficult task to employ it satisfactorily 
as the basis. for distinguishing types. Several adaptations are often com- 
bined in the same leaf, and it is only where one of these is preeminently 
developed, as in the case of succulence, that the plant can be referred to a 
definite type. The latter does not happen in many species of the less 
