448 



THE ECOLOGY OF PLANTS. 



the motley carpet of the oak-wood is made up of many 

 species dependent on the larger forms for shelter and shade, 

 or living as epiphytes, parasites, and humus saprophytes, 

 and including not only flowering plants, but ferns, mosses, 

 lichens, and fungi. The vegetation of the oak-wood is thus 



Fig. 182. Section of Soil, showing how Yorkshire Fog Gr<oss, Bracken, and BluebelJ 

 live together, their underground parts being at different depths. 



a mixed community with complex relationships, its members 

 struggling for existence and dominance, but it is a coherent 

 whole and may be studied as a unit or Formation. 



" Within the limits of a formation, there exist smaller 

 societies, each one with dominant, subdominant, and de- 

 pendent species of its own. Thus the undergrowth of an 



