208 THE FORMATION 
other than mosses and hysterophytes. The lodge-pole pine formation (Pinus 
murrayana-hylium), 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 single 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: (1) 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 hygrophilous, and those found upon 
soils that lose water readily as verophilous. Those which seemed to grow 
indifferently upon either were termed ubiquitous. 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 ‘natural 
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 ty 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 plants 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 
1Grundziige einer allgemeinen Pflanzengeographie, 157. 1823. 
*Essai de phytostatique, etc. 1849. 
5] ¢., 116. 1896. 
“loc, 3. 1898. 
