20 ASSOCIATION 



denser the forest. In the Otowanie Woods {Quercus-Eicoria 

 hylium), layering usually begins at a light value of .1 (1= 

 normal sunshine in the open). Thornber' has found the 

 same value to obtain in the thickets of the Missouri bluffs. 

 In these, again, layers disappear at a value of .005, the 

 extreme diffuseness making assimilation impossible except 

 for occasional mosses and algae. A number of herbaceous 

 plants are present in the spring, but these are all prevernal 

 or vernal bloomers, which are safely past flowering before 

 shade conditions become extreme. In the traxinus-Catalpa- 

 alsium, all inferior holophytic vegetation disappears between 

 the light value of .004 and that of .003. The spruce-uine 

 formation (Picea-Pinus-hylium) of the Rocky Mountains, with 

 a light value of .01, usually contains but a few scattered 

 herbs, mostly evergreen: in some cases there are no subord. 

 inate plants, 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 formations 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. 



Water content association {associatio aqualis). De 

 CandoIIe (1820:14) summarized the characteristics of water 

 plants and dry land plants, without, however, recognising 

 them as two distinct groups. Schoww (J823:t57) was the 

 first to give definite expression to the value of the moisture 

 content of the soil in the grouping of plants. He established 

 three groups: (1) swamp plants {plantae paiudosae) including 

 also slime plants {plantae Umosae), which he regarded as 

 passing readily into aquatic plans (plantae emersae); (2) 

 "plants which grow in m.oist meadows" {plantae uliginosae, 



^Thornber, J. J. The Prairiegrass Formation in Region I. Rep. 

 Bot. Surv. Neb. 5:36, 46 1901 ^ ^ 



