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 siibterranean 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 to 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 wuth respect to water-content. Ocean, forest, grassland, 

 and desert correspond exactly to hydrophytia, hylophytia, poophytia, and 

 xerophytia. Th'^, 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. 



