208 THE FORMATION 



Other than mosses and hysterophytes. The lodge-pole pine formation (Pinus 

 murrayaua-hyluim) , with light values often less than .005, is nearly or quite 

 destitute of all but hysterophytic undergrowth. Such extremely dense for- 

 mations are examples of coordinate association merely, since the formation 

 is reduced to a sinj^le superior layer, in which the individuals of the facies 

 bear the same spatial relation to incident light. In layered formations, in 

 addition to the subordinate relation of other species to the facies, there is, of 

 course, a kind of coordinate association manifested in each layer. 



258. Water-content association. Schouw' was the first to give definite 

 expression to the value of the water-content of the soil for the grouping of 

 plants. He established four groups: (i) water plants, (2) swamp plants, 

 (3) plants of moist meadows, (4) plants of dry soils. The first he termed 

 hydrophytes, introducing the term halophytes to include all saline plants. 

 Thurmann^ recognized the fundamental influence of water-content upon as- 

 sociation, and further perceived that the amount of water present was deter- 

 mined primarily by the physical nature of the soil. He distinguished plants 

 which grow in soils that retain water as liygrophiloiis, and those found upon 

 soils that lose water readily as xerophiloits. Those which seemed to grow 

 indififerently upon either were termed ubiqtdtotis. The latter correspond in 

 some measure to mesophytes, but they are really plants possessing a con- 

 siderable range of adaptability, and do not properly constitute a naturari 

 group. Warming^ proposed the term mesophytes to include all the plants 

 intermediate between hydrophytes and xerophytes. He recognized the para- 

 mount value of water-content association as the basis of ecology, and upon 

 this made a logical and systematic treatise out of the scattered results of 

 many workers. Schimper* placed the study of vegetation upon a new basis 

 by drawing a distinction between physical and physiological water-content, 

 and 1 y pointing out that the last alone is to be taken into account in the 

 study of plant life, and hence of plant geography. Accepting the easily 

 demonstrable fact that an excess of salts in the soil water, as well as cold, 

 tends greatly to diminish the available water of the soil, i, e., the chresard, 

 it is at once seen why saline and arctic pla^its are as truly xerophytic as those 

 that grow on rocks or in desert .sands. An anomalous case which, however, 

 physical factor records have explained fully, is presented by many plants 

 growing in alpine gravel slides, strands, blowouts, sandbars, etc., in which 

 the water-content is considerable, but the water loss excessive, on account 



'Grund/iige cincr allRcmcinen Pflanzengeographic, 157. 1823. 

 *Essai (le phytostatiquc, etc. 1849. 

 '/. c, 116. 1896. 

 */. c. i. 1898. 



