18 ASSOCIATION 



formation or region, resulting in species guilds' (Artengen- 

 ossenschafien). The association in this case is largely one of 

 cominunity of origin or area, and of concomitance of 

 migration. It is especially characteristic of areas adjacent 

 to formational and regional limits. As it is a direct result 

 of invasion, the species guild will be more fully considered 

 under this principle. 



Light association {assodatin luminalis.) Schouw (1823: 

 166) divided plants into three groups upon the basis of light 

 intensity in the habitats, using the terms plantae aphotistae, 

 subterranean or darkness plants, plantae umtnosae, shade 

 plants and plantae apricae, light or sunshine plants. Kabsch 

 (1865:111) distinguished two classes, light plants and night 

 plants, i. e., subterranean, dividing the former into sun and 

 shade plants. Wai-ming: (1895: J5) designated sun plants as 

 heliophilous, and shade plants as heliophobous. Pound and 

 Clements (1900:166) applied the term photophilous to sun 

 plants and sciophilous to shade plants, while Clements (1902: 

 166) has made a threefold division which corresponds 

 essentially to Schouw 's, namely, heliophyta, sciophyta and 

 scotophyta. 



The constituent species of formations show two funda- 

 mentally different groupings with respect to light In the 

 one case, the individuals are on the same level, or nearly so, 

 in such a way that each has direct access to sunlight. Such 

 an arrangement is characteristic of most grassland and her- 

 baceous formations. In the case of desert formations, 

 there is often considerable difference in the height of the 

 plants, but the distance between them is so great as to ad- 

 mit of direct illumination of all. This arrangement may be 

 termed coordinate association {associatio superior). ' 

 In forests, thickets, and many herbaceous wastes, the 

 height and density of certain species enable them to domin- 

 ate the formation. In a dense forest, the trees receive 



'Pound and Clements. The Phytogeographv of Nebraska 2 ed 175 

 1900. 



