CHAPTER XI 



THE PLANT FORMATION 



230. The nature of formations. A formation is an area of 

 vegetation, such as a meadow, a forest, a prairie, a bog, a cliff 



Fig. 79. A lichen formation on the face of a chff {Lecanora and Umbilicnria). 



covered with Hchens, or a pond of water-hlies. Its limits may 

 be sharply defined, as is true of the forest, cliff, and pond, or one 

 formation may pass almost insensibly into another. For example 



21.5 



