122 



THE PLANT 



increase of air-spaces is correlated with reduction of the pahsade, 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 

 extremes are always clearly 

 defined, however, and they 



Fig. 32. Mesophyll of Pedicidaris procera 

 (chresard, Vo%, light, 1). ;>< 130. 



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 



