ASSOCIATION 209 
of extreme heat or reduced’ air pressure. The effect of these conditions is 
to produce a plant xerophytic as to its aerial parts, and mesophytic or even 
hydrophytic as to subterranean parts. Such plants may, from their twofold 
nature, be termed dissophytes; they are especially characteristic of dysgeo- 
genous soils in alpine regions where transpiration reaches a maximum, but 
are doubtless tc be found in all gravel and sand habitats with high water- 
content. With these corrections, the concept of water-content association, 
which owes much to both Warming and Schimper, but is largely to be cred- 
ited to Thurmann, becomes completely and fundamentally applicable to all 
vegetation. 
Up to the present time, the general character of the habitat, together with 
the gross appearance of the plant itself, has been thought sufficient to deter- 
mine the proper position of a plant or a formation in the water-content clas- 
sification. Such a method is adequate, however, only for plants and forma- 
tions which bear a'‘distinct impress. For an accurate classification into the 
three categories, hydrophytes, mesophytes, and xerophytes, it is necessary 
to make exact determinations of the normal holard and chresard of the hab- 
itat, and to supplement this, in some degree at least, by histological studies. 
Except in the case of saline, acid, and frozen soils, the holard alone will be a 
fairly accurate index, especially in habitats of similar soil composition. For 
an exact and comprehensive classification, however, and particularly in com- 
parative work, the chresard must constitute the sole criterion. As the latter 
has been ascertained for very few formations, and in Nebraska and Colorado 
alone, the present characterization of many plants.and formations as hydro- 
phytic, mesophytic, or xerophytic must be regarded as largely tentative, and 
the final classification will be possible only after the thorough quantitative 
investigation of their habitats. 
The water-content groups, hydrophytia, mesophytia, and xerophytia, in- 
clude all formations found upon the globe. The exactness with which this 
classification applies to vegetation is made somewhat more evident by divid- 
ing mesophytia into forest and grassland. This is based primarily upon light 
association, but it also reflects water-content differences in a large degree. 
The groups thus constituted represent the fundamental zonation of the veg- 
etative covering with respect to water-content. Ocean, forest, grassland, 
and desert correspond exactly to hydrophytia, hylophytia, poophytia, and 
xerophytia. The difference is merely one of terminology: the first series 
takes into account the physiognomy of the vegetation itself, while the other 
emphasizes the causative factors, 
