24 ASSOCIATION 



logical water content, and not the physical, which deter- 

 mines the impress of the plant and of the formation. 

 Accepting the easily demonstrable fact that an excess of 

 salts or acid in the soil water, as well as cold, tends greatly 

 to diminish the available water of the soil, i. e., the physio- 

 logical water content, it is at once seen why saline, bog and 

 arctic plants are as truly xerophytic as those that grow on 

 rocks or in desert sands. An anomalous case, which, how- 

 ever, physical factor records have explained fully, is pre- 

 sented by many plants growing in alpine gravel slides, 

 strands, blowouts, sandbars, etc., in which the water con- 

 tent is considerable, but the water loss excessive, on account 

 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 subter- 

 ranean parts. Such plants may from their twofold nature 

 be termed dis so phytes {dissophyta, Svo-o-ds, double, <I>vt6v, 

 TO, plant) : they are especially characteristic of dysgeogenous 

 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 correc- 

 tions, the concept of water content association, which owes 

 much to both Warming and Schimper, but is largely to be 

 credited to Thurmann, becomes completely and fundamentally 

 applicable to all vegetation. 



The discrepancies in Warming's arrangement which have 

 just been mentioned have led to several attempts to rest a 

 fundamental distinction of groups upon some other factor 

 than that of water content. The most satisfactory of these 

 are the developmental classifications, such as those of Hult, 

 Graebner, Meigen and Cowles (termed physiographic by the 

 latter), and the nutrition content classification of Graebner. 

 Both classifications are valuable, especially the develop- 

 mental, because they furnish other view points of the many 

 relations of formations, but they have in no wise the funda- 

 mental value of the classification based upon water content. 

 The naturalisation and stabilisation of invading species by 



